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Human impacts alter landscapes with consequences for the distribution and availability of high‐quality food resources to populations inhabiting those landscapes, which may impact on the reproductive output of individuals in those populations. The sensitivity of wild populations to changes in food resources may vary among stages of the annual cycle. For example, in birds, effects are likely to be greater during costly stages such as egg production. Here we compare assimilated diet (from stable isotope analysis of chick feathers) and egg traits (egg size, shape, eggshell colour and maculation, using pattern‐analysis software) in Herring Gulls Larus argentatus, across seven colonies in southwest Scotland and Northern Ireland. The Herring Gull is an opportunistic, generalist forager on both marine and terrestrial resources which frequently exploits anthropogenic food sources such as fishery discards and human refuse. We found that larger eggs were laid in colonies where females consumed either a higher proportion of marine resources or terrestrial resources; smaller eggs were laid in colonies where females had an intermediate diet. In colonies where females consumed more marine items, they also laid eggs with higher maculation (intensity and size of spots) compared with colonies where females mainly consumed terrestrial food. We also found smaller and more pointed eggs, suggestive of resource shortages, in larger colonies. Generalist foragers are often thought to have the capacity to buffer themselves against changes in the food web, provided that enough alternative food is available. However, this study highlights that specializing on the most profitable or available resources has consequences for egg traits even in an opportunistic generalist forager exploiting a large range of habitats. If variation in egg traits is related to reproductive output, then understanding the impact of assimilated diet on reproduction early in the breeding season can provide important insights into how populations will respond to landscapes altered by human impact.

Methods to obtain moult data from wild birds have not changed much over the last century and most studies still depend on checking museum specimens or capturing birds. Here we assess the applicability of systematic field photography for detecting and scoring moult in adult Black Skimmers Rynchops niger from southern Brazil. Moult data extracted from photographs have a high within- (RGLMM = 0.98) and between- (RGLMM = 0.97) observer repeatability and show very good fit to current Underhill-Zucchini moult models (R² = 0.75). Photography offers the advantages of being less invasive, requiring less equipment and human effort, being feasible in areas where captures may not be possible, and causing less disturbance so enhancing the number of sampled individuals. This article is protected by copyright. All rights reserved.

Between-population variation of changes in numbers can provide insights into factors influencing variation in demography and how population size or density is regulated. Here, spatio-temporal patterns of population change of Herring Gull (Larus argentatus), Lesser Black-backed Gull (L. fuscus) and Great Black-backed Gull (L. marinus) in the British Isles are described. The aim of this study was to test for density-dependence and spatial variation in population trends as two possible, but not mutually exclusive, explanations of population changes with important implications for the understanding of these changes. Between 1969 and 2013, the three species showed different population trends with Herring Gulls showing a strong decline, Lesser Black-backed Gulls an increase until 2000 but then a decline since, and Great Black-backed Gulls showing no clear pattern. Population changes in Herring Gulls varied between different regions of the British Isles with decreases in the northern and western parts ...

The differential allocation hypothesis (DAH) predicts that an individual should vary its reproductive\ud investment according to the attractiveness of its mate. A recently revised version of the DAH makes\ud explicit that investment can be positive, i.e. higher for the offspring of attractive males which should be\ud of higher quality, or negative, i.e. higher for offspring of unattractive males, for example compensating for\ud inheriting poor paternal genes. Moreover, investment can be made by the father and the mother. Here,\ud we tested whether experimental manipulation of male attractiveness affected parental investment at\ud different reproductive stages and thus influenced fitness-related traits in offspring. In two aviaries, all\ud male zebra finches, Taeniopygia guttata, were given red leg rings to increase attractiveness and in two\ud aviaries all males received green leg rings to decrease attractiveness. This controlled for assortative\ud mating between treatments. Ring colour was merely an experimental manipulation of male attractiveness,\ud not paternal quality, so we might expect additional investment to elevate offspring quality. Eggs\ud were cross-fostered between and within treatments to allow differentiation of effects of investment in\ud eggs and nestlings. Clutch and brood sizes were standardized. Both positive and negative investment\ud were observed: Eggs from red-ringed fathers had higher yolk to albumen ratios than eggs from greenringed\ud fathers. Nestlings from eggs laid and incubated by parents in the red-ringed group had higher\ud hatching masses than those in the green-ringed group. Both parents in the green-ringed group fed\ud nestlings more frequently than red-ringed parents. Offspring performance was influenced by the\ud treatment of both foster and biological parents, but combined effects of these different investment\ud patterns on fitness-related traits were ambiguous. Male attractiveness appeared to affect patterns of\ud reproductive investment but not consistently across all forms of reproductive investment suggesting that\ud the costs and benefits of differential allocation vary among individuals and across contexts.

