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Avian incubation is a highly complex, adjustable behavior essential to embryo development and survival. When incubating, parents face a tradeoff between investing in incubation to maintain optimal temperatures for egg development or in self-maintenance behaviors to ensure their own survival and future reproduction. Because nest ectoparasites are costly and can reduce nestling quality and survival, infections could shift parental investment in current vs. future offspring. However, it is not well understood whether birds change investment in incubation in response to nest ectoparasitism, particularly in the context of other factors that are known to influence incubation behavior, such as ambient temperature, clutch size, and embryo development. We hypothesized that parents could respond to nest parasites by 1) investing more in incubation to promote the development of high-quality offspring to help offset the cost of parasites, 2) investing less in incubation or even abandoning their nest during incubation to save resources for future reproduction, or 3) being unresponsive to parasite infections, as incubation is more constrained by other factors. We tested these hypotheses by experimentally removing and adding mites in barn swallow nests at the start of incubation and deploying thermocouple eggs to measure egg temperatures at one-minute intervals until hatching. We found that while ambient temperature, clutch initiation date, embryo age, and clutch size were the main factors driving variation in egg conditions and parental incubation behavior, parasitized nests had higher mean egg temperatures, which could aid in nestling immune development. However, despite more optimal developmental temperatures, eggs in parasitized nests also had lower hatching success. Our results indicate that incubation is a dynamic behavior that is largely driven by the constraints of maintaining appropriate egg temperatures for development. Although quite costly upon hatching, ectoparasites appear to play a relatively minor role in driving variation in parental incubation investment.LAY SUMMARYIncubation is a complex behavior shaped by both internal and external factors.Ectoparasites often decrease quality and survival of nestlings and could influence investment in incubation because of tradeoffs in how parents spend energy.We manipulated parasites in barn swallow nests and used fake eggs with temperature sensors to collect data on egg temperatures and parental behavior throughout incubation.We found that other factors were the main drivers of variation in incubation behavior, but that eggs in parasite nests were warmer on average, which could help nestlings better cope with parasites upon hatching.Despite more optimal developmental temperatures, eggs in parasitized nests had lower hatching success.Although quite costly to nestlings, ectoparasites played a minor role in shaping parental incubation behavior.

Climate change is dramatically altering our planet, yet our understanding of mechanisms of thermal tolerance is limited in wild birds. We characterized natural variation in heat shock protein (HSP) gene expression among tissues and populations of free-living Tree Swallows (Tachycineta bicolor). We focused on HSPs because they prevent cellular damage and promote recovery from heat stress. We used quantitative PCR to measure gene expression of 3 HSPs, including those in the HSP70 and HSP90 families that have robust experimental connections to heat in past literature. First, to evaluate how tissues and, by extension, the functions that they mediate, may vary in their thermal protection, we compared HSP gene expression among neural and peripheral tissues. We hypothesized that tissues with particularly vital functions would be more protected from heat as indicated by higher HSP gene expression. We found that brain tissues had consistently higher HSP gene expression compared to the pectoral muscle. Next, we compared HSP gene expression across 4 distinct populations that span over 20° of latitude (>2,300 km). We hypothesized that the more southern populations would have higher HSP gene expression, suggesting greater tolerance of, or experience with, warmer local conditions. We observed largely higher HSP gene expression in more southern populations than northern populations, although this pattern was more striking at the extremes (southern Indiana vs. Alaska), and it was stronger in some brain areas than others (ventromedial telencephalon vs. hypothalamus). These results shed light on the potential mechanisms that may underlie thermal tolerance differences among populations or among tissues.

Studies of the evolutionary causes and consequences of variation in circulating glucocorticoids (GCs) have begun to reveal how they are shaped by selection. Yet the extent to which variation in circulating hormones reflects variation in other important regulators of the hypothalamic-pituitary-adrenal (HPA) axis, and whether these relationships vary among populations inhabiting different environments, remain poorly studied. Here, we compare gene expression in the brain of female tree swallows (Tachycineta bicolor) from populations that breed in environments that differ in their unpredictability. We find evidence of inter-population variation in the expression of glucocorticoid and mineralocorticoid receptors in the hypothalamus, with the highest gene expression in a population from an extreme environment, and lower expression in a population from a more consistent environment as well as in birds breeding at an environmentally variable high-altitude site that are part of a population that inhabits a mixture of high and low altitude habitats. Within some populations, variation in circulating GCs predicted differences in gene expression, particularly in the hypothalamus. However, some patterns were present in all populations, whereas others were not. These results are consistent with the idea that some combination of local adaptation and phenotypic plasticity may modify components of the HPA axis affecting stress resilience. Our results also underscore that a comprehensive understanding of the function and evolution of the stress response cannot be gained from measuring circulating hormones alone, and that future studies that apply a more explicitly evolutionary approach to important regulatory traits are likely to provide significant insights.

As urban areas expand, wildlife show adaptations to urban ecosystems. We tested two hypotheses for urban populations of red fox (Vulpes vulpes (Linnaeus, 1758)) in urban areas: the population pressure hypothesis, which posits that urban foxes make do with suboptimal habitat, and the urban island hypothesis, which presumes that urban areas provide high-quality habitat. We investigated habitat quality by investigating anthropogenic food in fox diets across a rural–urban gradient in Lancaster, Pennsylvania (USA). We used stable carbon isotopes because human food can have a distinct stable carbon isotope signature. We collected fox hair and stomach samples from 21 locations and extracted land use and land cover characteristics within a 100 ha buffer area. We found that higher δ13C values in fox hair were positively correlated with impervious surface cover and developed open spaces, key metrics of urbanization, and negatively associated with agricultural land cover, an indicator of rural habitats. Overall, fox hair δ13C was less related to urbanization and more related to the availability of developed open spaces that provide habitat with vegetation cover and access to nearby food sources. Our results suggest that urban habitats are high quality and support the growing literature revealing that certain species may thrive in urban areas.

Animals respond to sudden challenges with a coordinated set of physiological and behavioral responses that enhance the ability to cope with stressors. While general characteristics of the vertebrate stress response are well described, it is not as clear how individual components covary between or within individuals. A rapid increase in glucocorticoids coordinates the stress response and one of the primary downstream results is an increase in glucose availability via reduced glucose utilization. Here, we asked whether between- and within-individual variation in corticosterone directly predict variation in glucose. We collected 2673 paired glucose and corticosterone measures from 776 tree swallows (Tachycineta bicolor) from four populations spanning the species range. In adults, glucose and corticosterone both increased during a standardized restraint protocol in all four populations. Moreover, in one population experimentally increasing a precursor that stimulates corticosterone release resulted in a further increase in both measures. In contrast, nestlings did not show a robust glucose response to handling or manipulation. Despite this group-level variation, there was very little evidence in any population that between-individual variation in corticosterone predicted between-individual variation in glucose regulation. Glucose was moderately repeatable within individuals, but within-individual variation in glucose and corticosterone were unrelated. Our results highlight the fact that a strong response in one aspect of the coordinated acute stress response (corticosterone) does not necessarily indicate that specific downstream components, such as glucose, will show similarly strong responses. These results have implications for understanding the evolution of integrated stress response systems.

