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Free-ranging animals are often used as bioindicators of both short- and long-term changes in ecosystem health, mainly to detect the presence and effects of contaminants. Birds, and gulls in particular, have been used as bioindicators over a broad range of marine and terrestrial ecosystems. In this study, we standardise the conditions for the use of a suite of biochemical markers in non-destructive matrices of Lesser Black-backed Gull (Larus fuscus) to facilitate future biomonitoring of marine and terrestrial contaminants. We characterized cholinesterase (ChE) in plasma and optimized assay conditions for ChE activity as a marker of neurotoxic damage. Moreover, we quantified variation in activity of ChE, lactate dehydrogenase (LDH), glutathione-S-transferase (GST) and catalase (CAT) as well as variation ranges of lipid peroxidation (LPO), in free-ranging adults and captive chicks. The main ChE form present in plasma of both adults and chicks was butyrylcholinesterase (BChE) followed by acetylcholinesterase (AChE), whose relative proportion in plasma tended to decrease with increased chick age. LPO levels and GST activity in blood cells (BCs) decreased significantly with increasing chick age, while BChE and LDH activity in plasma were not age-dependent. CAT in BCs tended to decline non-significantly in older chicks. Results of this study underscore the importance of standardising assay conditions and assessing intrinsic baseline variation in biochemical markers, before biochemical quantification. Data presented here provide a foundation for future use of BChE and LDH activity in plasma, as well as oxidative stress markers (LPO, CAT and GST) in BCs, to monitor environmental stress effects in Lesser Black-backed gulls.

Human activities impact upon natural habitats used by birds for breeding and foraging, and lead to changes in the composition and spatial distribution of predator communities, mainly through loss, fragmentation and disturbance of formerly pristine habitat. Yet possible fitness consequences of such changes through impacts on bird nest‐site selection remain poorly known. Here we study nest‐site selection and reproductive success of Placid Greenbuls Phyllastrephus placidus in the Taita Hills, southeast Kenya. We show that habitat features associated with nest‐site selection by this insectivorous, open‐cup‐nesting bird species vary among forest fragments that are exposed to different levels of habitat disturbance. Such differences in sites selected for breeding result from a plastic response to fragment‐specific conditions or may be driven by fragment‐specific variation in the distribution and availability of certain habitat features. Given the overall high nest predation rates in our study area, we expected variation in nest‐site selection to correlate with reproductive success and nestling condition, but detected no such relationship. Because predator density and nest predation rates may vary strongly in space and time, a better understanding of spatio‐temporal variation in predator communities is needed to assess the possible adaptive value of nest‐site selection strategies for reducing the high predation rates that are typical for this and many other open‐cup‐nesting tropical passerines.

Current emission and mobilization rates of mercury (Hg) in the environment pose extensive threats to both wildlife and human health. Assessing the exposure risk and effects of Hg contamination in model species such as seabirds is essential to understand Hg risks at the population and ecosystem levels. The lesser black-backed gull (Larus fuscus), a generalist seabird species, is an excellent model species because it forages in both marine and terrestrial habitats, which in turn differ in their Hg exposure risk. To identify possible deleterious effects of Hg exposure on developingL. fuscuschicks, a dietary experiment was carried out and chicks were provided a marine, terrestrial, or mixed diet. The effects of embryonic and dietary Hg exposure on chick body condition and physiological state were assessed at different developmental stages until fledging age (30 d). Overall physiological condition was lower in chicks fed a predominantly marine diet, which coincided with higher Hg loads in blood and primary feathers. However, no effect of dietary uptake of Hg was observed on body condition or in terms of genotoxic damage. Body condition and genotoxic damage correlated instead with Hg exposure during embryonic development, which seems to indicate that embryonic exposure to Hg may result in carry-over effects on later chick development.Environ Toxicol Chem2020;39:2008-2017. (c) 2020 SETAC

Parent decisions about food allocation are usually based on simple time-saving rules that optimize their own fitness; however, they can sometimes vary depending on the prevailing ecological conditions both outside and inside the nest. Parent-offspring interactions also become more complex when parents suffer from brood parasitism, which implies that they care for the parasite's eggs and unrelated young. The great spotted cuckoo (Clamator glandarius) is a specialist brood parasite that uses the magpie (Pica pica) as its primary host. Here, by filming food allocation by magpie parents in natural non-parasitized and experimentally parasitized and non-parasitized magpie nests, we have found that magpie provisioning behaviour is highly complex including two types of feedings apart from normal ones. First, false feedings, when the parent touched the chick's beak but did not leave any food, occurred more frequently when feeding a cuckoo than when feeding magpie nestlings. Second, two types of what we have called coax feedings: (2a) when magpie parents induce a nestling to beg by waking it up by touching it softly with the beak, and (2b) when parents disregard begging signals (always from brood parasitic great spotted cuckoos) while coaxing one non-begging nestling (always one of their own) to feed it. We suggest that brood parasitism, involving selfish excessively begging nestlings, could have acted as a selective pressure for both false and coax feedings to evolve, as both imply ignoring nestlings that beg too much. We also discuss that these parental responses could have evolved either by a discrimination without recognition mechanism, or, more probably, by a recognition-based discrimination mechanism.

Monthly monitoring data between 2013 and 2018 are used to investigate and document the recovery of the hyperbenthic community in the Sea Scheldt, the Belgian part of the Scheldt estuary after a long-lasting state of hypoxia, and to identify ecological drivers of the current community. Hyperbenthos was present in all studied salinity zones and was taxonomically dominated by juvenile fish, Amphipoda, mysids and decapod shrimps. Species richness and guild diversity were comparable to that of other European estuaries. Variation in community composition was primarily governed by salinity zone and seasonality. In addition, hyperbenthos densities related negatively to recent river discharge, likely through flush effects or marinisation. Hyperbenthos abundance was positively affected by particulate organic matter density and was higher near neap tide. Seasonal migrations and changes in age and size distributions illustrate that the upper Scheldt estuary resumed its functions as spawning, nursery and feeding area. Given the observed rapid recolonization, we speculate that estuarine resident taxa may have persisted in refuges within the estuary from which recolonization could occur. Evidence is given of the contribution of the freshwater tidal zone to the estuarine food production and biodiversity maintenance functions, justifying maintenance and restoration of these rare habitats.

