Plasticity in laying dates of Canada Geese in response to spring phenology

Observed phenological changes can be explained either by individual phenotypic plasticity or by evolutionary changes, but there is more evidence pointing towards phenotypic plasticity to explain the mechanism behind changes in bird phenology. However, most studies on phenology have been conducted on insectivorous bird species for which breeding is closely tied to temperature and insect emergence. In this study, we examined the consequences of climatic conditions on the nesting phenology of temperate breeding Canada Geese Branta canadensis maxima, which rely on a continuous food supply, during a 14‐year period (2003–16). We determined whether laying dates were plastically adjusted to spring environmental conditions, and whether this adjustment resulted in a laying date advancement. We further estimated the strength and shape of selection acting on breeding timing, by looking at the effect of laying date on the relative number of young successfully hatched in a nest. We found that Geese plastically adjusted their laying date to spring maximum temperature (and not to precipitation or ice break‐up), resulting in a 9‐day advancement of laying date in the population for that period. Laying date was also moderately repeatable (r = 0.23) and subject to directional selection, but stabilizing selection was negligible. We thus demonstrate how Canada Geese plastically adjust laying dates to temperature, which may further be beneficial to nesting success. Evolutionary change of laying date to selection related to climate change, however, is still possible.     

Variation in phenotypic plasticity and selection patterns in blue tit breeding time: between- and within-population comparisons

1.?Phenotypic plasticity, the response of individual phenotypes to their environment, can allow organisms to cope with spatio-temporal variation in environmental conditions. Recent studies have shown that variation exists among individuals in their capacity to adjust their traits to environmental changes and that this individual plasticity can be under strong selection. Yet, little is known on the extent and ultimate causes of variation between populations and individuals in plasticity patterns. 2.?In passerines, timing of breeding is a key life-history trait strongly related to fitness and is known to vary with the environment, but few studies have investigated the within-species variation in individual plasticity. 3.?Here, we studied between- and within-population variation in breeding time, phenotypic plasticity and selection patterns for this trait in four Mediterranean populations of blue tits (Cyanistes caeruleus) breeding in habitats varying in structure and quality. 4.?Although there was no significant warming over the course of the study, we found evidence for earlier onset of breeding in warmer years in all populations, with reduced plasticity in the less predictable environment. In two of four populations, there was significant inter-individual variation in plasticity for laying date. Interestingly, selection for earlier laying date was significant only in populations where there was no inter-individual differences in plasticity. 5.?Our results show that generalization of plasticity patterns among populations of the same species might be challenging even at a small spatial scale and that the amount of within-individual variation in phenotypic plasticity may be linked to selective pressures acting on these phenotypic traits.     

Natural selection and inheritance of breeding time and clutch size in the collared flycatcher

Many characteristics of organisms in free-living populations appear to be under directional selection, possess additive genetic variance, and yet show no evolutionary response to selection. Avian breeding time and clutch size are often-cited examples of such characters. We report analyses of inheritance of, and selection on, these traits in a long-term study of a wild population of the collared flycatcher Ficedula albicollis. We used mixed model analysis with REML estimation ("animal models") to make full use of the information in complex multigenerational pedigrees. Heritability of laying date, but not clutch size, was lower than that estimated previously using parent-offspring regressions, although for both traits there was evidence of substantial additive genetic variance (h2 = 0.19 and 0.29, respectively). Laying date and clutch size were negatively genetically correlated (rA = -0.41 +/- 0.09), implying that selection on one of the traits would cause a correlated response in the other, but there was little evidence to suggest that evolution of either trait would be constrained by correlations with other phenotypic characters. Analysis of selection on these traits in females revealed consistent strong directional fecundity selection for earlier breeding at the level of the phenotype (beta = -0.28 +/- 0.03), but little evidence for stabilising selection on breeding time. We found no evidence that clutch size was independently under selection. Analysis of fecundity selection on breeding values for laying date, estimated from an animal model, indicated that selection acts directly on additive genetic variance underlying breeding time (beta = -0.20 +/- 0.04), but not on clutch size (beta = 0.03 +/- 0.05). In contrast, selection on laying date via adult female survival fluctuated in sign between years, and was opposite in sign for selection on phenotypes (negative) and breeding values (positive). Our data thus suggest that any evolutionary response to selection on laying date is partially constrained by underlying life-history trade-offs, and illustrate the difficulties in using purely phenotypic measures and incomplete fitness estimates to assess evolution of life-history trade-offs. We discuss some of the difficulties associated with understanding the evolution of laying date and clutch size in natural populations.     