Maintaining avian eggs and young at optimum temperatures for development can increase hatching success and nestling condition, but this maintenance requires parental energetic demands. Bird nests, which often provide a structure to safely hold the eggs and nestlings and protect them from predators, can additionally be designed to help maintain eggs’ optimum temperatures by minimising heat loss, especially in climates where eggs cool rapidly when unattended. We collected and measured Prairie Warbler ( Setophaga discolor) nests in western Massachusetts, U.S. in 2009 and conducted a climate-controlled, nest-cooling experiment to determine how nest characteristics affect thermal properties for small, open-cup nesting birds. We then assessed if nests with better insulation properties resulted in any fitness benefits, and also tested if nest structural characteristics affected birds’ fitness. We found that nest characteristics influenced their thermal properties, with thicker, heavier, and larger nests having slower cooling rates and higher predicted equilibrium egg temperatures. Both nest cup depth and clutch size significantly declined over the breeding season, and we observed a trend, although non-significant, that nests with shallower cups had smaller clutches. Contrary to studies on cavity-nesting birds, we found no significant effects of nest thermal properties or nest structure on hatching and fledging success, nestling condition, brood parasitism, or nest survival. Prairie Warblers in our study site may already be adapted to build nests within a range that maximises their fitness. Furthermore, studies have shown that open-cup nests of other species are relatively thicker and more insulated in colder environments at higher latitudes than our study. Instead of building nests to solely minimise heat loss, open-cup nesting birds in temperate climates may also be driven by opposing selection pressures when building their nests, such as to prevent nestlings from overheating during hot days.

The ability to respond appropriately to challenges is an important contributor to fitness. Variation in the regulation of glucocorticoid hormones, which mediate the phenotypic response to challenges, can therefore influence the ability to persist in a given environment. We compared stress responsiveness in four populations of tree swallows (Tachycineta bicolor) breeding under different environmental conditions to evaluate support for different selective pressures in driving the evolution of glucocorticoid regulation. In accordance with the environmental unpredictability hypothesis, stronger stress responses were seen in more unpredictable environments. Contrary to the reproductive value hypothesis, the stress response was not lower in populations engaging in more valuable reproductive attempts. Populations with stronger stress responses also had stronger negative feedback, which supports a “mitigating” rather than a “magnifying” effect of negative feedback on stress responses. These results suggest that combining a robust stress response with strong negative feedback may be important for persisting in unpredictable or rapidly changing environments.

Animals must balance various costs and benefits when deciding when to breed. The costs and benefits of breeding at different times have received much attention, but most studies have been limited to investigating short-term season-to-season fitness effects. However, breeding early, versus late, in a season may influence lifetime fitness over many years, trading off in complex ways across the breeder’s lifespan. In this study, we examined the complete life histories of 867 female tree swallows (Tachycineta bicolor) breeding in Ithaca, New York, between 2002 and 2016. Earlier breeders outperformed later breeders in short-term measures of reproductive output and offspring quality. Though there were weak indications that females paid long-term future survival costs for breeding early, lifetime fledgling output was markedly higher overall in early-breeding birds. Importantly, older females breeding later in the season did not experience compensating life history advantages that suggested an alternative equal-fitness breeding strategy. Rather, most or all of the swallows appear to be breeding as early as they can, and differences in lay dates appear to be determined primarily by differences in individual quality or condition. Lay date had a significant repeatability across breeding attempts by the same female, and the first lay date of females fledged in our population was strongly influenced by the first lay date of their mothers, indicating the potential for ongoing selection on lay date. By examining performance over the entire lifespan of a large number of individuals, we were able to clarify the relationship between timing of breeding and fitness and gain new insight into the sources of variability in this important life history trait.

Summary 1.Glucocorticoid steroid hormones play a central role in regulating the metabolic state of animals, especially when they cope with unanticipated stressors in their environment. The cort-adaptation hypothesis predicts that baseline concentrations of glucocorticoids are adjusted upward to match energetic needs and promote fitness when individuals are faced with physiological challenges, including those associated with reproduction. 2.We tested the cort-adaptation hypothesis in the violet-green swallow (Tachycineta thalassina) by experimentally increasing flight costs during the offspring rearing period. Individual females were assigned to one of three treatments: light feather clipping (removal of 4 wing feathers), heavy feather clipping (removal of 8 wing feathers), or a sham-clip control. 3.We measured baseline corticosterone and body mass immediately prior to implementing treatments during the offspring rearing period and then 10 days after initial manipulations took place. We also quantified risk-taking behavior, offspring feeding rate, and the number of offspring fledged. Finally, we examined how treatments influenced offspring phenotype via measurements of nestling body mass and baseline corticosterone, as both measures have been associated with post-fledging survival. 4.We found that handicapped females significantly increased baseline corticosterone between the two sampling periods, with the magnitude of change in the light clipping and heavy clipping treatments 2.5x and 6.1x greater than controls, respectively. All individuals lost mass between the two sampling periods, but the degree of loss was greater for females in both clipping treatments relative to unmanipulated controls. 5.In contrast, we found no evidence of treatment differences in female risk-taking, offspring provisioning, or in the number of offspring fledged. Offspring raised by females in both handicapped treatments did have significantly elevated baseline corticosterone relative to those in control broods, but we detected no treatment differences in offspring body mass. 6.Our study found that handicapped females increased circulating glucocorticoids and were able to maintain critical parental care behaviors and raise a similar number of offspring as unmanipulated controls. Thus, increases in baseline cort of handicapped females appeared to have allowed them to maintain fitness despite increased physiological challenges, providing support for the cort-adaptation hypothesis. This article is protected by copyright. All rights reserved.

The vibrant colors of many birds' eggs, particularly those that are blue to blue-green, are extraordinary in that they are striking traits present in hundreds of species that have nevertheless eluded evolutionary functional explanation. We propose that egg pigmentation mediates a trade-off between two routes by which solar radiation can harm bird embryos: transmittance through the eggshell and overheating through absorbance. We quantitatively test four components of this hypothesis on variably colored eggs of the village weaverbird (Ploceus cucullatus) in a controlled light environment: (1) damaging ultraviolet radiation can transmit through bird eggshells, (2) infrared radiation at natural intensities can heat the interior of eggs, (3) more intense egg coloration decreases light transmittance ("pigment as parasol"), and (4) more intense egg coloration increases absorbance of light by the eggshell and heats the egg interior ("dark car effect"). Results support all of these predictions. Thus, in sunlit nesting environments, less pigmentation will increase the detrimental effect of transmittance, but more pigmentation will increase the detrimental effect of absorbance. The optimal pigmentation level for a bird egg in a given light environment, all other things being equal, will depend on the balance between light transmittance and absorbance in relation to embryo fitness.

