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Parasites are a major risk for group-living animals and seabirds are notoriously susceptible to ectoparasite infestations because they commonly nest in dense colonies. Ticks parasitize seabirds across all biogeographical regions and they can be particularly harmful to nestlings, but the ecological factors that affect their transmission to chicks are little studied and poorly understood. Here we show that abundance of tick larvae in blue-footed booby (Sula nebouxii) broods varies with local nest synchrony and density, and also with habitat structure: abundance increased with local breeding synchrony, was linearly and quadratically related to local nest density, and was highest toward the southern end of the study area which has suitable (boulder-rich) habitat for ticks. Also, with increasing chick age infestation first increased and then declined. The results of this study highlight how local physical and social environmental factors influence infestation of seabird nestlings by ticks. This article is protected by copyright. All rights reserved.

Variability in population numbers is a central issue in evolutionary ecology and also in biodiversity conservation. However, for most seabirds this information is lacking and tropical populations are virtually unstudied. Long-term studies are warranted because world's seabird populations exhibit an overall declining trend since 1950. Using data spanning 23 yr, we investigated how adult survival, local recruitment, and their relative contributions to population growth (λ) vary over time in the blue-footed booby Sula nebouxii, a long-lived locally foraging seabird that breeds in tropical waters. In addition, we investigated whether booby demographic rates exhibit the same declining trend observed in other seabirds, whether these rates are impacted by hurricanes, and whether these potential impacts differ between sexes. Our analysis of 4608 capture–recapture histories revealed that survival and recruitment were nearly equal between males and females, exhibited a declining trend over the last 23 yr, and in both sexes, these vital rates improved after a hurricane. The declining trend in recruitment was slightly more attenuated in males. These results add to the current evidence for an overall declining trend in world's seabird populations and extend its confirmation to the warm eastern tropical Pacific. Moreover, they provide the first evidence that hurricanes may favor natural populations. As a result of the declining trend and variation in survival and recruitment, λ exhibited a slight decline and substantial variation over the 23 yr. However, most λ values were equal to or higher than 1, and the long-term average indicates population increase. The ability of blue-footed boobies to maintain a positive population balance despite of negative trends in their vital rates might result from canalization of adult survival (the vital rate that contributes most to λ and shows lower variation compared to recruitment) against environmental variability.

Mercury (Hg) and cadmium (Cd) concentrations in eggs of 8 seabird species inhabiting 5 coastal ecosystems in Sinaloa, Mexico were determined during 2 breeding seasons (2012 and 2013): blue-footed booby (Sula nebouxii), brown booby (Sula leucogaster), double-crested cormorant (Phalacrocorax auritus), magnificent frigatebird (Fregata magnificens), brown pelican (Pelecanus occidentalis), royal tern (Thalasseus maximus), laughing gull (Leucophaeus atricilla), and Heermann's gull (Larus heermanni). The interspecific differences found in the concentrations of both metals were attributed to the diet and foraging ecology of the species. The highest Hg concentrations were detected in piscivorous species (brown pelican, 0.42 µg/g; brown booby, 0.31 µg/g; blue-footed booby, 0.26 µg/g; and double-crested cormorant, 0.23 µg/g); whereas species with more varied diets presented the highest Cd concentrations (Heermann's gull, 0.31 µg/g; laughing gull, 0.27 µg/g; and magnificent frigatebird, 0.27 µg/g). Cadmium concentrations were significantly greater in 2013 than 2012 for most species, and brown pelican and laughing gull also had higher Hg concentrations in 2013 in Santa Maria Bay, suggesting a relationship as a result of the changes either in oceanographic conditions or in continental runoff. Mercury concentrations in brown pelican and Cd concentrations in Heermann's gull and laughing gull were above threshold levels for adverse effects on reproduction and survival. Environ Toxicol Chem 2016;35:2330-2338. �� 2016 SETAC.