Selection and evolutionary potential of spring arrival phenology in males and females of a migratory songbird

The timing of annual life‐history events affects survival and reproduction of all organisms. A changing environment can perturb phenological adaptations and an important question is if populations can evolve fast enough to track the environmental changes. Yet, little is known about selection and evolutionary potential of traits determining the timing of crucial annual events. Migratory species, which travel between different climatic regions, are particularly affected by global environmental changes. To increase our understanding of evolutionary potential and selection of timing traits, we investigated the quantitative genetics of arrival date at the breeding ground using a multigenerational pedigree of a natural great reed warbler (Acrocephalus arundinaceus) population. We found significant heritability of 16.4% for arrival date and directional selection for earlier arrival in both sexes acting through reproductive success, but not through lifespan. Mean arrival date advanced with 6 days over 20 years, which is in exact accordance with our predicted evolutionary response based on the breeder's equation. However, this phenotypic change is unlikely to be caused by microevolution, because selection seems mainly to act on the nongenetic component of the trait. Furthermore, demographical changes could also not account for the advancing arrival date. Instead, a strong correlation between spring temperatures and population mean arrival date suggests that phenotypic plasticity best explains the advancement of arrival date in our study population. Our study dissects the evolutionary and environmental forces that shape timing traits and thereby increases knowledge of how populations cope with rapidly changing environments.     

Natural selection and genetic variation for reproductive reaction norms in a wild bird population

Many morphological and life-history traits show phenotypic plasticity that can be described by reaction norms, but few studies have attempted individual-level analyses of reaction norms in the wild. We analyzed variation in individual reaction norms between laying date and three climatic variables (local temperature, local rainfall, and North Atlantic Oscillation) of 1126 female collared flycatchers (Ficedula albicollis) with a restricted maximum likehood linear mixed model approach using random-effect best linear unbiased predictor estimates for the elevation (i.e., expected laying date in the average environment) and slope (i.e., adjustment in laying date as a function of environment) of females' reaction norms. Variation in laying date was best explained by local temperature, and individual females differed in both the elevation and the slope of their laying date-temperature reaction norms. As revealed by animal model analyses, there was weak evidence for additive genetic variance of elevation (h2 +/- SE = 0.09 +/- 0.09), whereas there was no evidence for heritability of slope (h2 +/- SE = 0.00 +/- 0.01). Selection analysis, using a female's lifetime production of fledglings or recruits as an estimate of her fitness, revealed significant selection for a lower phenotypic value and breeding value for elevation (i.e., earlier laying date at the average temperature). There was selection for steeper phenotypic values of slope (i.e., greater plasticity in the adjustment of laying date to temperature), but no significant selection on the breeding values of slope. Although these results suggest that phenotypic laying date is influenced by additive genetic factors, as well as by an interaction with the environment, selection on plasticity would not produce an evolutionary response.     

Migration and wintering strategies in vulnerable Mediterranean Osprey populations

A broad range of migration strategies exist in avian species, and different strategies can occur in different populations of the same species. For the breeding Osprey Pandion haliaetus populations of the Mediterranean, sporadic observations of ringed birds collected in the past suggested variations in migratory and wintering behaviour. We used GPS tracking data from 41 individuals from Corsica, the Balearic Islands and continental Italy to perform the first detailed analysis of the migratory and wintering strategies of these Osprey populations. Ospreys showed heterogeneous migratory behaviour, with 73% of the individuals migrating and the remaining 27% staying all year round at breeding sites. For migratory individuals, an extremely short duration of migration (5.2 ± 2.6 days) was recorded. Mediterranean Ospreys were able to perform long non‐stop flights over the open sea, sometimes overnight. They also performed pre‐ and post‐migratory trips to secondary sites, before or after crossing the sea during both autumn and spring migration. Ospreys spent the winter at temperate latitudes and showed high plasticity in habitat selection, using marine bays, coastal lagoons/marshland and inland freshwater sites along the coasts of different countries of the Mediterranean basin. Movements and home‐range areas were restricted during the wintering season. The short duration of trips and high levels of variability in migratory routes and wintering grounds revealed high behavioural plasticity among individuals, probably promoted by the relatively low seasonal variability in ecological conditions throughout the year in the Mediterranean region, and weak competition for non‐breeding sites. We stress the importance of considering the diversity in migration strategies and the particular ecology of these vulnerable populations, especially in relation to proactive management measures for the species at the scale of the Mediterranean region.     