Our previous research on range-expanding house sparrows in Kenya revealed that (i) range-edge birds released more corticosterone (CORT) in response to a stressor than range-core birds, ii) that range-edge birds were more exploratory than range-core birds, and that (iii) all birds exhibited extensive variation in genome-wide DNA methylation among individuals, regardless of their position along the range expansion. Within the hippocampus, mediators of neural plasticity such as brain-derived neurotrophic factor (BDNF), can influence and be influenced by CORT, hippocampus-associated behaviors and regulatory epigenetic modification enzymes. Here, we investigated whether individuals and populations colonizing a new geographic range, Senegal, vary in the expression of BDNF and DNA methyltransferases (DNMTs) within the hippocampus and the release of CORT in response to a stressor. DNMT expression is an important mediator of epigenetic potential, the propensity of a genome to capacitate phenotypic variation via mechanisms such as DNA methylation. We surveyed three populations across Senegal, predicting that hippocampal BDNF and DNMT expression would be highest at the range-edge, and that BDNF and DNMT would be inversely related to one another, but would each positively covary with CORT within individuals. We found a nonlinear relationship between CORT and BDNF expression within individuals. Moreover, we found that CORT positively covaried with DNMT1 expression in a more recently established population, while the reverse was true in the oldest population (i.e. at the range-core). Our study is among the first to explore whether and how variation in CORT regulation contributes to variation in mediators of neural plasticity and epigenetic potential within the hippocampus of a range-expanding vertebrate.

Nest prospecting, that is, visiting potential future nest sites, may be a widespread bird behavior. Here we describe apparent prospecting while nesting by white-rumped swallows (Hirundinidae, Aves). In southern Brazil, birds tagged with passive-integrated transponders (PIT-tags) and breeding in nest boxes with PIT-tag readers: (1) often visited nest boxes that were in use by apparently unrelated birds (54 % of nests were visited at least once), (2) visited other boxes while caring for their own nestlings, (3) tended to have smaller broods than birds that were not recorded visiting, and (4) did not have a preference for any particular box. These patterns do not indicate any clear explanation for nest visiting, but they do show that prospecting is done by floaters (non-breeders) as well as actively breeding individuals. We suggest that these visits occur when time is available and that visitors may be assessing the availability of future nest sites.

Intermittentfemaleincubation,whereeggsareleftunattendedperiodicallywhilethefemaleforages,iscommon among passerines. In extremely cold environments, unattended eggs may be at risk of freezing or exposed to suboptimal developmental temperatures. Our aim was to examine incubation behaviour of Chilean Swallows nesting in a cold environment(dailyaveragetemperatures~10 � C)atthesoutherntipofArgentina,andthetemperatureregimeofincubation. Femaleshadbare,vascularisedbrood-patches,whereasmaleshadfalsebrood-patches,wherefeatheringoftheabdomenwas somewhat sparse. Video-cameras were used at three nests, and at all three two adults were observed to attend eggs, day and night, confirming incubation by males. Temperature records of eggs showed that one adult (assumed to be the female) had high rates of attendance and maintained egg temperatures that approached 38� 39 � C when incubating, and the other adult (assumed male) had lower and variable rates of attendance and maintained egg temperatures generally above 34 � C when incubating.Theassumedfemalerarelylefttheeggsunlesstheassumedmalewastheretoreplaceher.Researchersshouldbe cautious when making conclusions usingdata fromtemperature loggersinsituations where male attendanceat nests during incubation is possible.

A central assumption in ecological immunology is that immune responses are costly, with costs manifesting directly (e.g., increases in metabolic rate and increased amino acid usage) or as tradeoffs with other life processes (e.g., reduced growth and reproductive success). Across taxa, host longevity, timing of maturity, and reproductive effort affect the organization of immune systems. It is reasonable, therefore, to expect that these and related factors should also affect immune activation costs. Specifically, species that spread their breeding efforts over a long lifetime should experience lower immune costs than those that mature and breed quickly and die comparatively early. Likewise, body mass should affect immune costs, as body size affects the extent to which hosts are exposed to parasites as well as how hosts can combat infections (via its effects on metabolic rates and other factors). Here, we used phylogenetic meta-regression to reveal that, in general, animals incur costs of immune activation, but small species that are relatively long-lived incur the largest costs. These patterns probably arise because of the relative need for defense when infection risk is comparatively high and fitness can only be realized over a comparatively long period. However, given the diversity of species considered here and the overall modest effects of body mass and life history on immune costs, much more research is necessary before generalizations are appropriate.

In a study of almost 16 000 nest records from seven swallow species across the entire Western Hemisphere, clutch sizes decline with relative laying date in each population, but the slope of this decline grows steeper with increasing distance from the equator. Late-laying birds at all latitudes lay clutches of similar sizes, suggesting that latitudinal differences may be driven primarily by earlier-laying birds. Focused comparisons of site-years in North America with qualitatively different food availability indicate that food supply significantly affects mean clutch size but not the clutch size-lay date regression. Other studies on the seasonality of swallow food also indicate that steeper clutch size-lay date declines in the North are not caused by steeper earlier food peaks there. The distribution of lay dates grows increasingly right-skewed with increasing latitude. This variation in lay-date distributions could be due to the predominance of higher quality, early-laying (and large-clutched) individuals among populations at higher latitudes, resulting from latitudinal variation in mortality rates and the intensity of sexual selection. Our results underscore the importance of studying clutch size and lay date in tandem and suggest new research into the causes of their joint geographic variation. Fil: Winkler, David W.. Cornell University; Estados Unidos Fil: Ringelman, Kevin M.. University Of Delaware; . Cornell University; Estados Unidos Fil: Dunn, Peter O.. University Of Wisconsin-milwaukee; Fil: Whittingham, Linda. University Of Wisconsin-milwaukee; Fil: Hussell, David J. T.. Ontario Ministry Of Natural Resources; Canadá Fil: Clark, Robert G.. Environment Canada; Canadá Fil: Dawson, Russell D.. University Of Northern British Columbia; Canadá Fil: Johnson, L. Scott. Towson University; Fil: Rose, Alexandra. University Colorado Museum; Fil: Austin, Suzanne H.. State University of Oregon; Estados Unidos Fil: Robinson, W. Douglas. State University of Oregon; Estados Unidos Fil: Lombardo, Michael P.. Grand Valley State University; Fil: Thorpe, Patrick A.. Grand Valley State University; Fil: Shutler, Dave. Acadia University; Canadá Fil: Robertson, Raleigh J.. Queen's University, Kingston; Canadá Fil: Stager, Maria. Cornell University; Estados Unidos. University of Illinois. Urbana - Champaign; Estados Unidos Fil: Leonard, Marty. Dalhousie University Halifax; Canadá Fil: Horn, Andrew G.. Dalhousie University Halifax; Canadá Fil: Dickinson, Janis. Cornell Lab Of Ornithology; Fil: Ferretti, Valentina. Cornell University; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”; Argentina. Cornell Lab Of Ornithology; Fil: Massoni, Viviana. Universidad de Buenos Aires; Argentina Fil: Bulit, Florencia. Universidad de Buenos Aires; Argentina Fil: Reboreda, Juan Carlos. Universidad de Buenos Aires; Argentina Fil: Liljesthrom, Marcela. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina Fil: Quiroga, Martin Anibal. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Ciencias Veterinarias del Litoral. Universidad Nacional del Litoral. Facultad de Ciencias Veterinarias. Instituto de Ciencias Veterinarias del Litoral; Argentina Fil: Rakhimberdiev, Eldar. Cornell University; Estados Unidos. Lomonosov Moscow State University; . Royal Netherlands Institute For Sea Research - Nioz; Fil: Ardia, Daniel R.. Franklin And Marshall College, Lancaster