Co-existence between great tits Parus major and blue tits Cyanistes caeruleus, \ud but also other hole nesting taxa, constitutes a classic example of species\ud co-occurrence resulting in potential interference and exploitation competition\ud for food and for breeding and roosting sites. However, the spatial and temporal\ud variation in co-existence and its consequences for competition remain poorly\ud understood. 2.We used an extensive database on reproduction in nest boxes by\ud great and blue tits based on 87 study plots across Europe and Northern Africa\ud during 1957-2012 for a total of 19,075 great tit and 16,729 blue tit clutches to \ud assess correlative evidence for a relationship between laying date and clutch\ud size, respectively, and density consistent with effects of intraspecific and\ud interspecific competition. 3.In an initial set of analyses, we statistically\ud controlled for a suite of site specific variables. We found evidence for an\ud effect of intraspecific competition on blue tit laying date (later laying at\ud higher density) and clutch size (smaller clutch size at higher density), but no\ud evidence of significant effects of intraspecific competition in great tits, nor\ud effects of interspecific competition for either species. 4.To further control for\ud site-specific variation caused by a range of potentially confounding variables,\ud we compared means and variances in laying date and clutch size of great and blue \ud tits among three categories of difference in density between the two species. We \ud exploited the fact that means and variances are generally positively correlated. \ud If interspecific competition occurs, we predicted a reduction in mean and an\ud increase in variance in clutch size in great tit and blue tit when density of\ud heterospecifics is higher than the density of conspecifics and for intraspecific \ud competition this reduction would occur when density of conspecifics is higher\ud than the density of heterospecifics. Such comparisons of temporal patterns of\ud means and variances revealed evidence, for both species, consistent with\ud intraspecific competition and to a smaller extent with interspecific competition.\ud 5.These findings suggest that competition associated with reproductive behaviour \ud between blue and great tits is widespread, but also varies across large spatial\ud and temporal scales. This article is protected by copyright.

Body temperature (Tb) is a valuable parameter when assessing the physiological state of animals, but its widespread measurement is often constrained by methods that are invasive or require frequent recapture of animals. Alternatives based on automated remote sensing of peripheral Tb show promise, but little is known about their strengths and limitations. We measured peripheral Tb in great tits Parus major with subcutaneously implanted passive integrated transponders (PIT tags) and externally attached radio transmitters to determine repeatability of measurements, sensitivity of each method to variation in ambient temperature (Ta) and wind speed, the relationship between methods, and their ability to capture circadian variation in Tb. Repeatability of measurements by radio transmitters was high (> 80%) when readings were taken within 20 min, but reduced to 16% when measures were spaced 3.5 h apart. PIT tag data for the 3.5 h interval were more repeatable (33%) and less variable (cv). Data were affected by Ta with a stronger effect on the externally attached transmitters, but the influence of wind speed was small for both methods. There was a significant positive relationship between transmitter- and PIT tag temperature during both days and nights. Both methods were equally suited to detect diel changes in peripheral Tb. However, transmitters offered longer detection distance and better temporal resolution. These qualities should be considered when deciding how to collect Tb data remotely. If properly deployed, both methods allow measurement of peripheral Tb over a wide range of natural systems and conditions in small, free-ranging, birds.

Our understanding of fundamental organismal biology has been disproportionately influenced by studies of a relatively small number of ‘model’ species extensively studied in captivity. Laboratory populations of model species are commonly subject to a number of forms of past and current selection that may affect experimental outcomes. Here, we examine these processes and their outcomes in one of the most widely used vertebrate species in the laboratory – the zebra finch (Taeniopygia guttata). This important model species is used for research across a broad range of fields, partly due to the ease with which it can be bred in captivity. However despite this perceived amenability, we demonstrate extensive variation in the success with which different laboratories and studies bred their subjects, and overall only 64% of all females that were given the opportunity, bred successfully in the laboratory. We identify and review several environmental, husbandry, life-history and behavioural factors that potentially contribute to this variation. The variation in reproductive success across individuals could lead to biases in experimental outcomes and drive some of the heterogeneity in research outcomes across studies. The zebra finch remains an excellent captive animal system and our aim is to sharpen the insight that future studies of this species can provide, both to our understanding of this species and also with respect to the reproduction of captive animals more widely. We hope to improve systematic reporting methods and that further investigation of the issues we raise will lead both to advances in our fundamental understanding of avian reproduction as well as to improvements in future welfare and experimental efficiency.