Small‐scale spatial variation in evolvability for life‐history traits in the storm petrel

The evolutionary potential in the timing of recruitment and reproduction may be crucial for the ability of populations to buffer against environmental changes, allowing them to avoid unfavourable breeding conditions. The evolution of a trait in a local population is determined by its heritability and selection. In the present study, we performed pedigree‐based quantitative genetic analyses for two life‐history traits (recruiting age and laying date) using population data of the storm petrel over an 18‐year period in two adjacent breeding colonies (only 150 m apart) that share the same environmental conditions. In both traits, natal colony effect was the main source of the phenotypic variation among individuals, and cohort variance for recruitment age and additive genetic variance for laying date were natal colony‐specific. We found significant heritability only in laying date and, more specifically, only in birds born in one of the colonies. The difference in genetic variance between the colonies was statistically significant. Interestingly, selection on earlier breeding birds was detected only in the colony in which heritable variation in laying date was found. Therefore, local evolvability for a life‐history trait may vary within a unexpectedly small spatial scale, through the diversifying natural selection and insulating gene flow. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 439–446.     

Climatic effects on arrival and laying dates in a long-distance migrant, the Collared Flycatcher Ficedula albicollis

Long‐distance migrants may respond to climate change in breeding, wintering or staging area by changing their phenology. The geographical variation in such responses (e.g. coastal vs. continental Europe) and the relative importance of climate at different spatial scales remain unclear. Here we analysed variation in first arrival dates (FADs) and laying dates of the Collared Flycatcher Ficedula albicollis in a central European population, from 1973 to 2002. The North Atlantic Oscillation (NAO) index correlated weakly with local temperature during the laying period. Decreasing spring temperatures until 1980 were associated with a trend towards later laying. The rate of warming (0.2 °C per year) and laying advancement (0.4 days per year) since 1980 are amongst the highest values reported elsewhere. This long‐term trend in laying date was largely explained by the change in climatic factors. The negative effect of local spring temperature on laying was relatively stronger than that of NAO. The number of clutches initiated on a particular day was marginally affected by the temperature 3 days prior to laying and the response of females to daily variation in temperature did not change over years. Correspondence between the average population‐level and the individual‐level responses of laying date to climate variation suggests that the advancement of laying was due to phenotypic plasticity. Despite warmer springs and advanced laying, FADs did not change over years and were not correlated with local spring temperature. Marginal evidence suggests later departure from wintering grounds and faster migration across staging areas in warmer conditions. Advancement of arrival was probably constrained by low local temperatures in early spring just before arrival that have not changed over years. The interval between first arrival and laying has declined since 1980 (0.5 days per year), but the increasing temperature during that period may have kept the food supply approximately unchanged.     

Intrinsic determinants of a population trend in timing of breeding in the wandering albatross

Numerous studies of wild animal species have documented that population level responses to environmental change are underpinned by individual level phenotypic plasticity. However, where the relationship between an individual trait and a climate variable occurs when both show a trend over time, phenotypic plasticity may be confounded by ageing. We investigated between and within individual change in laying date in the wandering albatross Diomedea exulans, a long‐lived species experiencing a dramatic decline in population size. Laying date has advanced over the last three decades. A mean‐centering analysis demonstrated that this pattern was driven by within‐individual changes as opposed to appearance or disappearance of phenotypes. Furthermore, a lack of between individual effect suggested the change resulted from ageing as opposed to phenotypic plasticity. Females varied significantly in rate of advance, such that those with low past reproductive rates exhibited a negative temporal trend in laying date, whereas birds with moderate to high past reproductive performance showed little change. The population trend was therefore driven by a subset with low past breeding success. An analysis of effects of timing of breeding on breeding success revealed stabilizing selection for relative laying date. Furthermore, current breeding success was positively related to past success rate, which suggests that there may be indirect selection against plasticity in this population. Our results show that population trends can arise from individual level change unrelated to prevailing environmental conditions, thus demonstrating the importance of longitudinal analyses in the interpretation of climate change effects.     