Urban sprawl increasingly affects the ecology of natural populations, including host-microbiota interactions, with observed differences in the gut microbiota between urban and rural hosts. While different mechanisms could explain this pattern, dietary uptake constitutes a likely candidate. To assess the contribution of diet in explaining urban-rural variation in gut microbiota, we performed an aviary experiment in which urban and rural house sparrows were fed with mimics of urban or rural diets. Before the experiment, rural sparrows hosted more diverse gut communities, with a higher relative abundance of Enterococcaceae and Staphylococcaceae and lower abundance of genes involved in xenobiotic degradation and lipid metabolism than their urban counterparts. The experimental diets significantly altered gut microbiota alpha- and beta-diversity and taxonomic composition, with the strongest shifts occurring in individuals exposed to contrasting diets. Overall, diet-induced shifts resembled initial differences between free-ranging urban and rural hosts. Furthermore, rural diet had a positive impact on urban host body mass but only in hosts with the highest initial gut diversity. Overall, our results indicate that diet constitutes an important factor contributing to differences in gut microbiota along the urbanization gradient and provide new insights on possible fitness consequences of a reduced gut diversity in urban settings.

While urbanization exposes individuals to novel challenges, urban areas may also constitute stable environments in which seasonal fluctuations are buffered. Baseline and stress-induced plasma corticosterone (cort) levels are often found to be similar in urban and rural populations. Here we aimed to disentangle two possible mechanisms underlying such pattern: (i) urban environments are no more stressful or urban birds have a better ability to habituate to stressors; or (ii) urban birds developed desensitized stress responses. We exposed wild-caught urban and rural house sparrows (Passer domesticus) to combined captivity and diet treatments (urban versus rural diet) and measured corticosterone levels both in natural tail feathers and in regrown homologous ones (cortf). Urban and rural house sparrows showed similar cortf levels in the wild and in response to novel stressors caused by the experiment, supporting the growing notion that urban environments are no more stressful during the non-breeding season than are rural ones. Still, juveniles and males originating from urban populations showed the highest cortf levels in regrown feathers. We did not find evidence that cortf was consistent within individuals across moults. Our study stresses the need for incorporating both intrinsic and environmental factors for the interpretation of variation in cortf between populations., Summary: Corticosterone in natural and regrown feathers was similar between urban and rural birds. Sex and age related to corticosterone in regrown feathers. Feather corticosterone was not consistent across different moults.

While urbanization exposes individuals to novel challenges, urban areas may also constitute stable environments in which seasonal fluctuations are buffered. Baseline and stress-induced plasma corticosterone (cort) levels are often found to be similar in urban and rural populations. Here we aimed to disentangle two possible mechanisms underlying such pattern: (i) urban environments are no more stressful or urban birds have a better ability to habituate to stressors; or (ii) urban birds developed desensitized stress responses. We exposed wild-caught urban and rural house sparrows (Passer domesticus) to combined captivity and diet treatments (urban versus rural diet) and measured corticosterone levels both in natural tail feathers and in regrown homologous ones (cortf ). Urban and rural house sparrows showed similar cortf levels in the wild and in response to novel stressors caused by the experiment, supporting the growing notion that urban environments are no more stressful during the non-breeding season than are rural ones. Still, juveniles and males originating from urban populations showed the highest cortf levels in regrown feathers. We did not find evidence that cortf was consistent within individuals across moults. Our study stresses the need for incorporating both intrinsic and environmental factors for the interpretation of variation in cortf between populations.

Inter- and intra-clutch variation in egg corticosterone (CORT), the major glucocorticoid in birds, may provide insights into how maternal stress levels vary with the timing of breeding and with laying order. Common analytical methods (e.g. immunoassays), however, suffer from cross-reaction with other steroids, leading to potential overestimation of CORT concentrations which can obscure true hormone-environment relationships and complicate among-study comparisons. We here apply a new LC-MS/MS technique, which has recently been shown to avoid the problem of cross-reactivity due to its high specificity, to quantify CORT concentrations in yolk and albumen in clutches of lesser black-backed gulls Larus fuscus. We found that CORT concentration exhibited a previously unreported U-shaped relationship with time of breeding, which we explain as a potential interplay of two forces exerting extra strain on the early and late breeders. Furthermore, results showed an increase in CORT with laying order indicating the energetic expense of egg production. The levels of CORT assessed in this study were significantly lower than those previously reported in studies using immunoassays for CORT analysis. This supports the fact that incorporating chromatography effectively reduces overestimation of CORT due to cross-reactivity with other steroid hormones, particularly in egg yolk.

Background & aim – In general, biodiversity has positive effects on ecosystem functioning. In forests, understorey vegetation is influenced by both the composition and species richness of the overstorey through species-specific effects on environmental conditions at the forest floor. Forest fragmentation is also known to influence understorey vegetation composition and richness. However, the combined effects of tree species diversity and forest fragmentation have not been studied yet. With the TREEWEB research platform, consisting of 53 forest plots along a tree species diversity and forest fragmentation gradient, we aim to unravel the combined effects of tree species diversity, tree species identity and forest fragmentation on the understorey composition and diversity.Methods – The TREEWEB platform includes forest plots of three tree species richness levels, containing all possible species combinations of Quercus robur, Quercus rubra and Fagus sylvatica. Complete dilution is avoided in the design, allowing separation between tree species identity and diversity effects. Vegetation surveys were conducted in all plots to investigate the species richness, species diversity, compositional turnover and cover of the herb layer as well as the shrub layer cover.Key results – Within the TREEWEB platform, overstorey-understorey diversity relationships were mainly characterised by tree species identity effects. No clear effects of tree species diversity and forest fragmentation on understorey composition and diversity were found.Conclusion – Tree species identity effects were most important in explaining the observed patterns in the understorey vegetation. Further in-depth research will allow us to disentangle which mechanisms underlie these patterns and whether effects of fragmentation are more pronounced at higher trophic levels.

Host defences against cuckoo parasitism and cuckoo trickeries to overcome them are a classic example of antagonistic coevolution. Recently it has been reported that this relationship may turn to be mutualistic in the case of the carrion crow (Corvus corone) and its brood parasite, the great spotted cuckoo (Clamator glandarius), given that experimentally and naturally parasitized nests were depredated at a lower rate than non-parasitized nests. This result was interpreted as a consequence of the antipredatory properties of a fetid cloacal secretion produced by cuckoo nestlings, which presumably deters predators from parasitized host nests. This potential defensive mechanism would therefore explain the detected higher fledgling success of parasitized nests during breeding seasons with high predation risk. Here, in a different study population, we explored the expected benefits in terms of reduced nest predation in naturally and experimentally parasitized nests of two different host species, carrion crows and magpies (Pica pica). During the incubation phase non-parasitized nests were depredated more frequently than parasitized nests. However, during the nestling phase, parasitized nests were not depredated at a lower rate than non-parasitized nests, neither in magpie nor in carrion crow nests, and experimental translocation of great spotted cuckoo hatchlings did not reveal causal effects between parasitism state and predation rate of host nests. Therefore, our results do not fit expectations and, thus, do not support the fascinating possibility that great spotted cuckoo nestlings could have an antipredatory effect for host nestlings, at least in our study area. We also discuss different possibilities that may conciliate these with previous results, but also several alternative explanations, including the lack of generalizability of the previously documented mutualistic association., This work was partially supported by the Junta de Andalucía (research project CVI-6653) and the Spanish Ministerio de Economía y Competitividad/FEDER (research project CGL2011-25634/BOS to MS). There was no additional funding received for this study.