Nest microclimate, particularly temperature, can affect energy balance of both parents and offspring. Here I investigate patterns of correlation between internal temperatures of artificial nestboxes and fitness in Tree Swallows ( Tachycineta bicolor). Nests with cold overnight temperatures were less likely to fledge offspring. Conversely, nests with high daytime temperatures were also less likely to fledge offspring. In particular, nest success declined when internal nestbox temperatures exceeded 35°C. To investigate a potential physiological cause for reduced fitness, I tested whether haematocrit, a rough indicator of blood volume, was related to warmer nest temperatures. Whilst variable, nests with higher temperatures had higher average haematocrit of nestlings on day 13. This may indicate greater evaporative heat loss reflected in lower plasma volumes. These results as a whole indicate that temperatures outside the thermo-neutral zone may influence energy balance of both parents and offspring and may have an important effect on fitness.

The nest environment can have important influences on incubation behavior and nestling development in birds. Nest thermal properties, particularly nest composition and size, can have a major influence on heat loss. To examine the role of nest size and insulation on clutch cooling rates, we collected tree swallow Tachycineta bicolor nests and measured the cooling rate of eggs in a controlled thermal environment. We also examined the thermal benefits of nest feathers by comparing the cooling rates of nests with and without feathers. Nests with more feather insulation and heavier, deeper cupped nests cooled at slower rates. In addition, nests with feathers cooled at much slower rates than did the same nests without feathers. Our results show that nest insulation and size play important roles in nest cooling rates, which may ultimately affect incubation costs and thus reproductive performance.

Carotenoids are finite resources that animals can allocate to self-maintenance, attractiveness, or reproduction. Here we test how carotenoids affect the acute phase response (APR), an intense rapid systemic response characterized by fever, sickness behavior, and production of acute phase proteins, which serves to reduce pathogen persistence. We conducted a 2x2 factorial design experiment in captive adult male and female zebra finches (Taeniopygia guttata) to determine the effects of carotenoid supplementation on the intensity of the APR. We measured changes in feeding rate, activity level, and body temperature of the birds. We found that, relative to unsupplemented controls, carotenoid-supplemented birds exhibited less severe reductions in feeding and activity, smaller increases in body temperature, and lower circulating levels of haptoglobin (an acute phase protein) 24 h after inducing an APR. Among supplemented individuals, those with higher blood carotenoid levels exhibited a lower reduction in activity rate after 24 h. Forty-eight hours after APR induction, birds exhibited a significant decrease in plasma carotenoid levels and a decrease in bill hue, with less reduction in hue in carotenoid-supplemented individuals. These results demonstrate that carotenoids can alleviate several important behavioral and physiological effects of an APR and that bill color can change rapidly following induction of the costly APR immune defense. In particular, immune activation may have caused birds to preferentially draw down carotenoids from the bloodstream, ostensibly for use in health. Rapid bill color changes over a 48-hr period support growing evidence that bills may serve as short-term signals of health and condition.

There are at least two reasons to study traits that mediate successful range expansions. First, dispersers will found new populations and thus impact the distribution and evolution of species. Second, organisms moving into new areas will influence the fate of resident communities, directly competing with or indirectly affecting residents by spreading non-native or spilling-back native parasites. The success of invaders in new areas is likely mediated by a counterbalancing of costly traits. In new areas where threats are comparatively rare, individuals that grow rapidly and breed prolifically should be at an advantage. High investment in defenses should thus be disfavored. In the present study, we compared the energetic, nutritional and collateral damage costs of an inflammatory response among Kenyan house sparrow (Passer domesticus) populations of different ages, asking whether costs were related to traits of individuals from three different capture sites. Kenya is among the world's most recent range expansions for this species, and we recently found that the expression of Toll-like receptors (TLRs), leukocyte receptors that instigate inflammatory responses when bound to microbial elements, was related to the range expansion across the country. Here, we found (contrary to our expectations) that energetic and nutritional costs of inflammation were higher, but damage costs were lower, in range-edge compared to core birds. Moreover, at the individual level, TLR-4 expression was negatively related to commodity costs (energy and a critical amino acid) of inflammation. Our data thus suggest that costs of inflammation, perhaps mediated by TLR expression, might mitigate successful range expansions.

Summary 1. In wild birds, incubation period shortens and the general pace of life quickens with distance from the equator. Temperature and various biotic factors, including adult behaviours, cannot fully account for longer incubation periods of equatorial birds and only explain some of the variation between tropical and temperate life histories. Here we consider the role of differences in light in driving variation in incubation period. In poultry, incubation periods can be experimentally shortened by exposing eggs to light. The positive influence of light on embryonic growth, called photoacceleration, can begin within hours after an egg is laid. 2. We artificially incubated house sparrow (Passer domesticus) eggs under photoperiods similar to those found at temperate (18Light : 6Dark) and tropical (12L : 12D) latitudes. We also measured embryonic metabolic rate during light and dark phases. 3. Eggs of house sparrows collected from the wild developed more rapidly under ‘temperate’ than ‘tropical’ photoperiods and had higher metabolic rates during phases of light exposure than during phases of darkness. Metabolic rates during light phases were high enough to account for a 1 day difference in incubation periods between temperate and tropical birds. 4. Based on a synthesis of photoacceleration studies on domesticated galliformes and our experimental results on a wild passerine, we provide the first support for the testable hypothesis that differences in photoperiod may influence variation in the rate of embryonic development across latitudes in birds.