What prevents phenological adjustment to climate change in migrant bird species? Evidence against the “arrival constraint” hypothesis

Phenological studies have demonstrated changes in the timing of seasonal events across multiple taxonomic groups as the climate warms. Some northern European migrant bird populations, however, show little or no significant change in breeding phenology, resulting in synchrony with key food sources becoming mismatched. This phenological inertia has often been ascribed to migration constraints (i.e. arrival date at breeding grounds preventing earlier laying). This has been based primarily on research in The Netherlands and Germany where time between arrival and breeding is short (often as few as 9 days). Here, we test the arrival constraint hypothesis over a 15-year period for a U.K. pied flycatcher (Ficedula hypoleuca) population where laying date is not constrained by arrival as the period between arrival and breeding is substantial and consistent (average 27 ± 4.57 days SD). Despite increasing spring temperatures and quantifiably stronger selection for early laying on the basis of number of offspring to fledge, we found no significant change in breeding phenology, in contrast with co-occurring resident blue tits (Cyanistes caeruleus). We discuss possible non-migratory constraints on phenological adjustment, including limitations on plasticity, genetic constraints and competition, as well as the possibility of counter-selection pressures relating to adult survival, longevity or future reproductive success. We propose that such factors need to be considered in conjunction with the arrival constraint hypothesis.     

Gene expression plasticity evolves in response to colonization of freshwater lakes in threespine stickleback

Phenotypic plasticity is predicted to facilitate individual survival and/or evolve in response to novel environments. Plasticity that facilitates survival should both permit colonization and act as a buffer against further evolution, with contemporary and derived forms predicted to be similarly plastic for a suite of traits. On the other hand, given the importance of plasticity in maintaining internal homeostasis, derived populations that encounter greater environmental heterogeneity should evolve greater plasticity. We tested the evolutionary significance of phenotypic plasticity in coastal British Columbian postglacial populations of threespine stickleback (Gasterosteus aculeatus) that evolved under greater seasonal extremes in temperature after invading freshwater lakes from the sea. Two ancestral (contemporary marine) and two derived (contemporary freshwater) populations of stickleback were raised near their thermal tolerance extremes, 7 and 22 °C. Gene expression plasticity was estimated for more than 14 000 genes. Over five thousand genes were similarly plastic in marine and freshwater stickleback, but freshwater populations exhibited significantly more genes with plastic expression than marine populations. Furthermore, several of the loci shown to exhibit gene expression plasticity have been previously implicated in the adaptive evolution of freshwater populations, including a gene involved in mitochondrial regulation (PPARAa). Collectively, these data provide molecular evidence that highlights the importance of plasticity in colonization and adaptation to new environments.     

Disentangling plastic and genetic changes in body mass of Siberian jays

Spatial and temporal phenotypic differentiation in mean body size is of commonplace occurrence, but the underlying causes remain often unclear: both genetic differentiation in response to selection (or drift) and environmentally induced plasticity can create similar phenotypic patterns. Studying changes in body mass in Siberian jays (Perisoreus infaustus) over three decades, we discovered that mean body mass declined drastically (ca. 10%) over the first two decades, but increased markedly thereafter back to almost the initial level. Quantitative genetic analyses revealed that although body mass was heritable (h² = 0.46), the pronounced temporal decrease in body mass was mainly a product of phenotypic plasticity. However, a concomitant and statistically significant decrease in predicted breeding values suggests a genetic component to this change. The subsequent increase in mean body mass was indicated to be entirely due to plasticity. Selection on body mass was estimated to be too weak to fully account for the observed genetic decline in body mass, but bias in selection differential estimates due to environmental covariance between body mass and fitness is possible. Hence, the observed body mass changes appear to be driven mainly by phenotypic plasticity. Although we were not able to identify the ecological driver of the observed plastic changes, the results highlight the utility of quantitative genetic approaches in disentangling genetic and phenotypic changes in natural populations.     

CONTRASTING PATTERNS OF PHENOTYPIC PLASTICITY IN REPRODUCTIVE TRAITS IN TWO GREAT TIT (PARUS MAJOR) POPULATIONS

Phenotypic plasticity is an important mechanism via which populations can respond to changing environmental conditions, but we know very little about how natural populations vary with respect to plasticity. Here we use random‐regression animal models to understand the multivariate phenotypic and genetic patterns of plasticity variation in two key life‐history traits, laying date and clutch size, using data from long‐term studies of great tits in The Netherlands (Hoge Veluwe [HV]) and UK (Wytham Woods [WW]). We show that, while population‐level responses of laying date and clutch size to temperature were similar in the two populations, between‐individual variation in plasticity differed markedly. Both populations showed significant variation in phenotypic plasticity (IxE) for laying date, but IxE was significantly higher in HV than in WW. There were no significant genotype‐by‐environment interactions (GxE) for laying date, yet differences in GxE were marginally nonsignificant between HV and WW. For clutch size, we only found significant IxE and GxE in WW but no significant difference between populations. From a multivariate perspective, plasticity in laying date was not correlated with plasticity in clutch size in either population. Our results suggest that generalizations about the form and cause of any response to changing environmental conditions across populations may be difficult.     