Generalist species can potentially exploit a wide variety of resources, but at the individual level they often show a certain degree of foraging specialization. Specific foraging strategies, however, may increase exposure to environmental contaminants that can alter the cost-benefit balance of consuming particular food items. The Lesser Black-backed Gull (Larus fuscus) is known to opportunistically feed on a wide range of marine and terrestrial prey that differ in contaminant load, such as mercury (Hg) that strongly biomagnifies through the aquatic food web. The hypothesis tested in this study were: i) a predominant use of marine prey by females during egg-formation and by both parents during chick rearing increases the exposure to Hg during embryonic development and chick growth, and ii) this affects parental investment in clutch volume, chick growth and body condition. Total Hg burden and isotopic signatures of carbon (delta C-13) and nitrogen (delta N-15) were determined for eggs, down feathers, and primary feathers of L. fuscus chicks collected at a coastal colony in Belgium. As expected, eggs and feathers of chicks from parents with a stable isotope signature that suggested a predominantly marine diet had higher levels of Hg. The use of marine resources by females during the egg-formation period positively correlated to maternal investment in egg size, though entailing the cost of increased Hg-concentrations which in turn negatively affected clutch volume. Furthermore, it is shown that the use of chick down feathers is a suitable matrix to non-lethally estimate Hg concentrations in eggs. Contrary to our expectations, no relationship between Hg exposure and chick growth or chick body condition was found, which may be due the low concentrations found. We conclude that currently Hg contamination does not constitute a risk for development and condition of L. fuscus offspring at the levels currently observed at the Belgian coast. (C) 2017 Elsevier B.V. All rights reserved.

For avian group living to be evolutionary stable, multiple fitness benefits are expected. Yet, the difficulty of tracking fledglings, and thus estimating their survival rates, limits our knowledge on how such benefits may manifest postfledging. We radio-tagged breeding females of the Afrotropical cooperatively breeding Placid greenbul (Phyllastrephus placidus) during nesting. Tracking these females after fledging permitted us to locate juvenile birds, their parents, and any helpers present and to build individual fledgling resighting datasets without incurring mortality costs or causing premature fledging due to handling or transmitter effects. A Bayesian framework was used to infer age-specific mortality rates in relation to group size, fledging date, maternal condition, and nestling condition. Postfledging survival was positively related to group size, with fledglings raised in groups with four helpers showing nearly 30% higher survival until independence compared with pair-only offspring, independent of fledging date, maternal condition or nestling condition. Our results demonstrate the importance of studying the early dependency period just after fledging when assessing presumed benefits of cooperative breeding. While studying small, mobile organisms after they leave the nest remains highly challenging, we argue that the telemetric approach proposed here may be a broadly applicable method to obtain unbiased estimates of postfledging survival.

Several studies on birds have proposed that a lack of invertebrate prey in urbanized areas could be the main cause for generally lower levels of breeding success compared to rural habitats. Previous work on house sparrows Passer domesticus found that supplemental feeding in urbanized areas increased breeding success but did not contribute to population growth. Here, we hypothesize that supplementary feeding allows house sparrows to achieve higher breeding success but at the cost of lower nestling quality. As abundant food supplies may permit both high‐ and low‐quality nestlings to survive, we also predict that within‐brood variation in proxies of nestling quality would be larger for supplemental food broods than for unfed broods. As proxies of nestling quality, we considered feather corticosterone (CORT f), body condition (scaled mass index, SMI), and tarsus‐based fluctuating asymmetry (FA). Our hypothesis was only partially supported as we did not find an overall effect of food supplementation on FA or SMI. Rather, food supplementation affected nestling phenotype only early in the breeding season in terms of elevated CORT f levels and a tendency for more variable within‐brood CORT f and FA. Early food supplemented nests therefore seemed to include at least some nestlings that faced increased stressors during development, possibly due to harsher environmental (e.g., related to food and temperature) conditions early in the breeding season that would increase sibling competition, especially in larger broods. The fact that CORT f was positively, rather than inversely, related to nestling SMI further suggests that factors influencing CORT f and SMI are likely operating over different periods or, alternatively, that nestlings in good nutritional condition also invest in high‐quality feathers.

In cuckoo-host coevolution, rejection of parasite eggs based on visual discrimination is the key host-defensive mechanism reducing the costs of parasitism. Although host discriminatory tasks often occur in variable environmental conditions, the influence of nest light variation on the perceptual processes involved in egg discrimination has been seldom considered. Here, we combine visual modeling, experimental manipulation of nest ambient light, and egg recognition experiments with model eggs differing in background color (cream vs. blue) to explore the possibility that variation in ambient light in magpie (Pica pica) domed nests may affect the perceptual process involved in discrimination of foreign eggs. We found that the architecture of magpie nests affects the quality of ambient light for egg recognition and that changes in luminosity did not differently affect rejection of blue and cream models. However, we found that ejection of model eggs declined throughout the season in nests in which luminosity remained unmodified, but that magpies rejected eggs at a similar rate over the season in nests in which luminosity was increased. These results therefore suggest that variation in ambient light at the nests might potentially influence the perceptual processes involved in visual detection of parasite eggs by cuckoo hosts.

Corticosterone (CORT) is the dominant plasma glucocorticoid in birds. There has been increasing interest in the function of CORT in avian egg yolk and in the potential to use CORT concentrations in eggs to quantify stress and to assess the effect of maternal stress on offspring. The concentration of CORT in egg yolk is most frequently assessed using enzyme or radioimmunoassays, alone or in combination with high-performance liquid chromatography. However, the quantification of CORT is frequently hampered by the presence of high concentrations of other steroid hormones which cross-react with the CORT antibody. As an alternative, we developed a sensitive and specific LC-MS-MS method. The sample-preparation procedure consisted of a protein-lipid precipitation step, followed by defatting and clean-up using a C18 SPE column. Chromatography was performed on an Acquity C18 BEH column (50 mm × 2.1 mm i.d., dp: 1.7 μm, run-time: 6 min), using 0.1% formic acid in both water (A) and acetonitrile (B) as mobile phases. The MS-MS instrument was operated in the positive-electrospray-ionization mode. The method was validated in-house according to European Guidelines (linearity, accuracy and precision, limits of quantification and detection, specificity, stability) and the results fell within the accepted ranges. The method was successfully used for the analysis of CORT in yolk and albumen of eggs collected from eight breeding lesser black-backed gulls at a Flemish coastal colony. CORT concentrations were in the range 42.4-166.3 pg g(-1) in albumen and LOQ (75 pg g(-1))-762.5 pg g(-1) in yolk.

