Environmental conditions experienced early in life can shape growth, morphology and survival, and can have lasting effects on individuals that survive to adulthood. Understanding how early-life conditions affect fitness components across the life cycle is therefore essential for predicting variation in life-history trajectories and their ecological and evolutionary consequences. We experimentally tested for short- and long-term effects of early growth conditions by manipulating brood size in a wild population of the long-lived Alpine swift Tachymarptis melba. Enlarging or reducing brood size alters sibling competition and per capita resource availability, with potential consequences on resource allocation between growth and self-maintenance. We manipulated 45 pairs of broods between 2003 and 2006 and followed the offspring throughout their lives until 2019. Brood enlargement reduced average growth and survival, while increasing phenotypic variability among nestlings. Nestlings from enlarged broods were smaller, fledged later, and had lower survival to fledging and recruitment. Recruitment probability increased with nestling size, indicating strong selection on developmental phenotype. Consequently, nestlings from enlarged broods that recruited were a non-random subset of bigger, higher-quality individuals, and morphological differences between treatments disappeared among adults. Despite lower recruitment, individuals from enlarged broods showed equal or higher lifetime reproductive output, primarily through more frequent breeding attempts. Age-specific analyses showed that reproductive performance patterns reflected both within-individual changes and selective disappearance among individuals. We discuss how early-life conditions shape life-history trajectories through both developmental effects and selection acting before adulthood, arguing that apparent advantages in adult performance among individuals from harsher developmental environments arise from viability selection rather than beneficial effects of early-life stress. Our findings show how missing-fraction problems can bias inferences about fitness when early-life mortality is ignored, highlighting the need to integrate early survival when evaluating the lifelong consequences of developmental environments.

The ecological and selective processes acting on cognitive traits have received recent attention because of the potentially key role of cognition in rapid behavioural adjustments in response to environmental variation. Innovation, a cognitive process involved in the development of new behaviour, has been linked to fitness in several species but the proximate mechanisms explaining such links are still unknown. We investigated whether problem-solving performance, a proxy of innovation ability, predicts provisioning performance (provisioning rate, prey number, size and type) and used path analyses to explore whether problem-solving was related to reproductive success (number of nestlings, nestling survival) through provisioning performance in 107 female and 92 male great tits (Parus major). Solvers delivered prey at a higher rate and brought a higher estimated prey biomass to their nestlings than non-solvers. Path analyses confirmed this link in both sexes, and showed that, in turn, higher provisioning rate underlay higher reproductive success but only in males. Our study highlights the role of problem-solving performance in provisioning performance as a potential underlying mechanism linking innovation to reproductive success, and that this mechanism could be sex dependent. These results support the hypothesis of the adaptive value of problem-solving ability in the context of foraging ecology.

Polar and sub-polar animals evolved to thrive in cold climates and may thus be particularly sensitive to rising temperatures associated with climate change. Penguins may be especially vulnerable, due to their dual habitat, alternating between foraging in cold waters and breeding/moulting on an increasingly warm land. Here, we characterized heat stress occurrence in breeding king penguins through behavioural observations (e.g. panting occurrence) and body temperature measurements. We observed that behavioural signs of heat stress are frequent in king penguins breeding in the sub-Antarctic region (> 20% of observations at mid-day), and that subcutaneous temperatures increase under high heat load, especially in penguins observed panting. Subcutaneous and core body temperatures were moderately correlated and both increased with heat load. Yet, their responses were not parallel since core body temperature is markedly less sensitive to heat load than subcutaneous temperature. Air temperature alone was a poor predictor of heat stress occurrence, whereas the combination of high solar radiation, low wind speed and high air temperatures provided the strongest predictive power. Finally, reproductive failures were more likely to occur on warmer days, suggesting that heat stress may have significant sublethal effects on adults that could ultimately affect population dynamics.