In many animal species, females select a mate on the basis of the expression of secondary sexual traits. A prevalent theory suggests that male ornaments are reliable indicators of immunocompetence, because the cost of immune function prevents cheating. However, sexual signalling is a component of male reproductive effort, and an immune challenge may also alter his perceived future prospects and hence signalling effort. In this study, blue-footed booby males (Sula nebouxii) were inoculated with a diphtheria-tetanus vaccine during courtship to investigate the consequences of mounting an immune response on signalling effort. We found that, after this immune challenge, on average, males increased their signalling effort but lost more body mass compared with control males. Importantly, vaccination affected the partner's reproductive decisions: compared with control females, females paired with vaccinated males laid eggs earlier and increased clutch volume in pairs that laid early. Overall, our results suggest that blue-footed booby males invest more in sexual signals when future breeding opportunities are at risk, eliciting a greater reproductive investment by their partners. Increased signalling effort by infected individuals may contrast the idea of sexual ornaments as signals of infection status.

Experiments on birds, fish and mammals have shown that adverse conditions during infancy can produce diverse long-term and delayed deficits during adulthood, prejudicing both the individual and its offspring. Natural selection should prepare animals to cope with adversity of the type, magnitude and timing that commonly occur in their natural habitat, but very little is known about such evolved developmental buffering against natural ‘poor starts’ in life. In two-chick broods of the blue-footed booby Sula nebouxii, the junior (younger) chick usually experiences aggressive subordination, reduced nutrition and growth and elevated circulating corticosterone. To test whether this poor start produces long-term, delayed or intergenerational deficits in body size, body condition or cell-mediated immune response, we measured 3–8 yr old female breeders banded as chicks, and their infant offspring. Results failed to support our predictions. Compared to former seniors and former singletons (solitary nestlings), former juniors showed no deficit in cell-mediated immune response at any age. They showed an 8.04% deficit in body condition at age 4–6 yr but this deficit disappeared completely by age 7–8 yr. Furthermore, their offspring showed no deficits in body size, body condition or immune response. Junior chicks are affected by their poor start, but their developmental resilience, also confirmed by studies of post-fledging survivorship, recruitment, natal dispersal, aggressive nest defense and reproduction, is evidence of evolved developmental buffering against predictable adversity during infancy.

Hybrid zones represent natural laboratories in which the processes of divergence and genetic isolation can be examined. The generation and maintenance of a hybrid zone requires mispairing and successful reproduction between organisms that differ in one or more heritable traits. Understanding the dynamics of hybridization between two species requires an understanding of the extent to which they have diverged genetically, the frequency of misparing and hybrid production, and the extent of introgression. Three hundred and twenty one blue-footed Sula nebouxii and Peruvian S. variegata boobies from the eastern tropical Pacific Ocean were analyzed using 19 putatively neutral genetic markers to evaluate interspecific differentiation, to classify morphological hybrids using Bayesian assignments, and to characterize hybridization using cline theory and Bayesian assignments. The species were well differentiated at mitochondrial and nuclear microsatellites, the hybrid zone was bimodal (contained a high frequency of each parental species but a low frequency of hybrids), and morphologically intermediate individuals were most likely F1 hybrids resulting from mating between female Peruvian boobies and male blue-footed boobies. Clines in allele frequency could be constrained to share a common geographic centre but could not be constrained to share a common width. Peruvian and blue-footed boobies hybridize infrequently, potentially due to strong premating reproductive isolation; however, backcrossing appears to facilitate introgression from blue-footed to Peruvian boobies in this hybrid system.