Parent–offspring conflict theory posits that parents and offspring differ in the optimal duration of parental care. Accordingly, the timing of the termination of parental care could be contested between parents and offspring. The outcome of this conflict might be related to the degree of parental sensitivity to offspring demand. By cross-fostering zebra finch, Taeniopygia guttata, chicks of different ages among breeding pairs, we examined whether parents continue parental behaviour for a fixed time period, or are sensitive to offspring age. Parents provisioned chicks in the nest for longer when they received younger foster chicks and for less time when they received older foster chicks. All chicks fledged and became independent at the same age. Our results show that zebra finch parents extend the period of parental care in response to protracted demands from their brood.

Whether parental effort can be negotiated between partners over ecological time and adjusted across different contexts is not well understood. We manipulated male extra-pair copulation (EPC) opportunity in captive zebra finches, Taeniopygia guttata, to test whether males adjust incubation effort to the mating context and to examine how females respond to their partner’s effort. Birds without previous breeding experience were paired randomly and bred with the same partner twice. In the first breeding attempt, half the males received EPC opportunities with ‘extra-pair females’ during incubation, while the other half did not. Males that received EPC opportunities in the first breeding attempt did not in the second breeding attempt and vice versa. We recorded incubation effort on days when EPC opportunities were not presented. In their first breeding attempt, males with EPC opportunities incubated less than those without. Females compensated fully for the deficit in male care so that a pair’s combined incubation effort was unchanged. In the second attempt, when a male’s opportunity for EPCs was switched, individuals showed the same level of incubation effort that they had previously, irrespective of the current availability of extra-pair females. This suggests that division of effort was negotiated in the first breeding attempt and maintained without significant adjustments in the second attempt. The effects of male EPC opportunity in the first breeding attempt on subsequent incubation effort suggests that individual parental decisions can be shaped by previous experience and this may partly explain conflicting results in studies where individuals’ histories were not known.

The Black Skimmer Rynchops niger is a New World's coastal migratory bird with three subspecies based on distinctive morphometrics, plumage, and breeding distribution. Its population is decreasing in North America while numbers are poorly known in South America. Here we conducted an extensive review of the literature on the Black Skimmer to fill gaps in the knowledge of each subspecies' natural history and suggest where future research should focus. We used 41,247 records collected from databases and literature to establish the overall occurrence and location of breeding colonies for each subspecies. We also collected information on the phenology of life-history events described in 150 papers and books to compare the annual cycle of the three subspecies. The North American subspecies R. n. niger breeds, migrates and stages along the coast with few records inland. The South American R. n. intercedens and Amazonian R. n. cinerascens subspecies breed inland and migrate mostly to coastal staging sites. The pre...

Models of climate change predict that its effects on animal populations will not always be negative, but most studies indicate negative associations between changes in climate and the phenology of animal migration and reproduction. For some populations, however, climate change may render particular environments more favourable, with positive effects on population growth. We used a 30-year population dataset on over 2000 Common Eiders Somateria mollissima at a colony in southwest Iceland to examine the response of this species to climate fluctuations. Eiders are strongly dependent on suitable climatic conditions for successful reproduction and survival. Temperatures in southwest Iceland, in both winter and summer, have generally increased over the past 30 years but have shown considerable fluctuation. We show that females laid earlier following mild winters and that year-to-year variation in the number of nests was related to the temperature during the breeding season 2 years previously. Milder summers could have positive effects on breeding success and offspring survival, producing an increase in nest numbers 2 years later when most Eiders recruit into the breeding population. In this part of their range, Eiders could benefit from a general warming of the climate.