It has been suggested that a high aerial lifestyle makes it difficult for males to limit mating opportunities for their partners. This would explain the particularly high levels of extra‐pair paternity (EPP) observed in several species of swallows and martins (Hirundinidae, Passeriformes), but EPP in the other bird lineages with a high aerial lifestyle, such as swifts (Apodiformes), is largely unknown. Here, we investigated EPP in the Alpine swift Tachymarptis melba. We found nine cases of EPP in 9 broods out of 214 (4.2%) nestlings and 87 (10.3%) broods analysed with information on fathers. This low incidence of extra‐pair paternities is similar to the only estimate reported so far in Apodiformes (i.e. 4.5% of nestlings in the common swift Apus apus). We suggest that life‐history traits (biparental incubation, long lifespan, and frequent mate retention across years), rather than their aerial lifestyle, may explain the low incidence of EPP in swifts.

In many avian species, variation in breeding phenology is known to affect reproductive success. In king penguins, the breeding cycle lasts more than a year, and the start of egg‐laying extends over three months, with early‐breeders laying eggs around December and late‐breeders around February. Consequently, late‐born chicks have less time to grow and build‐up their energy reserves before the austral winter. Variation in metabolic rate is one important pathway affecting differences in growth patterns, as it is directly involved both in energy allocation processes and fitness. The conversion of resources into energy occurs in mitochondria, and the efficiency of this conversion is likely to play a fundamental role in explaining individual variation in growth and survival. The purposes of this study were to investigate the differences in red blood cell mitochondrial metabolism between early‐ and late‐born king penguin chicks and to assess whether growth patterns could be explained by differences in mitochondrial metabolism. Late chicks expressed higher mitochondrial metabolism than early chicks at 100 days post‐hatching, probably linked to the stress related to winter environmental conditions and a decrease in parental feeding rates. We did not find a clear association between chick mitochondrial metabolism and growth patterns, suggesting that environmental conditions mostly contributed in explaining different metabolic phenotypes. As king penguin populations in Crozet may face changing breeding conditions (changes in feeding area and foraging trips duration) in relation to global changes, studying the physiological traits and adaptations underlying the chick growth and survival may help in understanding the response of king penguins facing challenging conditions.

Stable isotope analysis has been used extensively in migratory bird studies to provide ecological insights that may otherwise be difficult to obtain. However, an understanding of moult is critical for appropriate feather sampling, and here we make the first assessment of its relevance for examining the non‐breeding ecology of the Alpine Swift Tachymarptis melba, a long‐distance Afro‐Palaearctic migrant. We sampled three feather types from birds found dead after their return from migration and investigated variability in δ13C, δ15N and δ2H within and among feathers and between age classes. We found isotopic evidence supporting an interrupted primary moult in adults, with the innermost primary of adults (known to be moulted on the breeding grounds) significantly depleted in 13C and 2H compared with either the outermost primary or outermost tail feather, both of which were representative of sub‐Saharan non‐breeding areas. In contrast, the absence of significant differences in immature birds suggests the probable existence of a non‐migratory strategy within this age class. These isotopic insights into moult and life history highlight the potential for stable isotope analysis as a tool for investigating non‐breeding strategies in the Alpine Swift.

When approached by predators, prey must decide whether to flee or remain and fight. The economics of such decisions are underlain by the trade-off between current and residual fitness. The trade-off predicts that (i) breeders should be less prone than non-breeders to flee from approaching predators, as breeders can lose their investment into current reproduction; (ii) among breeders, parents should increasingly defend their offspring with increasing investment into the brood (brood value hypothesis), at least until the offspring can independently take part in anti-predator defenses; and (iii) for a similar investment into reproduction, breeders with lower perspectives to fledge or wean their young should invest less into offspring defense. We tested these predictions in a colonially breeding seabird, the king penguin (Aptenodytes patagonicus). Specifically, we considered how antipredator behaviors varied according to life history stage (molting, courting, breeding), offspring age and their dependence on parents for antipredator defenses, and the timing of breeding, with late breeders being very unlikely to fledge offspring in this species. Using non-lethal human approaches to mimic the threat of predation, we approached > 500 penguins and measured their alert and flight initiation distances, as well as the distance fled. We found that birds showed increasingly stronger antipredator behaviors as they initiated and increased their investment into reproduction, from non-reproductive stages to courting and brooding small, thermo-dependent chicks. However, once offspring gained thermal independence and freedom of movement, parents reduced their antipredator behaviors. Late breeders were more likely to flee from the approaching threat than early breeders. Altogether, our results demonstrate that parental antipredator responses are dynamic and shaped by the levels of investment into current reproduction, the ability of offspring to defend themselves, and the perceived future value of the brood.