Most tropical booby species complete breeding foraging trips within daylight hours, thus avoiding nights at sea. Nazca Boobies Sula granti are unusual in this respect, frequently spending one or more nights away from the nest. We used GPS dataloggers, time-depth recorders, and changes in body weight to characterize foraging trips and to evaluate potential influences on the decisions of 64 adult Nazca Boobies to spend a night at sea, or to return to their chicks on Isla Espanola, Galapagos, in daylight hours. The tagged birds foraged east of Isla Espanola, undertaking both single-day (2‐15 h, 67% of trips) and overnight trips (28 h‐7.2 days, 33%), and executing 1‐19 foraging plunge-dives per single-day trip. Birds might forage longer if they are in nutritional stress when they depart, but body weight at departure was not correlated with trip length. Birds might be expected to return from longer trips with more prey for young, but they returned from single-day and overnight trips with similar body weights, consistent with previous indications that Nazca Boobies forage until accumulating a target value of prey weight. Birds with a lower dive frequency during the first 5 h of a trip were more likely to spend the night at sea, suggesting that they might choose to spend the night at sea if prey capture success was low. At night, birds almost never dived and spent most of their time resting on the water’s surface (11.8‐12.1 h, > 99% of the time between civil sunset and civil dawn). Thus, the night is an unproductive time spent among subsurface predators under low illumination. The birds’ webbed feet provided evidence of this risk: 24% of birds were missing > 25% of their foot tissue, probably due to attacks by predatory fish, and the amount of foot tissue lost increased with age, consistent with a cumulative risk across the lifespan. In contrast, other tropical boobies (Blue-footed Sula nebouxii and Brown Boobies Sula leucogaster), which do not spend the night on the water, showed no such damage. These results suggest that chick-rearing Nazca Boobies accept nocturnal predation risk on occasions of low prey encounter during a foraging trip’s first day.

1. There is increasing interest in the impacts of El Nino Southern Oscillation (ENSO) on reproduction of apical predators such as seabirds and marine mammals. Long-term studies documenting ENSO effects on reproduction of seabirds in the warm tropics are scarce, and differential sensitivity of breeding parameters to ENSO has rarely been explored. 2. Analysis of 18 years of breeding data from a colony of the blue-footed booby Sula nebouxii (Milne-Edwards) showed a delay in onset of breeding when the global Southern Oscillation Index was negative; each unit of the atmospheric pressure differential (hPa) across the Pacific Ocean meant a delay of 7 days. 3. ENSO conditions also produced declines in breeding participation, clutch size, brood size, hatching success and fledging success, especially when surface waters surrounding the colony were warmer during winter and spring. Each additional degree (°C) of water temperature produced a reduction of 0.45 fledglings per nest. Different breeding parameters were sensitive to ENSO indices in different blocks of months. 4. Warming of local waters during the winter was associated with decline in ocean productivity in the current year and the following year, consistent with ENSO impacts on breeding parameters being mediated by effects on local productivity and prey availability. However, there was no evidence of lagged effects of ENSO on any breeding parameter. 5. Comparison of 5 years revealed that when local surface waters were warm, chicks grew more slowly, but no effects of ENSO on weight and size of eggs were evident in data of 9 and 7 years, respectively. 6. Our findings extend evidence of impacts of ENSO on seabird reproduction to the eastern tropical Pacific and indicate that several breeding parameters of blue-footed boobies (but not egg size) are affected in the short term by ENSO conditions, particularly by local anomalies in sea surface temperature associated with decline in ocean productivity.

Aim We assessed population differentiation and gene flow across the range of the blue-footed booby (Sula nebouxii) (1) to test the generality of the hypothesis that tropical seabirds exhibit higher levels of population genetic differentiation than their northern temperate counterparts, and (2) to determine if specialization to cold-water upwelling systems increases dispersal, and thus gene flow, in blue-footed boobies compared with other tropical sulids. Location Work was carried out on islands in the eastern tropical Pacific Ocean from Mexico to northern Peru. Methods We collected samples from 173 juvenile blue-footed boobies from nine colonies spanning their breeding distribution and used molecular markers (540 base pairs of the mitochondrial control region and seven microsatellite loci) to estimate population genetic differentiation and gene flow. Our analyses included classic population genetic estimation of pairwise population differentiation, population growth, isolation by distance, associations between haplotypes and geographic locations, and analysis of molecular variance, as well as Bayesian analyses of gene flow and population differentiation. We compared our results with those for other tropical seabirds that are not specialized to cold-water upwellings, including brown (Sula leucogaster), red-footed (S. sula) and masked (S. dactylatra) boobies. Results Blue-footed boobies exhibited weak global population differentiation at both mitochondrial and nuclear loci compared with all other tropical sulids. We found evidence of high levels of gene flow between colonies within Mexico and between colonies within the southern portion of the range, but reduced gene flow between these regions. We also found evidence for population growth, isolation by distance and weak phylogeographic structure. Main conclusions Tropical seabirds can exhibit weak genetic differentiation across large geographic distances, and blue-footed boobies exhibit the weakest population differentiation of any tropical sulid studied thus far. The weak population genetic structure that we detected in blue-footed boobies may be caused by increased dispersal, and subsequently increased gene flow, compared with other sulids. Increased dispersal by blue-footed boobies may be the result of the selective pressures associated with cold-water upwelling systems, to which blue-footed boobies appear specialized. Consideration of foraging environment may be particularly important in future studies of marine biogeography.