An organism’s pattern of development can have important long-term fitness effects. In species where the sexes differ in size or other phenotypic traits, they may also have different optimal developmental rates. This influences both parental sex allocation strategies and susceptibility of the sexes to early developmental conditions. However, sex differences in developmental rate and vulnerability to environment during the embryonic period are not well understood. In birds, sibling competition and hatching asynchrony may select for accelerated embryonic development of the last offspring in order to reduce their competitive disadvantage after hatching. They may advance their hatching in response to vocal stimuli by the older siblings. It is, however, unclear whether this flexibility in developmental rates is sex specific. In this study, we experimentally manipulated between-embryo contact and tested whether this affected the pre-natal developmental rate and post-hatching performance of male and female offspring from last-laid eggs in the herring gull. Post-hatching performance was measured both in competitive and non-competitive situations. Among young incubated in isolation, males hatched faster than females, but both sexes fledged in similar, relatively good condition. Among young incubated with normal between-embryo contact, hatching time did not differ between sexes, but males fledged in poorer condition than females, regardless of whether they were reared singly or in a brood. These results suggest that male and female offspring differ in their ability to mitigate the costs of hatching asynchrony.

The quality of conditions provided by avian parents will have consequences for both parental and offspring fitness. While many components of avian reproduction appear to vary with parental age, the effect of age on incubation has largely been ignored so far. In this study, we tested whether young herring gulls provide a different incubation environment from mature ones and whether this has consequences for offspring performance. Laying and rearing conditions were standardised using a cross-fostering protocol. Egg predation rates tended to be higher in the nests of young parents. However, nest site, nest construction and egg temperature during incubation did not vary with parental age. Overall, the duration of incubation was shorter in young compared to mature birds and this reflected the later laying date of the former, since incubation duration generally decreased across the season. However, male eggs incubated by young parents had longer incubation periods than predicted for their laying dates. In contrast, incubation length of female eggs incubated by young pairs, and of male and female eggs incubated by mature birds did not deviate from the expected for any given laying date. Offspring that had been incubated by young parents had considerably poorer survival than those incubated by mature pairs, despite being reared under standardized, favourable conditions (singly, by mature parents). This was due to increased mortality among female chicks that had been incubated by young parents. The chicks incubated as eggs by young and mature birds, which survived until fledging, did not differ in body mass and size growth, or body condition. The results of this study demonstrate that parental age can influence offspring performance via variation in incubation environment, and that females are more susceptible than males to conditions experienced during embryonic development.

Asynchronous hatching is believed to result from incubation starting before laying is completed and leads to a hierarchy in chick size, with detrimental effects on the small, last-hatching chick. If chicks have some control over the timing of hatching, however, they might be able to compensate at least partly for the hatching spread. In this study on lesser black-backed gulls, Larus fuscus, we examined whether the timing of hatching varied with laying order. To separate their effects, we manipulated the hatching order relative to the laying order within broods of three chicks. We exchanged eggs between nests, so that chicks from first-laid eggs hatched last, and chicks from last-laid eggs hatched first. When hatching last, chicks from last-laid eggs took significantly less time to hatch than chicks from first-laid eggs. However, hatching duration did not differ between chicks from first- and last-laid eggs when hatching first. Hence, only offspring from last-laid eggs had the ability to accelerate hatching, and they did this only when their siblings had already hatched. Accelerated hatching might benefit the last-laid offspring by reducing hatching intervals, but it also resulted in low hatchling mass. As this ability to accelerate hatching was not found in first-laid offspring, differences in intrinsic egg quality may be responsible for hatching duration. Differences in egg quality, however, arise from differential allocation of resources across the laying sequence by the parent and may hence suggest a parental strategy to alleviate the detrimental effects of within-brood hierarchies on the last-hatching chick.

Suboptimal conditions during embryonic development can affect offspring fitness. Both egg quality and incubation behavior can affect hatching success, hatching mass, and subsequent offspring performance. These effects may differ between male and female offspring. We manipulated the prebreeding body condition of Zebra Finches (Taeniopygia guttata), using diets of different protein content. To separate possible effects on egg quality of parental body condition and incubation conditions, we did a cross-fostering experiment. We analyzed embryo survival and hatching mass with respect to body condition of the egg-laying parent, body condition of the incubating foster parent, and offspring sex. Embryos were not affected by the condition of the egg-laying parent. Eggs incubated by parents in better condition suffered less embryo mortality than those incubated by parents in poorer condition, but only when overall embryo mortality was low. Hatching mass was also affected by the incubating foster parent’s body condition. And hatchlings incubated by parents in good condition were heavier than those incubated by parents in poor condition. Female hatchlings from late-laid eggs were heavier, in comparison with the size of the egg from which they hatched, than female hatchlings from earlier-laid eggs. No such effect was found for males. Therefore, male and female embryos may differ in their sensitivity to suboptimal conditions during embryonic development. These results suggest that parental body condition during incubation can affect offspring fitness.Efectos de las Condiciones de Incubación y el Sexo de las Crías sobre el Desarrollo Embrional y la Supervivencia en Taeniopygia guttata