Despite the advantages of lasting pair bonds and the prevalence of monogamy, at least in avian species, some individuals switch mates (divorce). Divorce is generally considered to be adaptive (i.e., conferring net fitness benefits), although its causes and consequences often remain unclear, most notably regarding the genetic basis of this behavior. Using more than 30 years of data in a long-lived bird with obligate biparental care, the Alpine swift, we first described the overall patterns of mate and nest site fidelity and investigated the predictors of between-year divorce. We show that 16.6% of pairings ended in divorce, with low reproductive success and young age as predictors of divorce, and that males retained the nest site more often than females. By then studying individual repeatability and heritability of divorce, we show moderate repeatability in females and low repeatability in males and little additive genetic variance in either sex. Finally, we assessed the fitness consequences of divorce and report that an active decision to modify the pair bond (divorce) may be more beneficial than reactionary re-pairing following a partner's death. Overall, divorce may provide some reproductive benefits for Alpine swifts, but no microevolutionary potential of this behavior is evident in this population.

When foraging, marine top predators rely on increasingly unpredictable oceanographic structures. Central place foragers are particularly affected. Their efficiency at replenishing their body reserves at sea while feeding their offspring on land relies on accurately targeting predictable foraging locations. Therefore, increased time and effort spent searching for resources is likely to compromise reproduction. Here, we used an experimental design to assess the flexibility of breeding king penguin (Aptenodytes patagonicus) foraging behavior in response to harsh conditions at sea, and examined the consequences on the growth and survival of their chick. We tested for behavioral adjustments to compensate for experimentally increased foraging workload, obtained by the application of a hydrodynamic drag effect. Compared to controls, treated adults more directly targeted a predictable hydrographic feature, the Polar Front, while limiting the increased costs of deep diving. Treated adults significantly increased hunting activity at shallower depths where the effect of treatment on diving efficiency was negligible. Our experiment resulted in decreased body mass gain during the brooding stage of chicks raised by treated parents compared to controls, with no direct effects on chick survival up to the winter period, but significant negative effects during winter. We identified two different strategies for foraging in king penguins: (1) foraging at the Polar Front where prey patches are more predictable and accessible at shallower depths or (2) foraging closer to the colony by targeting preys at deeper depths. These results highlight the possibility of a trade-off between distance and depth in breeding king penguin foraging behavior.

Parental age at conception can have both short- and long-term consequences on the health, survival, and reproduction of their offspring. To date, most of our knowledge comes from laboratory studies, and considers the effects of maternal age and a "snapshot" of the life history trajectory of the offspring. Here, we use a multigenerational demographic dataset in a free-living, long-lived (median lifespan is 7 years old) bird, the Alpine swift (Tachymarptis melba), to investigate the effects of maternal and paternal age on offspring traits, from nestling to adulthood, and considering all major life history traits, from growth and age at first reproduction to reproductive success and lifespan. Parental age affected offspring phenotype before fledging and lifespan, but differently so for sons and daughters. Offspring from old-age mothers (≥11 years old) and fathers (≥9 years old) were bigger and less infested by ectoparasites before fledging, except sons from old-age fathers that show no reduction in ectoparasite load. We also report evidence of negative effects of paternal age on the lifespan of their offspring (i.e., Lansing effect), with sons (but not daughters) from old-age fathers having shorter lifespans. Our findings highlight the importance of the transgenerational effects of parental age at conception on the reproductive performance, survival, and phenotype of their offspring.