The day and night visual capacities (photopic and scotopic spectral sensitivity) of several species of boobies (family Sulidae) were analyzed using a behavioral psychophysical testing technique. On the basis of these data there is evidence for sensitivity to near-ultraviolet light in the order Pelecaniformes. The scotopic spectral sensitivity functions of three booby species conform closely to the spectral absorption characteristics of avian rhodopsin. There are marked differences between the photopic sensitivities of a coastal booby species with that of a pelagic species; these differences appear correlated with their respective feeding ecologies. Lastly, a brief review of avian spectral sensitivity and a consideration of individual variation in avian vision studies are presented.

Contradictory patterns of density-dependent animal dispersal can potentially be reconciled by integrating the conspecific attraction hypothesis with the traditional competition hypothesis. We propose a hypothesis that predicts a U-shaped relationship between density and both natal and breeding dispersal distance. Using 10 years of observations on a breeding colony of the Blue-footed Booby (Sula nebouxii), the hypothesis was confirmed by documenting simultaneous positive and negative density-dependent dispersal distances in natal and breeding dispersal of males and breeding dispersal of females within the colony. Point-pattern analyses demonstrated that the breeding sites of Blue-footed Boobies were highly aggregated in all years within a large study area, and aggregation presumably resulted in heterogeneity in patch density throughout the colony. As predicted, at moderate to high densities, dispersal distances showed positive density dependence, with individuals moving to lower density patches. In contrast, at low to moderate densities, dispersal distances showed negative density dependence, with individuals moving to higher density patches. In both sexes of the 1994 cohort, the higher the mean density in patches used by an individual over the long term (up to age 11 years), the fewer fledglings it produced. A positive effect of density on long-term reproductive success was not detected, possibly because birds that failed during pair formation or incubation were not sampled. Density of conspecifics may be an important influence on habitat selection of breeders, and dispersal may tend to carry individuals to patches where pair formation opportunities are better and negative effects of competition on reproductive success are reduced.

We examined androgens in clutches of two booby species that differ in their sibling conflict. Blue-footed booby Sula nebouxii chicks show an aggression-submission relationship, aggression is normally moderate and siblicide is facultative. Brown booby Sula leucogaster chicks show an aggression-aggression relationship, aggression of both chicks can be relentless and siblicide is obligate. The parental favoritism hypothesis predicts that egg mass, yolk mass and yolk androgens should decline with laying order less in the blue-footed booby than in the brown booby, to promote the survival of the former's junior chick. The eggs of the blue-footed booby had higher yolk concentrations of 5α-dihydrotestosterone (DHT) and lower concentrations of testosterone (T); both species had similarly high yolk concentrations of androstenedione (A). Intra-clutch variation in yolk DHT, T and A failed to support our predictions. In both species, first and second eggs showed similar concentrations of all three hormones and were of similar size, although in the blue-footed booby (only) yolk masses declined with laying order. Brown booby mothers make junior chicks vulnerable to siblicide by hatching them 5 d after their broodmates, but not by differential allocation of egg androgens or nutrients. Blue-footed booby mothers appear to prepare junior chicks for thriving in a subordinate non-provocative role by hatching them 4 d after their broodmates and providing them with 10% less yolk. To orchestrate agonism within the brood, these boobies may rely more on hatch intervals and yolk provision than on androgens.