Gorman, H. E., Arnold, K. E. and Nager R. G. 2005. Incubation effort in relation to male attractiveness in zebra finches Taeniopygia guttata . / J. Avian Biol. 36: 413/420. The division of labour in parental care between the two sexes varies between and within species. In birds, parents have been shown to invest more into egg production and nestling care when paired with an attractive rather than an unattractive mate, as predicted by the differential allocation hypothesis. Here we investigate variation in the female’s and male’s share of incubation behaviour, a vital, and costly, period of parental care during which the embryo is vulnerable to perturbations in developmental conditions. We manipulated the attractiveness of male zebra finches Taeniopygia guttata , using red or green leg-rings. To simulate their natural social environment we allowed them to breed in outdoor aviaries. All males within an aviary were given the same coloured ring to avoid ring-colour related assortative mating. Males within a colony, however, were still expected to show some variation in attractiveness with the earliest laying females possibly pairing with the most attractive males. Indeed we found that both factors played a role in explaining female incubation effort. Among females mated to red ringed males, earlier laying females contributed significantly more to incubation than late laying females, but no such pattern was found in females mated to green ringed males. Overall, there were no differences in the level of incubation provided by both parents between treatment groups, suggesting some compensation within the pair. Hatching success was correlated with a pair’s total incubation effort. These results suggest that variation in the division of parental care between the sexes is in agreement with both increased effort of females mated with attractive males, and females compensating for the reduced effort of attractive males seeking further mating opportunities. These two factors can act at the same time in natural populations and both should be considered when explaining variation in division of labour between the sexes.

Females mated to attractive males are predicted to produce male-biased broods. Previous studies on zebra finches, Taeniopygia guttata, in which colored leg rings were used to alter male attractiveness, support this hypothesis. However, because molecular sexing techniques were not available, it was not known when during development this bias arose. Also, because both attractive (red-ringed) and unattractive (green-ringed) males were within the same aviary, assortative mating between treatments may have confounded the results. Using two different experimental designs, we tested whether the sex ratio of zebra finch eggs and chicks differed in response to paternal ring color whilst controlling for assortative mating between treatments. In the aviary experiment, birds could interact socially, but all males in an aviary had the same leg ring color. In the cage experiment, each female was randomly assigned a red- or green-ringed mate, thus also eliminating assortative mating within treatments. Offspring were sexed based on plumage or using a molecular method. The sex ratio at laying did not differ between treatments in either the aviary (n = 313 eggs) or cage (n = 151 eggs) experiments, suggesting that female zebra finches do not manipulate the primary sex ratio in response to their mate's ring color. However, in the cage experiment we found greater male embryonic mortality in the attractive group, which resulted in a female-biased sex ratio at sexual maturity, that is, in the opposite direction to that found in previous studies. Possible explanations for the disparity between our results and those of previous studies are considered. Copyright 2005.

Egg composition, which is under maternal control, can have a profound effect on offspring fitness. The presence of maternal testosterone and carotenoids in avian egg yolk, for example, is thought to enhance the development and competitive ability of the offspring and protect the hatching and growing chick against oxidative stress. Egg quality often differs between females and such variation can be due to differences in maternal social environment, e.g. breeding density. However, this is confounded by the possibility that the quality of individuals breeding in high- or low-density areas may vary. We tested if maternal social environment influences egg composition in a colonial seabird, the lesser black-backed gull (Larus fuscus). To control for confounding effects of female quality, we experimentally manipulated maternal social environment during egg formation. We increased the frequency of intra-specific interactions (i.e. aggressive encounters with conspecifics other than nest mates) in which the females were involved, by placing an elevated platform in their territory. Females that took part in more intra-specific interactions produced a heavier last egg, but the yolk testosterone concentration in eggs laid by control and experimental females did not differ. Differences in yolk testosterone concentration in relation to embryo sex were found neither in the control nor in the experimental group. In contrast, within the control group, eggs with a male embryo contained more carotenoids than eggs with a female embryo. Moreover, experimental females that had been involved in more intra-specific interactions produced female eggs with higher carotenoid levels compared to female eggs of control birds. An experimental increase in carotenoid levels was not observed in eggs containing a male embryo. Our results suggest that intra-specific interactions experienced by female birds during egg formation can influence conditions for embryonic development.