Phenotypic variability can promote the evolution of adaptive plasticity by reducing the stringency of natural selection

Adaptive phenotypic plasticity is a potent but not ubiquitous solution to environmental heterogeneity, driving interest in what factors promote and limit its evolution. Here, a novel computational model representing stochastic information flow in development is used to explore evolution from a constitutive phenotype to an adaptively plastic response. Results show that populations tend to evolve robustness to developmental stochasticity, but that this evolved robustness limits evolvability; specifically, robust genotypes have less ability to evolve adaptive plasticity when presented with a mix of both the ancestral environment and a new environment. Analytic calculations and computational experiments confirm that this constraint occurs when the initial mutational steps towards plasticity are pleiotropic, such that mutant fitnesses decline in the environment to which their parents are well‐adapted. Greater phenotypic variability improves evolvability in the model by lessening this decline as well as by improving the fitness of partial adaptations to the new environment. By making initial plastic mutations more palatable to natural selection, phenotypic variability can increase the evolvability of an innovative, plastic response without improving evolvability to simpler challenges such as a shifted optimum in a single environment. Populations that evolved robustness by negative feedback between the trait and its rate of change show a particularly strong constraining effect on the evolvability of plasticity, revealing another mechanism by which evolutionary history can limit later innovation. These results document a novel mechanism by which weakening selection could actually stimulate the evolution of a major innovation.     

Phenological mismatch drives selection on elevation,but not on slope,of breeding time plasticity in a wild songbird

Phenotypic plasticity is an important mechanism for populations to respond to fluctuating environments, yet may be insufficient to adapt to a directionally changing environment. To study whether plasticity can evolve under current climate change, we quantified selection and genetic variation in both the elevation (RN_E) and slope (RN_S) of the breeding time reaction norm in a long‐term (1973–2016) study population of great tits (Parus major). The optimal RN_E (the caterpillar biomass peak date regressed against the temperature used as cue by great tits) changed over time, whereas the optimal RN_S did not. Concordantly, we found strong directional selection on RN_E, but not RN_S, of egg‐laying date in the second third of the study period; this selection subsequently waned, potentially due to increased between‐year variability in optimal laying dates. We found individual and additive genetic variation in RN_E but, contrary to previous studies on our population, not in RN_S. The predicted and observed evolutionary change in RN_E was, however, marginal, due to low heritability and the sex limitation of laying date. We conclude that adaptation to climate change can only occur via micro‐evolution of RN_E, but this will necessarily be slow and potentially hampered by increased variability in phenotypic optima.     

Will phenotypic plasticity affecting flowering phenology keep pace with climate change?

Rising temperatures have begun to shift flowering time, but it is unclear whether phenotypic plasticity can accommodate projected temperature change for this century. Evaluating clines in phenological traits and the extent and variation in plasticity can provide key information on assessing risk of maladaptation and developing strategies to mitigate climate change. In this study, flower phenology was examined in 52 populations of big sagebrush (Artemisia tridentata) growing in three common gardens. Flowering date (anthesis) varied 91 days from late July to late November among gardens. Mixed‐effects modeling explained 79% of variation in flowering date, of which 46% could be assigned to plasticity and genetic variation in plasticity and 33% to genetics (conditional R² = 0.79, marginal R² = 0.33). Two environmental variables that explained the genetic variation were photoperiod and the onset of spring, the Julian date of accumulating degree‐days >5 °C reaching 100. The genetic variation was mapped for contemporary and future climates (decades 2060 and 2090), showing flower date change varies considerably across the landscape. Plasticity was estimated to accommodate, on average, a ±13‐day change in flowering date. However, the examination of genetic variation in plasticity suggests that the magnitude of plasticity could be affected by variation in the sensitivity to photoperiod and temperature. In a warmer common garden, lower‐latitude populations have greater plasticity (+16 days) compared to higher‐latitude populations (+10 days). Mapped climatypes of flowering date for contemporary and future climates illustrate the wide breadth of plasticity and large geographic overlap. Our research highlights the importance of integrating information on genetic variation, phenotypic plasticity and climatic niche modeling to evaluate plant responses and elucidate vulnerabilities to climate change.     