1. Understanding the effects of individual and population factors on variation in breeding dispersal (the movement of individuals between successive breeding sites) is key to identifying the strategies behind breeders' movements. Dispersal is often influenced by multiple factors and these can be confounded with each other. We used 13 years of data on the locations, mates, breeding success and ages of individuals to tease apart the factors influencing breeding dispersal in a colonially breeding long-lived seabird, the blue-footed booby Sula nebouxii. 2. Breeding dispersal varied among and within years. Males dispersed further in years of higher population density, and late breeding males and females dispersed further than early breeders. This temporal variation related to changes in competition for territory was taken into account in all tests of individual factors influencing breeding dispersal. 3. Individuals that retained their mates from the previous year dispersed shorter distances than those that changed their mates. 4. The effect of previous breeding success depended on mate fidelity. Unsuccessful breeding induced greater dispersal in birds that changed their mates but not in birds that retained their mates, indicating that breeders who change mates may take their own previous breeding experience into account during habitat selection. Faithful individuals may have to stay close to their previous sites to encounter their mates. 5. Male divorcees dispersed over shorter distances than their former mates, possibly because males contribute more than females to establishing territories. 6. Dispersal of males and females declined with increasing age over the first 10-11 years of life, then increased in old age, possibly due to senescent decay in the ability to compete for mates and territories.

Quasi-parasitism occurs when a paired male facilitates dumping in its own nest by an extra-pair female with which it has recently copulated. Although numerous observations hint at quasi-parasitism in diverse avian species, direct behavioral confirmation of male complicity is required to exclude the alternative adaptive explanations enumerated by Griffith et al. [Behav. Ecol. Sociobiol. 56 (2004) 191]. Our direct observations on dumping by female blue-footed boobies (Sula nebouxii) show apparent male ambivalence: males were hostile to eggs dumped by their extra-pair partners but half-hearted in repelling those partners after the act of dumping. Hostility was evidenced when a host male that was present during dumping destroyed the extra-pair partner’s egg and when extra-pair partners selectively dumped when the host male was absent or distracted rather than when it was alone on the territory. Host males appear to deter dumping by their extra-pair partners rather than facilitating it, and their partial tolerance of females that have dumped may be a result of their general tolerance of unaccompanied females. Although paired male boobies sometimes copulate with females that dump into their nests, apparently this is not quasi-parasitism.

Summary 1. In species where size dimorphism emerges during parental care, individuals of the larger sex are presumably more expensive to produce than individuals of the smaller sex. 2. We evaluated whether female offspring of the blue-footed booby, because of their larger size, require greater parental feeding expenditure than males. 3. A field experiment compared growth rates of sons and daughters when hand-fed the same amount of food, and a descriptive field study compared the parental feeding rates to fledgling sons and daughters. 4. Females maintained greater rates of mass and ulna increase than males when fed the same amount of food. 5. First-hatched chicks had higher probability to be fed than second-hatched chicks. However, male and female fledglings were fed at similar rates by their parents. 6. No evidence that daughters of the blue-footed booby receive a greater feeding expenditure than sons was found. Faster motor development of males in this facultatively siblicidal species might explain why they grow more slowly than females with the same food budget.

Food limitation has been thought to explain the evolution of single-chick broods in pelagic seabirds. We compared the meal sizes, feeding rates, and time budgets of Masked Boobies (Sula dactylatra), typical pelagic seabirds, which raise one chick per breeding attempt, with those of sympatric Blue-footed Boobies (S. nebouxii), which often raise two or more chicks to fledging. Breeding adult Masked Boobies forage farther from land than do Blue-footed Boobies. If longer transit times caused Masked Boobies to deliver less food per day than did Blue-footed Boobies, and thus limited brood size, we expected Blue-footed Boobies, but not Masked Boobies, to be able to provision a two-chick brood. Analysis of chick meal size and adult foraging effort (time spent foraging) indicated that Blue-footed Booby parents could typically feed two, but not three, chicks; brood size thus matches feeding capacity in this species. Masked Boobies, however, appear to have sufficient excess foraging capacity to raise an additional chick, yet do not. The different brood sizes of these species are frequently cited to support the idea that foraging biology determines brood size in seabirds. In contrast, we conclude that Masked and Blue-footed Boobies have similar capacities to provision two-chick broods; some other factor limits Masked Boobies, but not Blue-footed Boobies, to fledging single chicks. Obligate siblicide clearly limits brood size at the proximate level in Masked, but not Blue-footed Boobies. A role for sibling competition also seems likely at the ultimate level, either through an interaction with delivery frequency or by driving genetic parent-offspring conflict over brood size.