State dependence is thought to be an important factor in resource allocation decisions, particularly in those decisions relating to parental care. Incubation behaviour is a costly part of avian parental care, and therefore likely to depend on the parent's body condition. We manipulated the body condition of zebra finches, Taeniopygia guttata, during incubation by using prelaying diets of different protein content, and induced birds to lay a similar number of eggs. We tested the hypothesis that birds in better body condition would invest more in incubation than birds in poorer condition when incubating on the same diet. Females that had received a high-protein prelaying diet lost more body mass than those that had received a low-protein diet. They also increased the length of their incubation bouts between early and middle incubation, whereas females that had received a low-protein prelaying diet increased bout length only between middle and late incubation. There were no differences between males from the two diet groups, and males were responsible for a lesser proportion of incubation than females. These results indicate that incubation behaviour is state dependent in female zebra finches, and that birds of different body condition adopt different incubation strategies. We found no differences in incubation duration and hatching success between the two incubation strategies in captivity, but the potential risk of nest predation in the wild may differ. We suggest that only females in good condition can afford to adopt a strategy of increasing bout length early in incubation; females in poorer condition first have to recover their body condition after having produced a clutch. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour

Summary In many incubating birds heat transfer from parent to egg is facilitated by the brood patch, an area of ventral abdominal skin that becomes highly vascularised, swells and loses its down feathers around the time of laying. Only the female develops a brood patch in most passerine species, but some males incubate and maintain the eggs at similar temperatures to females without a brood patch. Here we used a novel application of infra-red thermography (IRT) to examine sex differences in parental care from a physiological perspective. Using incubating male and female zebra finches (Taeniopygia guttata), a species in which the male lacks a brood patch, we measured the surface temperature of the ventral plumage overlying the abdomen and a reference area that does not contact the eggs (thorax) twice per pair. In half of the pairs clutch size was experimentally enlarged between the two sets of measurements to increase incubation demand. We found that the temperature differential between abdomen and thorax plumage was greater in females than in males, and that abdomen plumage was warmer after clutch enlargement than before it in females but not in males. These findings are consistent with morphological sex differences in brood patch development and suggest that male and female zebra finches differ in the way they regulate abdomen versus general body surface temperature in response to variation in incubation demand.

Studies of sexual selection have mostly focused on female choice and male–male competition. However, when a male contributes substantially to a breeding attempt, he would benefit from choosing a female whose reproductive potential is high in order to maximize his reproductive success. We examined male mate choice in zebra finches, Taeniopygia guttata, with respect to experimental manipulation of female fecundity. This was enhanced by a dietary supplement prior to contact with the males. When housed with females of differing fecundity, male zebra finches preferentially direct their courtship towards more fecund females. This outcome could be a consequence of female–female interactions or direct male preference. In this experiment, we gave males a choice between standard and supplemented females in an experimental arena where the females were spatially segregated, and could not interact with each other, thereby excluding the possibility of female–female competition. Males spent significantly more time with the more fecund female, they hopped more times on the perch opposite that female and were twice as likely to choose her than the standard female. The standard females were not, however, unreceptive to males, nor did they appear to differ in their activity patterns in response to the male. These results show that males are able to distinguish females that have higher fecundity, and that this is not the result of relative female competitiveness or dominance status.  2001 The Association for the Study of Animal Behaviour

Measurements of costs of reproduction are essential for our understanding of the evolution of reproductive effort. While in birds the effects of increased chick-rearing effort on subsequent survival and fecundity have been relatively well studied experimentally, costs associated with increased egg-production effort have received relatively little attention. We experimentally increased the egg-production effort of individually marked Lesser Black-backed Gulls Larus fuscus and followed their breeding performance in the next year. In the season following increased egg production, females, but not males, were less likely to be resighted in the study plot and those that did return were less likely to produce a clutch compared to control birds. It is unclear whether the observed effect on local return rate represents differential survival, differences in breeding propensity or differences in dispersal between experimental and control females. In any event, all of these would adversely affect the fitness of experimental females. In addition, those experimental females that did breed invested less in egg production the following season, which again is likely to affect breeding performance. Thus, this study provides evidence that there is an inter-brood trade-off between current egg-production effort and future fitness in birds.