Species that suffer from brood parasitism face a considerable reduction in their fitness which selects for the evolution of host defences. To prevent parasitism, hosts can mob or attack brood parasites when they approach the host nest and block the access to the nest by sitting on the clutch. In turn, as a counter-adaptation, brood parasites evolved secretive behaviours near their host nests. Here, we have studied great spotted cuckoo (Clamator glandarius) egg-laying behaviour and defence by their magpie (Pica pica) hosts inside the nest using continuous video recordings. We have found several surprising results that contradict some general assumptions. The most important is that most (71%) of the parasitic events by cuckoo females are completed while the magpie females are incubating. By staying in the nest, magpies force cuckoo females to lay their egg facing the high risk of being attacked by the incubating magpie (attack occurred in all but one of the events, n = 15). During these attacks, magpies pecked the cuckoo violently, but could never effectively avoid parasitism. These novel observations expand the sequence of adaptations and counter-adaptations in the arms race between brood parasites and their hosts during the pre-laying and laying periods.

Altricial nestlings are under strong selection pressures to optimize digestive efficiency because this is one of the main factors affecting nestling growth and survival. Bird species vary in their ability to assimilate different nutrients and current theory predicts that nestlings should also be able to adjust their nutritional physiology to feeding frequency. Variation in parental provisioning to nestlings would select for flexibility in nestling digestive physiology, which would allow maximization of nutrient assimilation. In the present study, by making use of a brood parasite–host study system in which great spotted cuckoo nestlings (Clamator glandarius) are reared by magpie (Pica pica) host foster parents when sharing the nest with host nestlings, we tested several predictions of the adaptive digestive efficiency paradigm. A hand-feeding experiment was employed in which we fed both great spotted cuckoo and magpie nestlings with exactly the same diet simulating one food abundance period and one food deprivation period. The results obtained show that cuckoo nestlings ingested more food, gained significantly more weight during the abundance period, and assimilated a higher proportion of the ingested food than magpie nestlings. These results demonstrate for the first time that cuckoo nestlings enjoy digestive adaptations that favour a rapid processing of the ingested food, thereby maximizing their intake rate but without decreasing digestive efficiency. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 280–289.

The post-fledging period is a critical phase for juvenile survival, and parental care provided during this period is a key component of avian reproductive performance. Very little is known about the relationships between foster parents and fledglings of brood parasites. Here, we present the results of a 5-year study about the relationships between fledglings of the non-evictor brood parasitic great spotted cuckoo (Clamator glandarius) and its magpie (Pica pica) foster parents. Sometimes, great spotted cuckoo and magpie nestlings from the same nest can fledge successfully, but most often parasitic nestlings outcompete host nestlings and only cuckoos leave the nest. We have studied several aspects of cuckoo post-fledging performance (i.e. feeding behaviour, parental defence and fledgling survival) in experimental nests in which only cuckoos or both magpie and cuckoo nestlings survived until leaving the nest. The results indicate that great spotted cuckoo fledglings reared in mixed broods together with magpie nestlings were disadvantaged by magpie adults with respect to feeding patterns. Fledgling cuckoos reared in mixed broods were fed less frequently than those reared in only cuckoo broods, and magpie adults approached less frequently to feed cuckoos from mixed broods than cuckoos from only cuckoo broods. These results imply that the presence of host's own nestlings for comparison may be a crucial clue favouring the evolution of fledgling discrimination; and furthermore, that the risk of discrimination at the fledgling stage probably is an important selection pressure driving the evolution of the arms race between brood parasites and their hosts.

Among the most eye-catching traits of avian eggs are their background coloration and pigmentation, consisting in many passerine birds of dark protoporphyrin spots. Although variation in protoporphyrin pigmentation among clutches has been shown to reflect female quality, within-clutch variation in egg pigmentation remains less well understood. Here, we hypothesize that female quality may also be reflected in within-clutch variation in egg pigmentation as a result of energetic constraints and/or increased susceptibility to oxidative stress, and test this hypothesis in a free-living population of Great Tits (Parus major). Within clutches, both pigment ‘darkness’ and ‘spread’ (reflecting intensity, distribution and size of pigment) increased with laying order. For pigment ‘darkness’, this was most strongly so in larger females and in females showing lysis (as a measure of constitutive innate immunity), suggesting that intra-clutch variation in pigment ‘darkness’ positively relates to both structural as well as condition-dependent female traits. In contrast, for pigment ‘spread’, no relationships were detected with body size, body condition, age, and two components of constitutive innate immunity. Among clutches, ‘darkness’ and ‘spread’ of pigments also varied. However, this variation was not related to any of the female characteristics we measured. To the best of our knowledge, this study is the first one to relate intra-clutch variation in protoporphyrin egg pigmentation to structural and condition-dependent traits of laying females. Further experimental study is, however, required to better understand the underlying causal mechanisms.

Environmental characteristics of neighboring locations are generally more similar than those of distant locations. Selection pressures due to parasitism and other environmental conditions shape life history traits of hosts; thus, the probability of parasitism should be associated with the strength of spatial autocorrelation in life history and defensive traits of their hosts. Here we test this hypothesis in three different subpopulations of Magpie (Pica pica) parasitized by the Great Spotted Cuckoo (Clamator glandarius) during three breeding seasons. In some of the years and study plots, we found evidence of positive spatial autocorrelations for clutch size and parasitism rate, but not for laying date. As predicted, brood parasitism was associated with the strength of these spatial autocorrelations. Magpies that bred close to each other in areas of high risk of parasitism responded similarly to experimental parasitic eggs. Moreover, an elevated risk of parasitism eliminated the spatial autocorrelation for clutch size, which became randomly distributed. We discuss possible mechanisms explaining these associations, which may have important consequences for estimating evolutionary responses of hosts to parasitic infections and, therefore, for epidemiological, ecological, and evolutionary studies of host-parasite relationships.