The association between the distribution of blue-footed boobies Sula nebouxii (BFB) and marine productivity around their breeding grounds is unclear. In Peru, they breed in a region seasonally influenced by the cold, nutrient rich Humboldt Current, and by warm, nutrient poor oceanic waters. The foraging range of BFB in Peru is unknown, although some evidence suggest that they may feed in warm offshore waters. During the austral winter 2002 and summer 2003, we tracked 26 BFBs breeding on Isla Lobos de Tierra, Peru (located approximately 15 km west of the mainland), during 59 feeding trips using small direction recorders to determine their marine habitat use. BFBs preferentially moved to the east of the island and foraged over the continental shelf in cold water masses where marine productivity and the abundance of the main prey, the Peruvian anchovy Engraulis ringens, are usually higher than in other areas. Overall, the median maximum foraging distance from the colony was 39 km (range 3� 109 km), with 90% of all trips located within 30 km from the mainland, and enclosed in an area of 13,113 km 2 . These results reveal that BFBs exploit the productive waters of the Peruvian Coastal Upwelling System, rather than open oceanic waters as previously suggested. BFBs travelled a greater distance, dispersed in a broader area, and visited more offshore waters during the winter 2002, than during the summer 2003, following the typical seasonal horizontal dispersion of the Peruvian anchovy.

In facultatively siblicidal bird species, the amount of food delivered by parent birds to their young ("food amount") has been assumed to be both an important proximate and ultimate cause of fatal aggression. The proximate "Food Amount Hypothesis" (FAH) contends that sibling aggression will vary inversely with the quantity of food delivered by the parents, presumably mediated by chick hunger. At the ultimate level, food shortages are expected to influence whether the combined effects of aggression and food control by older siblings will be fatal to the youngest brood member(s). We evaluated the effects of food amount on aggression and survival in facultatively siblicidal Ardeidae: (1) Sibling aggression in unmanipulated broods of Great Egrets (Cas- merodius albus) and Great Blue Herons (Ardea herodias) did not decline with increasing amounts of food, as had been predicted; indeed, this relationship may even be weakly positive. (2) In egret broods, the strongest predictors of total brood success were competitive vigor of the youngest sib and fighting rate. (3) Wild egret broods whose food amount was experimentally doubled did not reduce fighting rates, but experienced significantly lower brood reduction than control broods. (4) Captive egret broods whose food amount was experimentally decreased did not increase fighting rates, but suffered significantly more brood reduction than control broods. (5) In the wild, food amount appears not to be a sufficiently stable parameter on which to base prudent sib-aggression strategies. We conclude that food amount has little direct influence on fighting behavior in these birds, though it consistently influences chick survival. Thus the proximate effects of this ecological variable must be divorced from its ultimate role, at least in species such as these ardeids and in obligate-siblicidal species. However, the FAH may well account for sib- fighting variation in other facultatively siblicidal birds, such as Blue-footed Boobies (Sula nebouxii), Black-legged Kittiwakes (Rissa tridactyla), Ospreys (Pandion haliaetus), and South Polar Skuas (Catharacta maccormicki). It is predicted that sibling aggression will be food-amount dependent in species whose food fluctuations are relatively slow. An attempt to manipulate prey monopolizability (bolus size) in captive Great Egrets was unsuccessful, either because sib aggression in the laboratory was so inflated as to swamp prey-size effects or because this species simply lacks sufficient developmental flexibility in its intrabrood fighting behavior. If the latter were true, it would reveal a striking difference between Great Egrets and Great Blue Herons, which have shown considerable (adaptive) plasticity in this regard.