There has been a recent call for an Extended Synthesis in evolutionary biology (Pigliucci 2007). The Modern Synthesis, which united Mendelian and Darwinian ideas about genes and natural selection, has been the cornerstone of evolutionary biology for the last 60 years (Pigliucci 2007). However, many important elements were left out of that synthesis, particularly the mechanisms whereby genetic variation is transduced into phenotypic variation. Such ‘black boxes’ still pervade biology, and for biological fields such as immunology, these black boxes are the explicit subject of interest. Many of these black boxes are so dizzyingly complex, though, that generalizations about their components, much less their architecture, are rare. This lack of generality has made it difficult to link genetic changes to immunological phenotypic variation in an eco-evolutionary context. Indeed, a few exceptions aside (Matzinger 1998), each host– parasite interaction is often considered as a case study. Enter ecological immunology or, more commonly, ecoimmunology. The focus of ecoimmunology has been to describe and explain natural variation in immune functions (Sheldon & Verhulst 1996), specifically why and how biotic and abiotic factors contribute to variation in immunity in free-living organisms. This approach is in striking contrast to most immunological research, which has typically controlled variation experimentally, sometimes at great lengths (via modifications of gene expression or the use of ultra-clean housing facilities), to ascertain the molecular and cellular details of parasite coping mechanisms. Comparative immunology has taken yet a different tack by investigating major changes in immune system organization among taxa (e.g. alternative B and T cell receptor diversity generating mechanisms) (Litman et al. 2007). These data, as well as those generated by classic immunology, have yielded great insight into how hosts control parasites (and vice versa). However, neither comparative nor mainstream immunology can explain the persistence of parasitism as the most common mode of life on Earth (Price et al. 1986) nor why some hosts remain susceptible to infection (Levin & Antia 2001). All extant and extinct organisms, no matter how elegant their immune systems, are affected by parasites (Hedrick 2004). It is in this area, understanding variation in susceptibility, that ecoimmunology has made the greatest contributions. Ecoimmunology proposes that susceptibility persists because immune defence is but one element of a context-dependent, integrated, whole-organism response to parasitism (Ardia, Parmentier & Vogel 2011; Baucom& deRoode 2011; Demas, Adamo & French 2011; Graham et al. 2011). Immune defences exist to prevent the spread of cancerous cells or impede infections, but food quantity and quality, weather, threat of predation of conflict with conspecifics, and a host of other factors can matter too (Schulenburg et al. 2009). If ecological demands are great, or if fitness can be maximized via growth or reproduction instead survival of infection, immune defences may be lowered, altered, or outright compromised. This contribution of ecology to our understanding of immunology has come in just the last 20 years (Fig. 1). Before this period, papers including the terms ecol* and immunol* together were rare in ISIWeb of Science (search 10 ⁄ 20 ⁄ 2010). Since about 1990, though, the field has grown rapidly (658 papers since 1991; Fig. 1), almost twice as fast as another ‘hot’ field, oxidative stress ecology (McGraw et al. 2010). The first paper in which the above search terms were used in a manner consistent with modern ecoimmunology concerned schistosomiasis in humans. This paper (Warren 1973), which reviewed whether age-intensity curves for schistosome infections represented evidence of memory of infections or something else, was a centrepiece of classic epidemiological theory (Anderson & May 1985; May & Anderson 1979). Not until the mid-1990s, however, did ecoimmunology mature into a recognized discipline. In 1996, one of the foundational papers for the field was published (Sheldon & Verhulst 1996), which has up until now been cited over 700 times. Sheldon & Verhulst (1996) invoked trade-offs, the allocation of limited resources among competing, costly physiological functions, as a prime cause of variation in immune defences. Trade-offs remain a prime focus in ecoimmunology, although the idea has been developed and extended quite a bit (Graham, Allen & Read 2005; Adelman & Martin 2009). Another pair of papers was also critical to the establishment of the field: one proposing a handicap hypothesis for sexually selected traits (Hamilton & Zuk 1982) and the other a physiological extension, the immunocompetence handicap hypothesis (Folstad & Karter 1992). These two papers have been cited over 1000 times each and continue to be the focus of much ecoimmunological research. Today, though, the breadth of ecoimmunological research has expanded. Trade-offs continue to take a central position, but their roles in species and individual variation (Ardia, *Correspondence author. E-mail: lmartin@cas.usf.edu

Summary 1. Life history theory predicts that immunity should be plastic and reflect environmental contexts. However, individual variation in immune investment may arise not just because of individual adjustment, but because of developmental, physiological, genetic or immunological constraints which lead to non-adaptive responses by limiting or eliminating flexibility in immune investment. Constraints can arise because organisms are single integrated units with interconnected and interacting components, in which physiological and genetic control mechanisms may limit or constrain immunity. We review some of the key underlying genetic and physiological factors that may constrain the occurrence and intensity of immune responses. 2. A major part of individual variability may rest on variation in genetic background. Genetic-based constraints can limit or influence immune responses, particularly through pleiotropy and epistatic interactions. In addition, genetic variation, an important driver of variation in antigen recognition and immune system polarization, can be constrained through linkage disequilibrium and genetic drift. Epigenetic changes can also constrain or limit immune responses in future generations based on individual experience. 3. The immune system itself can influence individual flexibility in immune investment. Throughout development individuals face tradeoffs within the immune system that favour the expression of one trait at the expense of another. Ontogenetic differences can cause juveniles and adults to produce entirely different immune responses to the same pathogen. T-helper 1 (Th1)/T-helper 1 (Th2) polarization during infection also imposes constraints upon an individual’s immune responsiveness, with the consequence that hosts cannot simultaneously mount strong responses using both Th1 and Th2 cells. In addition, evidence suggests that flexibility in immune responses becomes constrained with age through accumulation of memory cells at the expense of naive cells, decreased function of cells involved in adaptive and innate immunity, and programming of HPA-immune interactions. 4. In summary, selection on a particular immune trait can have effects on other immune components or phenotypic characters, as revealed by artificial selection studies. In particular selection for increased investment in compartments of the immune system leads to decreased investment in other competing life history functions and/or marked changes in other immune components. The role of past experience, even the past experience of parents, may limit and constrain immune responses through influencing the ontogeny of immunity, as well as through epigenetic influences.

Testosterone (T) can influence both male-male competition and mate choice displays. In zebra finches, female mate choice is based in part on bill color, and bill color has been shown to be enhanced by long-term testosterone supplementation. However, it is not clear whether bill color plays a role in male-male interactions and how bill color responds to shorter-term changes in T. We tested whether a single injection of testosterone propionate (TP) would influence male-male dominance interactions and lead to rapid (over a three-day period) changes in bill color. In addition, we tested whether bill color predicted aggression and dominance. We allowed birds in triads to establish hierarchies and then injected either dominant or subordinate individuals with TP, in addition to establishing sham control triads. We found that red chroma, but not hue, predicted aggressiveness of males. Exposure to TP led both dominant and subordinate birds to increase dominance scores over three days, longer than the

Acutely stressful experiences can have profound and persistent effects on phenotype. Across taxa, individuals differ remarkably in their susceptibility to stress. However, the mechanistic causes of enduring stress effects, and of individual differences in stress susceptibility, are poorly understood. Here, we tested whether brief, acute increases in glucocorticoid hormones have persistent effects on phenotype, and whether effects differ according to the magnitude or duration of elevation. We used a novel method to non-invasively manipulate hormone levels on short time scales: the application of corticosterone gel to a model egg secured in the nest. Free-living female tree swallows ( Tachycineta bicolor ) exposed to several brief corticosterone increases during incubation showed dose-dependent differences in behaviour throughout the reproductive period. Birds receiving treatments that simulated higher or longer acute stress responses later provisioned larger broods at lower rates; the resulting offspring were smaller in size. Treatment did not influence female body condition, oxidative stress, reproductive success or inter-annual survival, but exposed females maintained higher baseline corticosterone after treatments ceased. Overall, these results indicate that brief, acute elevations in glucocorticoids in adulthood can have long-term consequences. Furthermore, individuals that mount a greater or longer acute stress response may be more likely to experience lingering effects of stress.