Empirical evidence is growing that the offspring sex ratio in birds can be biased in relation to the body condition of parents during breeding. The sex ratio bias may come about because (1) the actual production of the two sexes may be skewed and/or (2) there may be a sex bias in early nestling mortality contingent on parental condition. By manipulating parental condition and giving them a control brood to rear, thereby eliminating effects operating via the eggs, we examined the extent to which parental condition influences the post-hatching survival of male and female lesser black-backed gulls, Larus fuscus. We found that the pre-fledging survival of male chicks was strongly reduced in all-male broods reared by parents in poor condition. Pre-fledging survival of female chicks was, however, unaffected by parental condition or brood sex composition. Thus, independently of any production biases, sex differences in nestling mortality alone can bias the offspring sex ratio at fledging in relation to the prevailing rearing conditions. In other studies on gulls we have, however, also shown that females in poor condition at laying preferentially produce female eggs. Clearly a bias in fledging sex ratio can occur within the same species due to a combination of differential production and differential post-laying mortality; the latter can involve a differential effect of poor egg quality on male and female offspring, differential effects of brood sex composition on their survival and a difference in the capacity of parents to rear males and females. All of these processes need to be taken into account in attempting to understand offspring sex ratios.

Naef-Daenzer, L., Naef-Daenzer, B. and Nager, R. G. 2000. Prey selection and foraging performance of breeding Great Tits Parus major in relation to food availability. ‐ J. Avian Biol. 31: 206‐214. We studied the nestling diet and the foraging performance of Great Tits in relation to prey abundance in the field. Numerous experimental studies present data on foraging decisions in captive Great Tits. Little is, however, known about prey selection in the field in relation to the food available and the consequences this has for the food delivery rate to nestlings. Since the foraging performance of the parents is one of the main determinants of fledging weight and juvenile survival, foraging behaviour is an important part of Great Tit reproduction. During the early breeding season up to 75% of the prey biomass delivered to the nestlings were spiders, which is in contrast with other studies. Only when caterpillars reached a size of 10‐12 mg (approximately the average size of the spiders caught at that time) did the Great Tits change their preferences and 80‐90% of the delivered prey masses were caterpillars, as reported by other authors. This ‘switching’ between prey occurred within a few days. It was not related to the changes in abundance but to size of caterpillars. The rate at which caterpillars were delivered to the nestlings (in mg:nestling:h) was strongly correlated with the caterpillar biomass available (in mg:m of branches) and nestling growth rate was significantly influenced by the mass of available caterpillars. The results provide evidence why perfect timing of breeding is so important for the Great Tit, and contribute to the understanding of the causal link between food supply, growth and breeding success.

1. We tested the hypothesis that protein availability rather than energy availability constrains egg formation in great tits (Parus major L.) by providing them with two food supplements of different protein content in the prelaying and laying period of 1991 and 1992. 2. Timing of breeding, clutch size and egg size did not differ between the two food supplements. Thus, the hypothesis that a low protein content of supplement restricts laying in great tits cannot be supported. 3. Food supplementation, independent of food type and year, led to an advancement of laying by 5.6 days. Average clutch size of supplemented birds, corrected for laying date and year, was 0.55 eggs above the population average. 4. Supplementary fed great tits did not breed more successfully than controls. This suggests that by adding food early in laying, birds were induced to lay at a less favourable time and/or produced clutches that were too large to care for successfully under normal food condition. Thus food early in the season might not only energetically restrict laying birds, but also provide clues about future conditions during chick rearing. 5. The comparison of results of published supplemental feeding experiments shows that the birds responded more strongly to additional feeding in poor environments. Almost two-thirds of the variation in changes of clutch size due to experimental treatment can be explained by year quality. Thus, only in poor environments do reproductive traits seem to be limited by energy availability early in the season, but other factors become more important in good environments [KEYWORDS: clutch size, egg formation, food quality, food supplementation, Parus major]

It has been proposed that female birds can influence the phenotype of their offspring by provisioning eggs with variable amounts of nutrients and maternal hormones. Egg quality is strongly influenced by maternal body reserves and the amount of food available at the time of egg formation. This study investigated the effects of maternal state and food availability on the capacity of female lesser black-backed gulls Larus fuscus to provision their eggs with macronutrients and steroid hormones. Maternal state was reduced by increasing egg-production effort, whereas extra food was provided to reverse this effect. Compared with eggs of first clutches, eggs of experimentally induced replacement clutches exhibited a lower yolk/albumen ratio and contained more yolk testosterone. During one of the three years in which the study was performed, replacement eggs also contained more 17β-estradiol. Food provisioning during the relaying interval did not affect changes in yolk/albumen ratio or steroid concentrations, but fed females produced bigger eggs in their replacement clutch. This study demonstrates significant within-female consistency in egg size, macronutrient content, and yolk steroid concentration, and it shows that these egg characteristics are influenced by maternal state, food availability, and the timing of breeding.