Because of their extended fasting period on land during breeding, male king penguins have been extensively studied in order to unravel the physiological adaptations that enable them to fast while having to find a partner, defend their territory, or brood their offspring. While the different phases of fasting and the nature of the metabolic fuels used are well characterized in male king penguins, few studies have focused on the efficiency of the conversion of the metabolic resources into energy at a cellular level through mitochondrial respiration. Furthermore, little information is available on females in general while they experience fasting periods. Here, we measured mitochondrial respiration rates of red blood cells (RBCs) at the beginning (3 d) and end (10 d) of a natural egg-incubation fast in male and female king penguins. We tested whether RBC mitochondrial metabolism and its efficiency are modulated by fasting duration in free-living king penguins but also assessed whether this modulation is sex specific. In response to fasting, the respiration allocated to ATP synthesis in RBCs decreased in both sexes. Interestingly, RBC mitochondrial metabolic rates were higher in females at any stage of fasting. Furthermore, RBC mitochondrial metabolism efficiency decreased in males after 10 d of fasting but remained constant in females. Our results demonstrate that RBC mitochondrial metabolism is context and state dependent, differing between sexes and changing with fasting. They underline the importance of taking both sexes into account in physiological studies, where females remain underrepresented.

Morphological differences between the sexes are frequently reported in wild populations, which can extend beyond overall body size and result in differences in the size and/or shape of specific traits. Sexually selected traits have historically been expected to display positive allometric scaling (i.e., relatively larger trait in bigger individuals), although recent works suggest that negative allometric scaling (i.e., relatively larger trait size in smaller individuals) are equally likely. We used a long-term dataset to quantify sexual dimorphism and sex-specific allometric scaling of morphometric traits in a wild bird described as monomorphic, the Alpine swift. We identified subtle sexual dimorphisms suggesting that the Alpine swift is rather a cryptically dimorphic species. Fork length was the most sexually dimorphic trait, with males displaying 7% longer forks than females. Furthermore, we found that the extent of sexual dimorphism in swifts has changed over the past two decades, such that male and female feather traits have become more similar. Finally, we show that fork length scaled negatively with wing length in both sexes, indicating that short-winged individuals had relatively larger forks. In line with selection on multiple sexual ornaments and the functional allometry hypothesis, which predicts that patterns of allometric scaling should depend on the function of the trait in question (i.e., negative allometric scaling does not need to accurately reflect body size but rather "attractiveness"), we suggest that short-winged individuals may have to compensate for their reduce attractiveness in body size by exaggerating their fork size.

The pace of life syndrome (POLS) hypothesis suggests that organisms’ life history and physiological and behavioural traits should co-evolve. In this framework, how glycaemia (i.e. blood glucose levels) and its reaction with proteins and other compounds (i.e. glycation) covary with life history traits remain relatively under-investigated, despite the well-documented consequences of glucose and glycation on ageing, and therefore potentially on life history evolution. Birds are particularly relevant in this context given that they have the highest blood glucose levels within vertebrates and still higher mass-adjusted longevity compared to organisms with similar physiology as mammals. We thus performed a comparative analysis on glucose and albumin glycation rates of 88 bird species from 22 orders in relation to life history traits (body mass, clutch mass, maximum lifespan, and developmental time) and diet. Glucose levels correlated positively with albumin glycation rates in a non-linear fashion, suggesting resistance to glycation in species with higher glucose levels. Plasma glucose levels decreased with increasing body mass, but, contrary to what is predicted in the POLS hypothesis, glucose levels increased with maximum lifespan before reaching a plateau. Finally, terrestrial carnivores showed higher albumin glycation compared to omnivores despite not showing higher glucose, which we discuss may be related to additional factors as differential antioxidant levels or dietary composition in terms of fibres or polyunsaturated fatty acids. These results increase our knowledge about the diversity of glycaemia and glycation patterns across birds, pointing towards the existence of glycation resistance mechanisms within comparatively high glycaemic birds.