Innate immunity plays an important role in protecting birds early in development. The strength of innate immunity in nestlings appears to be affected by heritable and early maternal effects, as well as later environmental effects. To explore factors affecting innate immunity, we conducted a partial cross-fostering experiment in nestling tree swallows Tachycineta bicolor. The ability of nestling blood plasma to kill Escherichia coli in vitro was influenced by heritable or early maternal effects and was unrelated to body condition of nestlings or brood size of the foster nest. The relative importance of heritable and early maternal effects versus environmental effects on nestling hematocrit (packed volume of red blood cells) and body condition were also examined. Hematocrit was significantly influenced by the nest in which nestlings were reared, though it was not related to nestling body condition. Body condition was significantly influenced by the nest in which nestlings were reared, though it was also weakly influenced by heritable or early maternal effects. In summary, heritable and early maternal effects are stronger predictors than rearing conditions of innate immunity in nestling tree swallows. Furthermore, hematocrit is largely environmentally determined but is not related to body condition or innate immunity, as measured in this study.

Breeding synchrony occurs when individuals simultaneously perform portions of the reproductive cycle more often than expected by chance. We tested for the role of social stimulation in driving breeding synchrony of European starlings, Sturnus vulgaris, in a spatially-structured population in New Zealand over 11 years. Natural spreads in laying in groups were consistently tighter relative to aggregations of individuals chosen at random. Rather than a decrease in synchrony with increasing density, as would be expected if competition reduces breeding condition, we found that birds in dense breeding areas showed tighter synchrony. As the number of active nests within 500 m of a breeding location (i.e. human-constructed concrete bunkers) increased, the spread in clutch initiation date decreased. There was a correlation between synchrony and fledging success; less synchronous locations had lower fledging success. These results indicate that local and larger-scale levels of social stimulation may help coordinate reproductive timing in group-living birds. While the link between fitness and fledging success may not be causal, our results suggest that factors driving synchrony may also be related to individual quality and reproductive success.

Understanding how genetic variation in the Tree Swallow (Tachycineta bicolor) is geographically structured is informative because this broadly distributed North American bird is increasingly used as a model for studies of mating systems, life-history traits, and physiology. We explored patterns of phylogeographic differentiation across the Tree Swallow's breeding range by using nine microsatellite loci and a mitochondrial DNA sequence marker. Contrary to this species' high population-level variation in life-history traits and other ecologically important characteristics, we found no genetic structuring across the majority of its distribution, spanning Tennessee, New York, and Alaska, but we found that birds from California form a distinct yet subtly differentiated genetic cluster. The Tree Swallow can be characterized as a species with both continent-wide genetic panmixia and slight differentiation at one edge of its breeding distribution. This pattern of genetic variation has implications for u...

Summary 1. Life-history decisions are strongly affected by environmental conditions. In birds, incubation is energetically expensive and affected significantly by ambient temperature. We reduced energetic constraints for female tree swallows ( Tachycineta bicolor ) by experimentally heating nests during incubation by an average of 6·9 ° C to test for changes in incubation behaviour. 2. Females in heated boxes (hereafter ‘heated females’) increased time spent incubating and maintained higher on-bout and off-bout egg temperatures. This indicates that female energetic constraints, not maximizing developmental conditions of offspring, determine incubation investment. Furthermore, this result suggests that embryonic developmental conditions in unmanipulated nests are suboptimal. 3. We found individual variation in how females responded to experimental heating. Early-laying (i.e. higher phenotypic quality) females with heated nests increased egg temperatures and maintained incubation constancy, while later-laying (lower quality) heated females increased incubation constancy. Changes in egg temperature were due to changes in female behaviour and not due directly to increases in internal nest-box temperatures. 4. Behaviour during the incubation period affected hatching asynchrony. Decreased variation in egg temperature led to lower levels of hatching asynchrony, which was also generally lower in heated nests. 5. Our study finds strong support for the prediction that intermittent incubators set their incubation investment at levels dictated by energetic constraints. Furthermore, females incubating in heated boxes allocated conserved energy primarily to increased egg temperature and increased incubation attentiveness. These results indicate that studies investigating the role of energetics in driving reproductive investment in intermittent incubators should consider egg temperature and individual variation more explicitly.

Carotenoid pigments are responsible for many examples of sexually attractive red, orange, and yellow coloration in animals and play an important role in antioxidant and immune defenses. Because vertebrates cannot synthesize carotenoids, limited dietary availability may impose a trade-off between maintaining ornamental coloration and health. We used an experimental approach to test the carotenoid trade-off hypothesis in the fighting fish Betta splendens, by examining whether carotenoid allocation strategies differ among conspecifics that exhibit a gradient of body coloration from blue to red. We found that male redness is underlain by carotenoids and that females preferred to associate with red males over blue males, suggesting a sexually-selected advantage to being red. Moreover, we found strong experimental support for the carotenoid trade-off hypothesis, as individuals that varied in color did not appear to allocate carotenoids equally to both immune response and coloration. Redder fish given supplemental carotenoids increased in both immune response (to a phytohemagglutination challenge) and redness compared with controls. In contrast, bluer fish given supplemental carotenoids did not become more red but instead benefited immunologically more so than either control or redder supplemented fish. These results enhance our understanding of the evolution and plasticity of carotenoid mobilization and utilization pathways in animals. Key words: carotenoids, coloration, immune response, pteridines, sexual selection. [Behav Ecol]

In species with biparental care, males tend to invest less in offspring than do females, likely because of differences in the costs and benefits associated with parental effort. Here I test for sex differences in the response of tree swallows ( Tachycineta bicolor (Vieillot, 1808)) to a brood-size manipulation at two locations differing in food resources, Alaska and New York. I tested sex and habitat differences in how swallows responded to changes in offspring demand. At both sites, both sexes increased effort when feeding enlarged broods, although Alaskan males increased feeding less than Alaskan females. Males decreased feeding effort more to reduced broods than females, but only in Alaska. Food abundance was higher in Alaska than in New York, and Alaskan tree swallows made more feeding visits than New York tree swallows. In New York, food availability did not predict feeding rate and there was no sex difference in the response to brood manipulation. In both sites, male feeding effort was linked with nestling residual body mass, while female feeding effort was correlated with nestling growth rate. This study demonstrates that male tree swallows differ from females by being the first to reduce feeding effort under certain conditions and that male and female feeding rate affects offspring quality differently.

Testosterone (T) is hypothesized to be an important honesty reinforcer of animal sexual signals. Owing to its immunosuppressive effects, only those individuals that can immunologically withstand high T levels can develop the most exaggerated traits. To date, few studies have isolated phenotypic or genotypic buffers that provide ‘high-quality’ animals with such an advantage. Dietary carotenoid pigments may in fact confer such a benefit because when in high supply carotenoids boost immunocompetence and coloration in animals like birds and fishes. We examined the experimental effect of T elevation on carotenoid and immune status in male and female zebra finches (Taeniopygia guttata) and found that T was immunostimulatory in a generalized cell-mediated challenge. We also detected a significant interaction between T treatment and the change in plasma carotenoids that occurred during the immune challenge; the relationship between blood carotenoid change and immunity was positive in controls and negative in T-implanted birds. This suggests that, while correlationally birds with high carotenoid stores were inherently better at mounting strong immune responses, experimentally administered T induced birds to deplete carotenoids for maximizing their health. Our findings highlight a nutrient-specific mechanism by which animals escape high immune costs of T elevation and thus can still elevate ornamentation.