Nest shelter in incubating birds is of major importance in providing protection against unfavourable conditions such as harshness of the environment and exposure to predators. We examined the link between nest shelter, baseline corticosterone (CORT) levels and hatching success in common eiders (Somateria mollissima) incubating at nest sites with different levels of shelter. Since more sheltered nest sites could be occupied by better-quality females, we also used an experimental manipulation of nest shelter to separate the effects of the physical attributes of the nest site from those of individual quality. We compared birds with naturally sheltered nests, exposed nests and exposed nests provided with artificial nest shelters and measured clutch size, body condition, CORT levels at the end of incubation and hatching success. If nest shelter reduces CORT levels, we predicted that CORT levels would be highest at the least sheltered sites, and that the provision of artificial shelter would reduce CORT levels. We found that nest shelter was not related to CORT levels in incubating eiders. Nest shelter, however, affected body condition, with females at exposed sites losing more body mass during incubation than females at naturally and artificially sheltered nests. Interestingly however, in those birds nesting at the exposed sites, with and without artificial shelter, those with the highest CORT levels had the lowest hatching success. This relationship was not evident in females nesting at naturally sheltered sites. These results suggest that the level of nest shelter does not directly affect CORT levels in females. Instead, we suggest that the relationship between CORT levels and hatching success is state-dependent. Exposed sites are occupied by individuals that laid smaller clutches, and hence are likely to be of lower quality, and the negative effects of elevated CORT on hatching success are more pronounced in these females.

10 pages; International audience; Wetlands in the Mediterranean area have become a rare habitat due to human impact. To model the ecology and breeding biology of birds depending on that habitat, we describe long-term studies on two wetland birds (Little Egret Egretta garzetta and Greater Flamingo Phoenicopterus (ruber) roseus) in the Camargue, France. The hydrological conditions in natural Mediterranean wetlands depend largely on the pattern of rainfall (winter) and evapotranspiration (summer), but have been substantially altered by human activities. In natural conditions, these wetlands are very diverse and therefore sustain a high diversity of breeding birds. At the same time their unpredictable nature favours opportunistic breeding and wintering strategies of their inhabitants. Colonial waterbirds depend on wetlands both for safe nesting sites and for foraging. Availability of suitable nesting sites may limit breeding numbers. Availability and diversity of wetland habitats as foraging sites affect the number and density of breeding species that an area can sustain. Unfavourable feeding conditions can reduce the number of birds that attempt to breed and the number and quality of offspring the birds produce. Apart from local environmental factors, dispersal between wetlands in the western Mediterranean and existence of a non-breeding pool also affect breeding numbers in the Camargue. Thus, the persistence of wetland birds in the Mediterranean depends both on a network of habitat types on a local scale and a network of wetlands on a regional scale.

We examined seasonal variation in the hatching sex ratio of Audouin's Gull (Larus audouinii). This species is sexually size dimorphic (males are 20% larger than females at fledging); it has a modal clutch of three eggs, which vary in size (the third egg is the smallest) and hatch asynchronously. These sex, egg size, and hatching patterns generate substantial within-brood differences in chick size that interact with the food provisioning of the parents to influence chick survival. Parental provisioning capacity depends on both parental quality and environmental conditions, both of which are known to decline with season. Consequently, the optimal brood composition is likely to vary within a season. Using molecular markers to sex newly hatched chicks, we found that offspring sex was influenced by an interaction between hatching date and hatching order, with the proportion of males among third-hatched chicks initially increasing and then decreasing later in the season.

Some fascinating questions are raised by Kurt and William Burnham and Ian Newton's findings, reported in this issue of Ibis, that 14C dating has shown some Gyrfalcon Falco rusticolus nests to have been occupied for around the past 2500 years. As the authors demonstrate, this finding could make an important contribution to our understanding of climate change in the high arctic. And stable isotope analysis also has the potential to record changes in prey selection and foraging habitat over long periods of time. Other detritus from nest-sites can also achieve this.