Although climate change is considered to be partly responsible for the size change observed in numerous species, the relevance of this hypothesis for ungulates remains debated. We used body mass measurements of 5635 yearlings (i.e. 1.5 years old) of Alpine chamois (Rupicapra rupicapra) harvested in September in the Swiss Alps (Ticino canton) from 1992 to 2018. In our study area, during this period, yearlings shrank by ca 3 kg while temperatures between May and July rose by 1.7°C. We identified that warmer temperatures during birth and the early suckling period (9 May to 2 July in the year of birth) had the strongest impact on yearling mass. Further analyses of year-detrended mass and temperature data indicate that this result was not simply due to changes in both variables over years, but that increases in temperature during this particularly sensitive time window for development and growth are responsible for the decrease in body mass of yearling chamois. Altogether, our results suggest that rising temperatures in the Alpine regions could significantly affect the ecology and evolution of this wild ungulate.

Chemical pollution is a major man-made environmental threat to ecosystems and natural animal populations. Of concern are persistent organic pollutants (POPs), which can persist in the environment for many years. While bioaccumulating throughout the lives of wild animals, POPs can affect their health, reproduction, and survival. However, measuring long-term effects of POPs in wild populations is challenging, and therefore appropriate biomarkers are required in wildlife ecotoxicology. One potential target is telomere length, since telomere preservation has been associated to survival and longevity, and stressors as chemical pollution can disrupt its maintenance. Here, we investigated the effects of different classes of POPs on relative telomere length (RTL) and its rate of change (TROC) in wild long-lived Alpine swifts (Tachymarptis melba). As both RTL and TROC are often reported to differ between sexes and with chronological age, we tested for sex- and age-specific (pre-senescent vs. senescent, ≥ 9 age of years, individuals) effects of POPs. Our results showed that senescent females presented longer RTL and elongated telomeres over time compared to pre-senescent females and males. These sex- and age-related differences in RTL and TROC were influenced by POPs, but differently depending on whether they were organochlorine pesticides (OCPs) or industrial polychlorinated biphenyls (PCBs). OCPs (particularly drins) were negatively associated with RTL, with the strongest negative effects being found in senescent females. Conversely, PCBs led to slower rates of telomere shortening, especially in females. Our study indicates diametrically opposed effects of OCPs on RTL and PCBs on TROC, and these effects were more pronounced in females and senescent individuals. The mechanisms behind these effects (e.g., increased oxidative stress by OCPs; upregulation of telomerase activity by PCBs) remain unknown. Our results highlight the importance in wildlife ecotoxicology to account for sex- and age-related effects when investigating the health effects of pollutants on biomarkers such as telomeres.

Although problem-solving tasks are frequently used to assess innovative ability, the extent to which problem-solving performance reflects variation in cognitive skills has been rarely formally investigated. Using wild breeding great tits facing a new non-food motivated problem-solving task, we investigated the role of associative learning in finding the solution, compared to multiple other non-cognitive factors. We first examined the role of accuracy (the proportion of contacts made with the opening part of a string-pulling task), neophobia, exploration, activity, age, sex, body condition and participation time on the ability to solve the task. To highlight the effect of associative learning, we then compared accuracy between solvers and non-solvers, before and after the first cue to the solution (i.e., the first time they pulled the string opening the door). We finally compared accuracy over consecutive entrances for solvers. Using 884 observations from 788 great tits tested from 2010 to 2015, we showed that, prior to initial successful entrance, solvers were more accurate and more explorative than non-solvers, and that females were more likely to solve the task than males. The accuracy of solvers, but not of non-solvers, increased significantly after they had the opportunity to associate string pulling with the movement of the door, giving them a first cue to the task solution. The accuracy of solvers also increased over successive entrances. Our results demonstrate that variations in problem-solving performance primarily reflect inherent individual differences in associative learning, and are also to a lesser extent shaped by sex and exploratory behaviour.