On the fate of seasonally plastic traits in a rainforest butterfly under relaxed selection

Many organisms display phenotypic plasticity as adaptation to seasonal environmental fluctuations. Often, such seasonal responses entails plasticity of a whole suite of morphological and life‐history traits that together contribute to the adaptive phenotypes in the alternative environments. While phenotypic plasticity in general is a well‐studied phenomenon, little is known about the evolutionary fate of plastic responses if natural selection on plasticity is relaxed. Here, we study whether the presumed ancestral seasonal plasticity of the rainforest butterfly Bicyclus sanaos (Fabricius, 1793) is still retained despite the fact that this species inhabits an environmentally stable habitat. Being exposed to an atypical range of temperatures in the laboratory revealed hidden reaction norms for several traits, including wing pattern. In contrast, reproductive body allocation has lost the plastic response. In the savannah butterfly, B. anynana (Butler, 1879), these traits show strong developmental plasticity as an adaptation to the contrasting environments of its seasonal habitat and they are coordinated via a common developmental hormonal system. Our results for B. sanaos indicate that such integration of plastic traits – as a result of past selection on expressing a coordinated environmental response – can be broken when the optimal reaction norms for those traits diverge in a new environment.     

Potential local adaptation in populations of invasive reed canary grass (Phalaris arundinacea) across an urbanization gradient

Urban stressors represent strong selective gradients that can elicit evolutionary change, especially in non‐native species that may harbor substantial within‐population variability. To test whether urban stressors drive phenotypic differentiation and influence local adaptation, we compared stress responses of populations of a ubiquitous invader, reed canary grass (Phalaris arundinacea). Specifically, we quantified responses to salt, copper, and zinc additions by reed canary grass collected from four populations spanning an urbanization gradient (natural, rural, moderate urban, and intense urban). We measured ten phenotypic traits and trait plasticities, because reed canary grass is known to be highly plastic and because plasticity may enhance invasion success. We tested the following hypotheses: (a) Source populations vary systematically in their stress response, with the intense urban population least sensitive and the natural population most sensitive, and (b) plastic responses are adaptive under stressful conditions. We found clear trait variation among populations, with the greatest divergence in traits and trait plasticities between the natural and intense urban populations. The intense urban population showed stress tolerator characteristics for resource acquisition traits including leaf dry matter content and specific root length. Trait plasticity varied among populations for over half the traits measured, highlighting that plasticity differences were as common as trait differences. Plasticity in root mass ratio and specific root length were adaptive in some contexts, suggesting that natural selection by anthropogenic stressors may have contributed to root trait differences. Reed canary grass populations in highly urbanized wetlands may therefore be evolving enhanced tolerance to urban stressors, suggesting a mechanism by which invasive species may proliferate across urban wetland systems generally.     

Individual changes underlie age-specific pattern of laying date and egg-size in female common terns Sterna hirundo

It has been suggested that breeding performance differs between young and old birds due to the appearance and disappearance of phenotypes through differential survival (selection hypothesis) or differential recruitment (delayed breeding hypothesis) of high-quality individuals, but each bird may show constant breeding performance over its life. We tested constant egg-volume and laying date by modelling their variability on the basis of the 109 known-age females of common tern Sterna hirundo with data available from 1 to 9 years. Longitudinal analyses showed a significant advancement of laying date, as well as a steady increase in egg-volume, in young age classes from 2 to 5–7 years old, indicating individual intrinsic changes in performance with age. In our model, female effect accounted for 74% and 8% of variance in egg-volume and laying date, respectively, suggesting that if correlation between breeding performance and survival or recruitment exists, population patterns of age-specific performance may emerge. However, we found no evidence that birds that did not return to breed during young age classes laid later or smaller eggs than returned breeders. Likewise, we found no evidence that recruiting birds laid earlier or larger eggs than same aged birds recruited in preceding years. Thus, this study shows that age-specific patterns in timing of breeding and egg-size in common terns result from individuals intrinsic changes, and we reject the selection and the delayed breeding hypotheses as a major factor shaping age-specific patterns at population level.Communicated by F. Bairlein