Some avian brood parasitic nestlings are highly virulent, destroying all host eggs or nestmates, while others accept growing up together with host nestmates. The traditional idea was that all brood parasitic nestlings would benefit from being alone in the host nest. Thus, why do nestlings of some brood parasitic species accept the company of host offspring in the nest? The trade-off hypothesis suggests that brood parasites must balance the costs and benefits of killing host young because of two major potential costs: risk of nest desertion and loss of begging assistance. Here, we test this hypothesis in a non-evictor cuckoo species, the great spotted cuckoo Clamator glandarius and its main host, the magpie Pica pica, by manipulating brood size (1–3 nestlings) and brood composition (only cuckoo, only magpie or mixed) during three consecutive breeding seasons. None of the broods were abandoned by host parents, and cuckoo nestlings alone in the nest tended to grow faster (i.e. wing length). Thus, none of the predictions of the two potential costs on which the trade-off hypothesis is based apply to the great spotted cuckoo–magpie system. Our experimental study could not directly test why chick killing has not evolved in great spotted cuckoos, but the results point in the direction of several possibilities. We suggest that chick killing in great spotted cuckoos may not be adaptive mainly because another, less costly strategy (i.e. outcompeting host nestmates for food), is efficient for successful parasitism of magpie hosts.

Parent–offspring conflict theory predicts that offspring desertion would be adaptive for parents when the fitness costs related to invest ment in the current brood exceed the expected fitness benefits. In several seabirds, a fixed parental care period has been observed, and a preprogrammed fixed parental care period could be a general life-history trait evolved because of parent–offspring conflict. A recent study suggested that this could also be the mechanism by which hosts could discriminate against brood parasitic chicks that need longer care periods, by abandoning a brood when the parental care period passed the one typically needed by their own brood (i.e., discrimination without recognition). Here, we experimentally tested if a fixed parental care period also existed in magpies (Pica pica), the primary host of the great spotted cuckoo (Clamator glandarius). None of the experimentally prolonged magpie broods were deserted, and neither were any of the control or shortened broods. These results suggest that a preprogrammed parental care period is not a general trait in birds and support the idea that brood parasitism could be a selective pressure for optimal (short) parental care periods to evolve, but only in species where brood parasitic nestlings need a longer care period than host nestlings (which was not the case for the great spotted cuckoo). Furthermore, nestlings reared by parents with prolonged parental care period showed a slower development. Increasing provisioning costs, reduced sensitiveness to begging signals, or sexual conflict could cause this result, upholding parental care periods as a challenging matter of research.

Environmental stressors have the potential to induce perturbations in the development of young individuals, leading to aberrant and unstable development. This may manifest as fluctuating asymmetry (FA; small, non-directional changes in the bilateral symmetry of morphological traits). Although widely regarded as a proxy for stress effects, the use of FA as a biomarker is still a topic of much debate. We investigated the applicability of FA as an indicator of nutritional stress (brought about by energetic constraints) by experimental manipulation of the diet composition and quantity during the growth of Lesser Black-backed Gull (Larus fuscus) chicks. FA as an endpoint was measured across the tarsus, wing and 10th primary feather when chicks reached 30 days of age. Although levels of asymmetry were found to increase with stress in the feather, relationships with tarsus and wing FA were mixed and mostly non-significant. Furthermore, we did not find any correlations in unsigned FA between traits, indicating the absence of organism-wide asymmetry. Our study was therefore unable to find unequivocal evidence in support of the application of FA as a reliable estimator of nutritional stress.

Brood parasitic nestlings usually exhibit an exaggerated begging behaviour, which is mainly attributed to reduced inclusive fitness costs since they typically share the nest with unrelated individuals. However, energetic costs also constrain begging expression and accordingly a relation between food requirements and intensity of begging behaviour could also exist in brood parasites, just as in nesting bird species. Here, we tested this hypothesis in the great spotted cuckoo Clamator glandarius and its main host, the magpie Pica pica, by studying the effect of an appetite enhancer, cyproheptadine hydrochloride, on nestling provisioning and development (size, body mass and cell-mediated immune response). To study nestling provisioning, neck-collars were meticulously placed around nestling necks allowing normal respiration but avoiding the ingestion of food delivered by adult magpies during ca 2.5 h. Loss in body mass during neck-collar trials was used as a proxy for energetic begging costs, while the amount of food received during these trials and growth during the whole nestling period were used as variables reflecting short- and long-term effects of the experimental treatment. During neck-collar trials, we found that experimental nestlings of both species received more food than control nestlings. However, experimental magpies, but not cuckoos, lost more body mass than control nestlings. These results suggest a short-term beneficial effect of an escalated begging behaviour in both species that would be energetically cheaper for cuckoos than for magpies. We found positive long-term effects of the appetite enhancer only in magpies (in terms of tarsus and wing length at fledging, but not in terms of immune response and body mass); suggesting that exaggerated begging would be beneficial for hosts only. We discuss the possible effect of begging behaviour on the risk of predation and on inclusive fitness, but also the possibility that our results may be explained by some kind of limitation in the capability of food assimilation by parasitic species.

Nestlings of non-evicting avian brood-parasites have to compete for food with foster parents' own nestlings. The outcome of these competitive contests is determined mainly by body size differences between parasitic and host nestlings. As part of the coevolutionary arms race between brood parasites and their hosts at the nestling stage, it has been reported that some host foster parents discriminate against parasitic chicks and are reluctant to feed them. Here, by experimentally creating size-matched broods of different composition (only magpie Pica pica chicks, only great spotted cuckoo Clamator glandarius chicks or mixed broods), we show that great spotted cuckoo chicks starved in 20.2 per cent (17 of 84) of the parasitized magpie nests even in absence of size asymmetries, while in none (0 of 72) of the nests a magpie chick starved. As far as we know, this is the first record of non-evictor brood parasitic nestlings starving without being smaller than their host nestmates in a frequently used host species. Nest composition had no effect on chick starvation. The cuckoo nestling starved even in two of the nests occupied by only one cuckoo chick. Our results could be explained by (1) magpies being reluctant to feed cuckoo chicks; (2) parasitic chicks receiving lower-quality food items or cuckoo nestlings being sensitive to some particular component of the diet (e.g. cereal grains); and (3) the existence of cuckoo chick discrimination ability by magpie foster parents.

Many passerine species lay eggs that are speckled with dark protoporphyrin pigmentation. Because protoporphyrin is mainly derived from the blood, we here formulate and test a new hypothesis that links an increase in anaemia along the laying sequence to within-clutch variation in egg pigmentation. More intense pigmentation is expected if pigments accumulate during enhanced red blood cell production in response to anaemia. Reduced pigmentation is expected if pigments are derived from the degradation of red blood cells that circulate in smaller numbers due to blood loss. To test this hypothesis, we manipulated anaemia in great tit (Parus major) females by infesting the nests with hen fleas (Ceratophyllus gallinae) prior to egg laying. Polychromatophil (i.e., immature red blood cells) percentage, as a measure of blood cell production, was positively correlated with parasite load confirming that female great tits experienced stronger anaemia when infested with haematophagous parasites during egg laying. We found a positive relationship between spot darkness and laying order that weakened under high parasite load. This result suggests that anaemia in females due to blood-sucking parasites led to diminished protoporphyrin from disintegrated red blood cells and hence a decreased deposition of protoporphyrin. However, the overall increase in pigment darkness along the laying sequence suggests that pigments also accumulate by enhanced red blood cell production caused by anaemia due to egg production itself.