Persistent organic pollutants (POPs) bioaccumulate in the food chain and can cause ecotoxicity. In wild bird populations, various tissues are used to determine POP levels, including invasive (e.g., brain, fat, kidney, liver, muscle) and minimally-invasive tissues (e.g., blood, feather, preen oil). Minimally-invasive sampling, which does not require the death of the animal, opens new prospects for sampling birds as sentinels of environmental pollution and its consequences on fitness. However, POP variability between tissues is understudied, which is an essential prerequisite for making a reasoned choice about which tissues to sample. Here, we performed a meta-analysis of eight tissues across 115 studies comparing tissues across POP groups. We demonstrate increased use of minimally-invasive measures between 1974 and 2020. When grouping tissue correlations into three groups, “invasive:invasive”, “invasive:minimally-invasive” and “minimally-invasive:minimally-invasive”, we found that all three groups produced moderate to strong positive correlations with no difference seen between comparison groups. We demonstrate (1) lower POP concentrations in preen oil than fat, but no difference in detection frequencies, supporting preen oil use; (2) blood showed high concentration variability dependent on POP group but detection frequencies were comparable to liver and kidney; and (3) feathers demonstrated a significantly lower detection frequency than other matrices measured. By further researching minimally-invasive tissues, we increase our understanding of whether minimally-invasive tissues are ecologically representative of body-level toxicity. Our study supports blood and preen oil as substitutes for invasive measures when sampling living bird populations as they represent internal POP concentrations and provide significant benefits both practically and ethically.

Polar and sub-polar animals evolved to thrive in cold climates and may thus be particularly vulnerable to the rising temperatures associated with climate change. Polar and sub-polar penguins may be especially vulnerable due to their dual habitat, alternating between foraging in cold waters and breeding/moulting on an increasingly warm land. Here, we characterized heat stress occurrence in breeding king penguins through behavioural observations and subcutaneous body temperature measurements. We show that heat stress is frequent ( > 20% of observations at mid-day) in king penguins breeding in the sub-Antarctic region, and that thermoregulatory mechanisms appear insufficient to maintain stable sub-cutaneous temperature. Air temperature alone was a poor predictor of heat stress occurrence, while the combination of high solar radiations, low wind speed and high temperatures was its best predictor. Importantly, reproductive failure occurred on days warmer than average, suggesting potential significant sublethal effects of heat stress being likely to affect population dynamics. ![Figure][1] ### Competing Interest Statement The authors have declared no competing interest. The datasets used in this manuscript are available on FigShare (Noiret et al. (2024). HotKing dataset: heat stress in breeding king penguins. figshare. Dataset. ). [1]: pending:yes

Assessing the physiological stress responses of wild animals opens a window for understanding how organisms cope with environmental challenges. Since stress response is associated with changes in body temperature, the use of body surface temperature through thermal imaging could help to measure acute and chronic stress responses non-invasively. We used thermal imaging, acute handling-stress protocol and an experimental manipulation of corticosterone (the main glucocorticoid hormone in birds) levels in breeding king penguins (Aptenodytes patagonicus), to assess: 1. The potential contribution of the Hypothalamo-Pituitary-Adrenal (HPA) axis in mediating chronic and acute stress-induced changes in adult surface temperature, 2. The influence of HPA axis manipulation on parental investment through thermal imaging of eggs and brooded chicks, and 3. The impact of parental treatment on offspring thermal's response to acute handling.Maximum eye temperature (Teye) increased and minimum beak temperature (Tbeak) decreased in response to handling stress in adults, but neither basal nor stress-induced surface temperatures were significantly affected by corticosterone implant. While egg temperature was not significantly influenced by parental treatment, we found a surprising pattern for chicks: chicks brooded by the (non-implanted) partner of corticosterone-implanted individuals exhibited higher surface temperature (both Teye and Tbeak) than those brooded by glucocorticoid-implanted or control parents. Chick's response to handling in terms of surface temperature was characterized by a drop in both Teye and Tbeak independently of parental treatment.We conclude that the HPA axis seems unlikely to play a major role in determining chronic or acute changes in surface temperature in king penguins. Changes in surface temperature may primarily be mediated by the Sympathetic-Adrenal-Medullary (SAM) axis in response to stressful situations. Our experiment did not reveal a direct impact of parental HPA axis manipulation on parental investment (egg or chick temperature), but a potential influence on the partner's brooding behaviour.