Variation in immune responses is an important part of life history variation. When correlations between multiple immune measures are reported, studies report different patterns. I tested whether the correlation between levels of immune response was consistent across a species range. The ability of tree swallows Tachycineta bicolor to simultaneously produce immune responses to both a humoral immune response and T-cell mediated local inflammation to PHA was tested at three sites that span the breeding range. Females in Tennessee maintained stronger PHA responses than did females in either New York or Alaska. In New York and Alaska, individuals that produced strong PHA responses produced low levels of antibodies to a humoral challenge of sheep red blood cells (SRBC). However, in Tennessee, individuals that showed strong local PHA inflammation also mounted strong responses to SRBC. Increasing daily daytime temperatures led to increased PHA response, but there were no differences in the effect of temperature among sites. These results indicate spatial and/or temporal variation occurs in the ability to produce multiple immune responses simultaneously; this pattern suggests important geographic (or temporal) differences in factors driving investment in immune activity. In addition, these results suggest that studies extrapolating results across populations should be careful to consider geographic variation in immune activity.

Individual variation in life-history trade-offs can be caused by differences in quality and age. We tested for individual variation in parental investment in incubating tree swallows (Tacyhcineta bicolor) subjected to a feather-clipping manipulation. Individual quality influenced how females were affected by feather clipping; lower quality clipped females showed a greater reduction in incubation and a greater loss of body condition than higher quality clipped females compared with controls. Most importantly, responses during incubation influenced nestling traits; lower quality clipped females, particularly those losing the most body mass, raised nestlings in the poorest condition. There was no difference in incubation patterns of control females, but older clipped females suffered self-maintenance costs and raised offspring in better condition. In contrast, younger clipped females passed costs on to offspring through lower egg temperatures and reduced nestling condition while maintaining their own condition. Overall, we found a strong individual quality effect: at the population level, there was a trade-off between investing in incubation and maintaining parental condition, but among individuals, there was a positive correlation between change in parental condition and offspring quality. Individual differences in parental strategy can be important causes of life-history variation, especially through subtle, but important, aspects of reproduction such as maintaining egg temperature during incubation. Key words: age differences, energetic constraint, incubation, individual quality, life-history trade-offs, tree swallow. [Behav Ecol]

Heritability is an important component of the ability of a trait to respond to natural selection; variation in heritability can lead to differences in how a trait responds to selection pressures. Here we test whether an important physiological trait, immune function, varies by comparing heritability estimates through cross-fostering brood manipulation at three wide-spread sites in the tree swallow (Tachycineta bicolor): Alaska, New York and Tennessee. In two of three sites, there was no additive genetic component to nestling immune response to the mitogen phytohaemagglutinin, while immune response had a heritable component in Tennessee. Bootstrapping revealed significant differences in estimated heritability. This conclusion was supported by mother–offspring regressions; in Tennessee breeding females mounting strong immune responses tended to have offspring with strong immune responses, while in New York and Alaska, there was no relationship between the immune responses of mothers and offspring. These results suggest that studies investigating the roles of common origin and rearing environment should consider yearly or spatial variation within a species.

Nestlings can exhibit considerable variation in developmental patterns both within and among locations due to differences in environmental conditions and parental investment. I investigated trade-offs between nestling growth rate and residual body mass (body condition) at three locations across the range of the Tree Swallow (Tachycineta bicolor). Nestlings at the northern extreme of the range in Alaska had slower growth rates, lower body mass, and higher residual body mass than nestlings in New York and Tennessee. High insect availability was correlated with increased growth rates of nestlings in New York and Tennessee, but not in Alaska. Conversely, nestlings in Alaska showed increased residual body mass with high insect availability, but nestlings in New York and Tennessee did not. The trade-off between growth rate and residual body mass varied among sites, with fast-growing nestlings in Tennessee maintaining a higher residual body mass than those in Alaska. These results suggest that factors affecting offspring growth and condition vary among sites, leading to geographical differences in offspring development trajectories.

Orientation of nests can influence nest microclimate, particularly temperature. However, few investigators have examined orientation preference and microclimate simultaneously. We examined the possible correlation between entrance orientation of artificial nest boxes used by Tree Swallows (Tachycineta bicolor) and the internal temperature of boxes. Tree Swallows showed a preference for east- and south-facing boxes, but only during the first half of the breeding season (before 1 June). During the second half of the breeding season (after 1 June), Swallows selected boxes based on availability. We found that east- and south-facing boxes were warmer than north- and west-facing boxes, but only during the first half of the breeding season when those boxes were preferred. Entrance orientation and box temperature were only correlated during the morning (06:00-12:00); the temperature of all boxes was similar during the afternoon. Our results suggest that Tree Swallows show a preference for nest boxes with a certain entrance orientation only when orientation influences microclimate, suggesting that warmer nest temperatures may provide fitness benefits. SINOPSIS. El orientaci´ on de cajas de anidamiento afecta su temperatura interna y la selectividad por parte de Tachycineta bicolor La orientaci´ on de nidos puede influir el microclima de estos, particularmente su temperatura. Sin embargo, son muy pocos los trabajos de investigaci´ on que han examinado preferencias de orientacion y aspectos microclim´ aticos en el mismo estudio. Examinamos la posible correlacientre preferencias en la orientacion del hueco de entrada y la temperatura interna de las cajas en la golondrina Tachicineta bicolor. Las aves mostraron preferencias por cajas orientadas hacia el este y hacia el sur, antes del 1 de junio. Posterior a dicha fecha, las golondrinas seleccionaron las cajas a base de la disponibilidad de estas. Encontramos que las cajas orientadas hacia el este y hacia el sur eran m´ as calidas que las que estaban orientadas al norte y al oeste, aunque fueron utilizadas con preferencia hasta el 1 de junio. La orientacide la entrada a la caja y la temperatura se correlacionaron durante la ma˜

Variation in egg size and composition can have important consequences for the quality of offspring. We investigated the factors influencing the yolk mass and egg mass of tree swallows breeding in Ithaca, NY. Using a nondestructive technique to estimate yolk mass via standardized digital-candler photographs, we compared yolk size and egg size of tree swallows Tachycineta bicolor in response to variation in food availability and individual quality. Insect availability one to three days prior to laying, but not four to six days, predicted yolk mass, while insect availability two to three days prior to laying predicted total egg mass. This suggests that, while eggs are formed over longer periods, food availability closer to time of laying has the greatest influence on egg size. These results were supported by collected eggs, as yolk rings revealed that tree swallow eggs are formed over 5–6 days. There was an influence of female quality as well, with early laying birds, independent of food availability, laying larger eggs. Eggs laid later in the laying sequence had larger yolks and greater egg mass. Overall, variation in egg quality appears to be due to a combination of environmental conditions, reflected in food resources, individual quality, and allocation tradeoffs during the laying period.