It is generally assumed that begging signals provide parents with reliable information on the state of their young, allowing them to decide accordingly how much to invest in their nestlings. Avian brood-parasitic nestlings exhibit begging displays that are highly exaggerated. In most species, they also increase their begging level in relation to their level of hunger, as predicted by models of honest signalling. However, it has been suggested that great spotted cuckoo, Clamator glandarius, nestlings are an exception to this general rule because they spit out food when satiated and continue begging at a high intensity. We tested this assertion by using both natural observations of food delivery by foster parents and a laboratory experiment in which we hand-fed both cuckoo and host magpie, Pica pica, and carrion crow, Corvus corone, nestlings while controlling for the degree of food deprivation. First, we found that cuckoo nestlings did not throw out food because they are satiated. Second, as in host magpie nestlings, during a period of food deprivation the cuckoo nestlings begged at a significantly higher frequency and intensity than when fed ad libitum (a period of food abundance) and begging signals increased with an increased level of hunger in both species. These results indicate that in this cuckoo species the frequency and intensity of begging are in accordance with food requirements, and, thus, that begging is not dishonest in the sense of being unreliable.

Long-term effects of unfavourable conditions during development can be expected to depend on the quality of the environment experienced by the same individuals during adulthood. Yet, in the majority of studies, long-term effects of early developmental conditions have been assessed under favourable adult conditions only. The immune system might be particularly vulnerable to early environmental conditions as its development, maintenance and use are thought to be energetically costly. Here, we studied the interactive effects of favourable and unfavourable conditions during nestling and adult stages on innate immunity (lysis and agglutination scores) of captive male and female zebra finches (Taeniopygia guttata). Nestling environmental conditions were manipulated by a brood size experiment, while a foraging cost treatment was imposed on the same individuals during adulthood. This combined treatment showed that innate immunity of adult zebra finches is affected by their early developmental conditions and varies between both sexes. Lysis scores, but not agglutination scores, were higher in individuals raised in small broods and in males. However, these effects were only present in birds that experienced low foraging costs. This study shows that the quality of the adult environment may shape the long-term consequences of early developmental conditions on innate immunity, as long-term effects of nestling environment were only evident under favourable adult conditions.

Studies of migration have revealed multiple trade-offs with other life-history traits that may underlie observed variation in migratory properties among ages and sexes. To assess whether, and to what extent, body size and ⁄or sex-specific differences in competition for resources (e.g. breeding territories or winter food) may shape variation in migration distance and timing of arrival in ecologically and phylogenetically related species, we combined over 30 000 sightings of individually marked, sexually mature males and females of Herring Gulls Larus argentatus and Lesser Black-Backed Gulls Larus fuscus with biometric measurements and phenological observations at a mixed breeding colony. In L. argentatus, larger males migrated further from the breeding colony, whereas migration distance was independent of body size in adult females. In L. fuscus, no relationship between body size and migration distance was apparent in either sex. Mean arrival dates at the breeding colony did not vary with migration distances but differed between males and females of L. argentatus (but not L. fuscus). As allometry at least partly explains sexual segregation in migration behaviour in L. argentatus, but not in L. fuscus, we conclude that the effect of body size on sex-related migratory strategies may vary between closely related, sympatric species despite similar size dimorphism.

SUMMARY It remains largely unknown which factors affect the innate immune responses of free-living birds. Nevertheless, the degree of innate immunity may play a crucial role in an individual's survival as it procures the first defence against pathogens. We manipulated the ectoparasite load of great tit (Parus major) nests by infesting them with hen fleas (Ceratophyllus gallinae) before egg laying. We subsequently quantified natural antibody (NAb) concentration and complement activation in nestlings and adult females during breeding and post-breeding periods. NAb concentrations increased in nestlings and adult females breeding in flea-infested nest boxes during the nestling provisioning period, but not in breeding females during incubation. In contrast, parasite abundance did not affect levels of complement activity in females. NAb levels of nestlings were already fully developed at the end of the nestling stage, but complement activation was only observed post-fledging. Concentrations of NAbs and complement activation of adult females were significantly lower during the breeding season compared with post-breeding levels, but did not differ between incubation and chick rearing. Further experimental studies in species that vary in life-history strategies will allow us to unravel the mechanisms underlying the observed variation in innate immune defences.

One of the hypotheses to explain egg colour variation in birds lays in the context of sexual selection, where egg colour may signal the female’s physical condition and antioxidant capacity. We tested one of the assumptions following from this hypothesis, that eggshell pigment deposition should be limited for females. The study was conducted in a captive house sparrow (Passer domesticus) population over several years under constant environmental conditions. This multi-brooded species lays eggs which vary in ground colour (biliverdin pigment) and in the intensity and distribution of brownish-red spots (protoporphyrin pigment). Spot darkness, spread and ground colour diminished along the laying sequence, suggesting that the deposition of both pigments was limiting for females over the short term. Also, the proportion of eggs with biliverdin diminished in consecutive clutches laid by the same female over the breeding season, suggesting a long-term cost of biliverdin deposition. On the other hand, spots were darker at the end of the breeding season, indicating that protoporphyrin deposition was probably not limited over the long term. This result could indicate a lower capacity for calcium deposition over the long term, which was compensated for by darker spots. Female age also significantly affected the proportion of bluish eggs and spot patterns. Egg pigmentation decreased with age, indicating that senescing passerine females lay less pigmented eggs. Clutch size was positively related to the proportion of bluish eggs and to spot patterns (darker and more evenly spotted). These results are in accordance with assumptions for the sexual selection hypothesis.

SUMMARY Mothers can strongly influence the development of their offspring, and if maternal resources are limited, they may influence optimal reproductive strategies. In birds, maternally deposited carotenoids are a prominent component of egg yolk and are vital for the development of the embryo. However, results of long-lasting fitness consequences of this early nutritional environment have been scarce and inconsistent. In addition,sex-biased sensitivity to different egg components is one of the mechanisms postulated to account for sex-linked environmental vulnerability during early life. However, this important aspect is usually not accounted for when investigating maternal investment in carotenoids. In this study we gave carotenoid (lutein) supplements to female Eurasian kestrels (Falco tinnunculus) before and during egg laying. The experiment increased female plasma carotenoids, but this effect was not apparent in hatchling and fledgling plasma carotenoid concentration. Also, results showed that carotenoid supplementation increased the high density lipoprotein to low density lipoprotein ratio in adult females, suggesting that dietary carotenoids may influence lipid metabolism. Furthermore, the effect of the treatment was manifested in several nestling health state parameters. Nestlings of carotenoid-supplemented females were infested by less intestinal parasite groups, had higher lymphocyte concentrations in blood plasma, and were less stressed (heterophile to lymphocyte ratio) than control nestlings. In addition, an interaction between the experimental treatment and nestling sex was apparent for globulin concentrations, favouring the smaller male nestlings. Thereby, suggesting that males benefited more than females from an increase in maternal carotenoid investment. Our study shows that an increase in carotenoids in the maternal diet during egg laying favours nestling development in kestrels, and may also affect nestlings in a sex-specific way.

Aggressive behaviour against conspecifics is considered a costly behaviour that forms part of reproductive investment. Previous studies have suggested that the intensity of agonistic behaviour may indicate individual quality since it exhibits a positive correlation with brood value. However, this relationship has been demonstrated after clutch completion, and by creating a nexus between efforts it has not been possible to disentangle other hypotheses that could explain such findings. Here, by presenting a trio of natural decoys (adult male, adult female and 1-year-old male) to Eurasian kestrels, Falco tinnunculus, we tested whether the aggressive behaviour to conspecifics prior to laying predicted posterior reproductive performance (clutch size). We also tested the effect of breeding stage and breeding density on this behaviour. In addition, we examined the effect of the presence of moulted versus unmoulted 1-year-old males on aggressive behaviour of breeders. Intrasexual agonistic behaviour, measured as attack intensity to the decoys, was positively correlated with clutch size in females, but negatively in males. In males, aggressive behaviour was lower during the female fertile stage than during previous stage, but in females, was similar over stages. Furthermore, agonistic behaviour increased with breeding density in females but not in males. We discuss these results in the context of sexual selection theory, and propose that the link between agonistic behaviour prior to laying and clutch size is via the connection to courtship feeding and that may be considered as an indicator of individual quality for both sexes.

Parasite pressure and nutrition are two of the most important factors affecting the trade-off between nestling growth and immune development. During development, energy and nutrients are often limited, and nestlings should only dedicate differentially more valuable resources to their immune system when the associated benefits are high (i.e. in situations of an increased risk of parasitism). In this study, we manipulated nutritional condition of Eurasian Magpie Pica pica nestlings by providing a food supplement. Additionally, the study area was subdivided into two categories of habitat (irrigated and arid) based on the presence of irrigation canals. Nestling diet composition was more varied in the irrigated compared with in the arid habitat. In addition, nestlings of the irrigated habitat showed a significantly higher infestation of both ectoparasites and blood parasites and a significantly higher cell-mediated immunity, but lower tarsus length compared with nestlings of the arid habitat. Food supplementation to nestlings did not affect tarsus length, but increased nestling cell-mediated immunity in the arid habitat only. Based on the recent demonstrated trade-off between growth and immunocompetence in nestlings, we suggest that differences between habitats in nestling diet and parasite prevalence may have caused different priority rules in the allocation of resources between both fitness traits.

Human disturbance can be considered to have similar effects as predation risk for animals. Thus, when disturbed, animal responses are likely to follow the same economic principles used by prey when encountering predators. We simulated predator attacks with different characteristics and in different situations to study the factors that determine the escape response of 1-year-old chinstrap penguins. The results indicate that 1-year-old penguins adjusted their escape behaviour according to the level of risk posed by the researcher acting as a potential predator. When 1-year-old penguins were close to a breeding subcolony, they started to escape later, and fled shorter distances, at lower speeds, and not fleeing directly into the subcolony. This contrasts with their fleeing behaviour far from subcolonies, when penguins fled sooner, for longer, and faster, and in a direction that maximized the distance between themselves and the experimenter, by fleeing directly away from the experimenter. This might suggest the existence of a trade-off between fleeing from the predator and avoiding entering the subcolony where 1-year-old penguins will receive aggressive responses from breeding adults. The type of approach was not important in deciding when to flee. However, penguins did escape for longer distances and faster when approached directly, showing that penguins were able to assess risk level based on predator behaviour. Our findings may have implications for management of penguin colonies visited by tourists. The delimitation of buffer areas and advice on how tourists should behave when approaching penguins might arise from studies of the factors that affect risk assessment of penguins.

Reproductive success in many avian populations declines throughout the breeding season. Two hypotheses have gained attention to explain such a decline: the "timing" hypothesis proposes that deteriorating food availability causes the decline in reproductive success (causal effect of breeding time), whereas the "quality" hypothesis proposes that individuals of lower phenotypic quality reproduce at the end of the breeding season, causing the correlation between breeding time and breeding success. We tested both of these hypotheses in a monogamous single breeder, the magpie Pica pica, by experimentally inducing some pairs to lay a replacement clutch, after removal of the first clutch. The first clutch was left in the nest of another magpie pair (matched by laying date and clutch size), and incubated and raised by these foster parents. In this way we obtained two clutches from the same magpie pair with full siblings raised in conditions of the first and second reproductive attempts. High quality pairs (with laying dates in the first half of the breeding season) reached similar breeding success in replacement clutches as compared to first clutches of the same female. In addition, experimental pairs reared significantly more offspring of similar quality in their replacement clutches as compared to late-season first clutches, thereby suggesting that late season first clutches were produced by pairs of lower phenotypic quality. Results indicate that high quality pairs trade-off clutch size for larger eggs in replacement clutches, which could help magpie pairs to partly compensate for poorer environmental conditions associated with a delayed breeding attempt.

Food availability is an important factor affecting breeding success in birds. Food supplementation experiments in birds have in general focused on the effects on reproductive success in terms of female investment (laying date, clutch size, egg size), however, it is also known that the estimation of mate quality based on sexually selected signals influences female reproductive investment. In the particular case of magpies, females use nest size, a post-mating sexually selected signal, to assess male's likelihood to invest in reproduction, and accordingly adjust reproductive investment (clutch size). Then, the possible effects of food supplementation on female reproductive investment could be mediated by other variables related to parental quality, such as nest size in magpies. In the present study, we explore if higher food availability in a magpie territory affected both male sexually selected traits (i.e. nest size) and female reproductive investment (laying date, egg size, clutch size). We performed a food supplementation experiment in which we experimentally increased food availability in several magpie territories, keeping others as controls. In food-supplemented territories, males built significantly larger nests and females significantly increased egg size by 4.1% compared to control females. Results suggest that the continuous provisioning of protein rich food allowed magpie females to increase egg size. However, laying date and clutch size did not differ between control and food-supplemented magpie pairs. Food availability also affected the relationship between female reproductive investment and nest size. In control territories, females decreased their egg size in response to a larger nest, whereas a tendency for the opposite relationship was revealed in food-supplemented territories. We discuss the possibility that magpie females adopt different strategies for reproductive investment according to food availability.