The onset of incubation varies in birds, with many species beginning incubation prior to clutch completion. Here we examine whether early onset is more likely to occur during high temperatures, a critical prediction of the egg-viability hypothesis, which suggest that birds begin incubation prior to clutch completion in order to maintain egg-viability. We examined onset of incubation in tree swallows Tachycineta bicolor at two locations at the extremes of their breeding range, Alaska and Tennessee. A majority of individuals (68%) began incubation prior to clutch completion. While females in Tennessee were more likely to begin incubation early, there was no difference between sites when differences in temperatures inside nestboxes were controlled in analyses. Rather, early onset of incubation was predicted by the proportion of daily temperatures above physiological zero during laying, a critical prediction of the egg viability hypothesis. Both warm weather and early onset led to shorter incubation periods and increased levels of hatching asynchrony. We found no effect of timing of nesting, female body condition index or clutch size on the probability of beginning incubation prior to clutch completion. Our results are consistent with the egg viability hypothesis, not consistent with a threshold clutch size rule, and do not support the hurry-up hypothesis, that individuals breeding later in the season would begin incubation early to reduce the time spent nesting. Overall, our results suggest that broad scale geographic differences in incubation behaviour may be explained by individual-level responses to environmental conditions.

Blood chemistry can be used to assess physiological state and condition. Levels of glycated hemoglobin (GHb), which integrates blood glucose levels over a period of weeks, may provide a way to assess resource intake. I tested whether GHb reflects offspring quality by comparing growth rates of nestling American kestrels (Falco sparverius) with GHb levels at 24 days of age. Nestlings that gained structural size faster had higher levels of GHb than did slower growing nestlings. There was no difference in GHb levels between males and females, although females are larger. In addition, I tested whether albumin levels, a measure of protein storage, were correlated with nestling growth and body condition (reflected in residual body mass). Larger individuals, measured by both absolute body mass and by residual body mass, had larger levels of albumin. This was due in part to females having higher albumin levels. Interestingly, there was no correlation between GHb and albumin, suggesting that both measures are necessary to assess physical condition in nestling kestrels. These results suggest that blood chemistry can reflect offspring condition measures and may provide a way to assess offspring quality that reflects conditions experienced by offspring over longer periods through measurements at a single time point.

Latitudinal differences in life histories are believed to be underlain by dif- ferences in trade-offs between current and future reproduction. I report differences in trade- offs between parent and offspring across the range of a widespread avian species, the Tree Swallow (Tachycineta bicolor). I manipulated parental effort and found that in Alaska, where yearly adult return rates are low, breeding females increase their reproductive in- vestment to maintain offspring quality, whereas in Tennessee, where yearly adult return rates are high, breeding females are either unable or unwilling to increase reproductive investment and, consequently, raise offspring of lower quality. I further investigated a critical mechanism of self-maintenance that may underlie differences in survival among sites: immunocompetence. Females breeding in Alaska mounted weaker immune responses when raising enlarged broods, whereas females in Tennessee did not, further suggesting a strategy in Alaskan females to incur costs (i.e., reduce future reproduction) in order to maintain offspring quality. Insect availability increased as the breeding season progressed in both sites, but more food was available in Alaska than in Tennessee. This is among the first studies to report geographic differences in immune function consistent with life history theory, which predicts that individuals with higher survival probabilities should invest more in self-maintenance.

Summary 1 Lipids are a major source of energy storage in vertebrates. A cross-fostering brood manipulation was conducted to assess the role of rearing environment vs common origin on lipid stores. 2 Rearing conditions, especially nest temperatures, explained more variation in lipid levels than did nest of origin. As nest temperatures increased, lipid levels in nestlings also increased. Larger individuals, reflected in both wet and lean dry mass, tended to store more lipids. 3 There was no trade-off between structural growth and lipid stores. However, body mass growth was linked with lipids. In addition, nestlings raised early in the season tended to maintain greater lipid levels, suggesting a role of individual quality. 4 Because the assessment of lipid stores is difficult, it is common to use residual body mass (RBM) as a surrogate. Multiple methods of calculating RBM were compared with actual lipid mass in nestling European Starlings (Sturnus vulgaris). RBM was correlated with lipid levels (R2 up to 0·66); models calculated using ordinary least squares regression predicted lipid stores better than reduced major axis regression. 5 These results indicate that RBM can be a non-invasive predictor of lipid levels in nestling birds and that studies examining variation in physiological stores should consider development conditions and individual quality.

Summary 1 Individual variation in the trade-off between self-maintenance and offspring quality was assessed in tree swallows, Tachycineta bicolor, by manipulating reproductive effort while simultaneously immunochallenging breeding females. 2 An experimental manipulation of parental effort was conducted by creating broods of, on average, three, five and eight nestlings. Breeding females were immunochallenged to mount a humoral immune response to sheep red blood cells (SRBC) and a cell-mediated response to phytohaemagglutinin (PHA). The consequences of adult decisions on offspring quality were assessed through immune responses to PHA, growth rates and residual body mass of nestlings. 3 Clutch initiation date, a strong measure of individual quality in tree swallows, was linked with immune responses, with earlier-nesting, presumably higher quality, females mounting stronger immune responses than did later-nesting birds. Increased reproductive effort led to decreased parental immune responses. There was a significant interaction between individual quality and reproductive effort treatment, with lower-quality individuals showing greater depression of humoral immune response to SRBC while raising enlarged broods, suggesting individual-level variation in trade-offs. 4 Breeding females raising enlarged broods tended to raise offspring of similar quality to control females, with only growth rate decreasing with increasing brood size, but not residual nestling body mass or nestling immunocompetence. This suggests that females are maintaining offspring quality at the cost of their own immune system maintenance.

Many animals use carotenoid pigments to color their integument and become sexually attractive. These colorants can also serve physiological functions, protecting cells and tissues from oxidative damage as well as stimulating the immune system. Because animals often acquire several different carotenoid pigments from their diet, there is the potential for different carotenoids to exhibit different free-radical-scavenging or immunoenhancing activity. We experimentally tested how two common dietary xanthophylls - lutein and zeaxanthin - may differentially affect the immune system in male zebra finches (Taeniopygia guttata). Male T. guttata derive their red sexual beak colorants from these two carotenoids, and prior studies with this species have shown that lutein and zeaxanthin together boost cell-mediated immunity. We experimentally elevated these two dietary carotenoids separately in two groups of non-breeding zebra finches, but found that lutein-supplemented and zeaxanthin-supplemented males mounted similar cell-mediated immune responses (to phytohaemagglutinin, or PHA). Although zeaxanthin is a more conjugated carotenoid than lutein and has the potential to be a more potent antioxidant, our study suggests that such a subtle structural difference between these two biochemicals does not differentially affect immune performance in this songbird.