Post-mating sexually selected signals are expected to indicate parental quality. The good parent model assumes that expression of the sexual character positively reflects parental ability, resulting in a potential link between the exaggeration of the character and nestling-fitness traits. We tested this prediction in a population of a monogamous passerine, the magpie (Pica pica), for which nest size is known to act as a post-mating sexually selected signal. We provided a food supplement to half of the magpie nestlings in each nest, keeping the other half as control nestlings. We found that food-supplemented nestlings experienced a significantly higher T-cell-mediated immune response and a tendency to an increased condition index. In accordance with the good parent model, we found that nest size was positively related to T-cell mediated immune response for control magpie, whereas this relationship was nonexistent in food-supplemented nestlings. In addition, the difference in T-cell mediated immune response between food-supplemented and control nestlings of the same nest was principally explained by nest size. Based on our results, we discuss that magpie pairs with large nests provided their nestlings with higher quality food as compared to pairs with smaller nests, nest size thereby being an indicator of parental ability. To our knowledge, this is the first study showing a link between a post-mating sexually selected signal and nestling immunocompetence, a trait closely related to fitness in birds. Copyright 2004.

A trade-off between immunity and growth has repeatedly been suggested, mainly based on laboratory and poultry science, but also from experiments where parasitism intensity was manipulated in field bird populations. However, as resource allocation to different activities (or organs) during growth is difficult to manipulate, this trade-off has only been experimentally tested by studying the effects of non-pathogenic antigens. By providing some nestling magpies (Pica pica) with methionine, a sulphur amino acid that specifically enhances T-cell immune response in chickens, we investigated this trade-off by directly affecting allocation of limited resources during growth. Results were in accordance with the hypothetical trade-off because nestlings fed with methionine showed a lower growth rate during the four days of methionine administration, but a larger response when fledglings were challenged with phytohaemagglutinin (a measure of the intensity of T-lymphocyte-mediated immune responsiveness) than control nestlings. Surprisingly, we found that control and experimental nestlings fledged with similar body mass, size and condition, but experimental nestlings suffered less from blood parasites (Haemoproteus) and had fewer lymphocytes (a widely used measure of health status) than control nestlings, suggesting a negative effect of blood parasites or other pathogens on nestling growth.

Nest size or nest-building activity has recently been hypothesized to be a postmating sexually selected signal in monogamous birds: females may assess a male’s parental quality and willingness to invest in reproduction by his participation in nest building. Females may thus adjust their reproductive effort (i.e. clutch size) not only to their own abilities but also to those of their mates. We investigated whether female magpies, Pica pica, use nest-building activity rather than nest size to adjust their reproductive effort during replacement breeding attempts. After we removed their first clutch, high-quality pairs that built a large nest for the first clutch were more capable of building a replacement nest and females adjusted their clutch size in relation to the time it took to build the nest rather than nest size. We also found support for the hypothesized trade-off between clutch size and egg size in magpies. In replacement clutches females decreased clutch size and increased egg volume, thereby probably improving the survival probability of their offspring in less favourable conditions.  2002 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour

Natural selection penalizes individuals that provide costly parental care to non-relatives. However, feedings to brood-parasitic fledglings by individuals other than their foster parents, although anecdotic, have been commonly observed, also in the great spotted cuckoo (Clamator glandarius) – magpie (Pica pica) system, but this behaviour has never been studied in depth. In a first experiment, we here show that great spotted cuckoo fledglings that were translocated to a distant territory managed to survive. This implies that obtaining food from foreign magpies is a frequent and efficient strategy used by great spotted cuckoo fledglings. A second experiment, in which we presented a stuffed-cuckoo fledgling in magpie territories, showed that adult magpies caring for magpie fledglings responded aggressively in most of the trials and never tried to feed the stuffed cuckoo, whereas magpies that were caring for cuckoo fledglings reacted rarely with aggressive behavior and were sometimes disposed to feed the stuffed cuckoo. In a third experiment we observed feedings to post-fledgling cuckoos by marked adult magpies belonging to four different possibilities with respect to breeding status (i.e. composition of the brood: only cuckoos, only magpies, mixed, or failed breeding attempt). All non-parental feeding events to cuckoos were provided by magpies that were caring only for cuckoo fledglings. These results strongly support the conclusion that cuckoo fledglings that abandon their foster parents get fed by other adult magpies that are currently caring for other cuckoo fledglings. These findings are crucial to understand the co-evolutionary arms race between brood parasites and their hosts because they show that the presence of the host's own nestlings for comparison is likely a key clue to favour the evolution of fledgling discrimination and provide new insights on several relevant points such as learning mechanisms and multiparasitism., This work was supported by the Spanish Ministerio de Economía y Competitividad/FEDER (research project CGL2011-25634/BOS).

Nest size has been suggested to be a sexually selected trait indicating parental ability of both males and females. To test whether a female’s reproductive decisions (e.g., clutch size and starting incubation) change in relation to experimental manipulation of nest size, as would be predicted if nest size is a sexually selected signal reflecting the male’s parental quality, we manipulated nest size in a population of monogamous magpies before laying by adding or removing about 20 cm of large sticks in the roof of magpie nests. On the one hand, we found that clutch size of reduced nests was smaller than that of control or enlarged nests. Moreover, clutch size was significantly related to nest size after manipulation, which indicates that females adjust clutch size to the final size of the nest, nest size thereby being a good candidate for a sexually selected trait. On the other hand, number of eggs hatched during the first day is hypothesized to be related to the expected available resources during nestling growth, and subsequent nestlings hatched are likely to die due to brood reduction if resources are not sufficient to raise welldeveloped nestlings. Nest size is hypothesized to inform females about a male’s willingness to invest in reproduction, and we found that in broods of experimentally reduced nests, females started to incubate earlier in the laying sequence than they did in broods of control or enlarged nests. Moreover, in experimentally reduced nests, fewer nestlings hatched during the first day, and the difference in body mass between the first and the fourth nestling hatched increased. This result is in accordance with the hypothesis that the female’s decision of when to start incubation in the laying sequence is mediated by nest size, a sexually selected trait signaling parental quality. We discuss alternative explanations for the results such as the possibility that nests of different treatments may differ in their thermoregulation properties or in their protection against predators. Key words: clutch size, female decision making, magpies, nest building, parental investment, Pica pica, sexual selection. [Behav Ecol 12:301–307 (2001)]