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Coexistence among sympatric, functionally similar species often hinges on niche differentiation, especially as resource competition intensifies during the breeding season. Individual specialisation (IS) can promote coexistence by narrowing individual niches or increasing divergence among individuals. In colonial seabirds, aggregation at limited breeding sites and central-place foraging amplify both intra- and interspecific competition. Here, we assess seasonal shifts in individual and population isotopic niche widths in two sympatric cormorant species to elucidate the mechanisms underlying their coexistence. We analysed isotopic composition ( δ 13 C and δ 15 N) in multiple-tissues to produce repeated measures within 111 individuals of red-legged cormorant ( Poikilocarbo gaimardi) and imperial shag ( Leucocarbo atriceps) captured on the Pirén Islet (Los Lagos, Chile) during breeding and non-breeding seasons. Multivariate generalised linear mixed models estimated isotopic niche components: total niche width (TNW), within-individual component (WIC), and between-individual component (BIC). We estimated IS ( i.e. , the extent to which individuals exploit a narrower subset of the population niche) as BIC/TNW. L. atriceps exhibited 2.2-fold greater TNW than P. gaimardi during non-breeding and 2-fold greater during breeding. IS differed markedly between species: L. atriceps showed a higher IS during non-breeding (0.541 vs 0.213 in P. gaimardi ), but decreased by 79.3% during breeding, whereas P. gaimardi increased IS by 52.1%. Niche width overlap was asymmetric and seasonally variable: P. gaimardi exhibited high overlap with L. atriceps (95.7% non-breeding, 89.6% breeding), whilst L. atriceps showed lower overlap (48.3% non-breeding, 43.7% breeding). Competition indices increased substantially during breeding in both species (305% in L. atriceps , 221% in P. gaimardi ). Results suggest that coexistence relies on multiple mechanisms, including subtle population niche differentiation, contrasting IS between species, and divergent resource-use strategies. The high niche width overlap and narrower niche of P. gaimardi suggest greater competitive vulnerability for this Near Threatened species. Conservation of foraging habitat heterogeneity and prey availability is crucial for maintaining ecological opportunities that sustain these coexistence mechanisms.

Temporal variation in resource availability, amplified by global change, may have strong impacts on species breeding at temperate and high latitudes that cue their reproduction to exploit seasonal resource pulses. This study examines how resource availability and parental care influence niche partitioning between and within age classes in the rufous‐collared sparrow, which provides extensive parental care. We hypothesized juveniles would exhibit narrower niches focused on high‐quality resources compared to adults, regardless of resource availability. We used stable isotope analysis to quantify individual and population niches in juveniles and adults across the breeding season in two cohorts experiencing contrasting resource landscapes. Contrary to our initial hypothesis, juveniles exhibited greater among‐individual diet variation and smaller total niche widths (i.e. higher levels of individual specialization, IS) during periods of high food availability in comparison to periods of food scarcity. Interestingly, total niche width and IS of adults remained stable across seasons despite a shift in trophic level, highlighting their potential role in providing a consistent diet for their young. These findings reveal a dynamic interplay between resource availability, parental care, and IS, with important implications for understanding population resilience under variable resource scenarios. The study also suggests that adult sparrows modify their provisioning strategies based on resources, potentially buffering offspring from environmental fluctuations. Understanding age‐specific responses to resource variation is crucial for predicting species responses to ecological conditions, particularly in regions like central Chile where seasonal resource limitation is expected to become more variable in response to climate change.

Several studies have suggested that penguins are undergoing a major restructuring of their feeding habits and distribution after drastic climatic changes in the Antarctic Peninsula region. With the objective of estimating potential medium-term and inter-annual variations in trophic niche, we measured δ 15 N and δ 13 C in feather samples of pygoscelid penguins from museum specimens (1982–1984) and in blood and feather samples from 2009/10–2011 collected from animals on Ardley Island. Current penguin feathers had lower δ 13 C and δ 15 N values and were more similar to Antarctic krill values, than feathers in 1982–1984 and blood from 2009/10-2011. Moreover, δ 13 C and δ 15 N values from museum feathers and modern samples occupied a larger isotopic space in Gentoo Penguins ( Pygoscelis papua ), compared to Adélie Penguins ( Pygoscelis adeliae ) and Chinstrap Penguins ( Pygoscelis antarctica ). Our results from feathers samples indicated that penguins have decreased their consumption of fish and other prey of higher trophic levels in the early interbreeding period (EIBP), while increasing the amount of euphausiids (Antarctic krill Euphausia superba ) taken. The isotopic values of the species suggest that foraging sites varied significantly over time and seasonally. We suggest that environmental changes may have modified the feeding habits of pygoscelid penguins, resulting in changed foraging behaviour in the EIBP, and altering the secondary prey consumption. Prey choice in breeding pygoscelid species is probably limited by the foraging range around the breeding colony and competitive exclusion between congeners.

The niche variation hypothesis (NVH) predicts that populations with broader niches should exhibit greater between-individual diet variation or individual specialization (IS) relative to populations with narrower niches. Most studies that quantify population niche widths and associated levels of IS typically focus on a single or few species, but studies examining NVH in a phylogenetically informed comparative analysis among species are lacking. Here we use nitrogen isotope (δ15N) analysis to measure population niche widths and IS in a single bird community composed of 12 passerine species representing different foraging guilds. We found support for the NVH at the interspecific level; species with broader population niche widths were comprised of more individual specialists. Moreover, our results suggest that this relationship is influenced by foraging guild; specifically, omnivores have higher degrees of IS for a given population niche width than insectivores. Finally, the levels of IS among passerine species, in contrast to population niche width, were associated with their relatedness, suggesting that the potential phylogenetic effect on the prevalence of IS is higher than previously recognized.

Oxidative status and immune function are energy-demanding traits closely linked to diet composition, particularly resource availability and nutritional value. In seasonal environments, nutrient availability and diet quality fluctuate, potentially influencing these traits. However, limited evidence exists regarding these dietary effects on immune function in seasonal environments. In this study, we employed stable isotope analysis to assess the impact of seasonal changes in niche width and trophic level (i.e., δ15N) on two immune variables (hemolysis and hemagglutination scores) and two oxidative status parameters (lipid peroxidation and total antioxidant capacity) in three passerine species: Zonotrichia capensis (omnivorous), Troglodytes aedon (insectivorous), and Spinus barbatus (granivorous). We found that hemolysis scores varied seasonally in Z. capensis, with higher values in winter compared to summer. Total antioxidant capacity (TAC) also increased during the winter in Z. capensis and S. barbatus. The isotopic niche width for Z. capensis and S. barbatus was smaller in winter than in summer, with the omnivorous species exhibiting a decrease in δ15N. Despite the seasonal shifts in ecological and physiological traits in Z. capensis, we identified no correlation between immune response and TAC with trophic level. In contrast, in the granivorous S. barbatus, the lower trophic level resulted in an increase in TAC without affecting immunity. Our findings revealed that dietary shifts do not uniformly impact oxidative status and immune function across bird species, highlighting species-specific responses to seasonal changes. This underscores the importance of integrating ecological and evolutionary perspectives when examining how diet shapes avian immunity and oxidative balance.

Small mammals use multiple foraging strategies to compensate for fluctuating resource quality in stochastic environments. These strategies may lead to increased dietary overlap when competition for resources is strong. To quantify temporal contributions of high (C₃) versus low quality (C₄) resources in diets of silky pocket mice (Perognathus flavus), we used stable carbon isotope (d¹³C) analysis of 1391 plasma samples collected over 2 years. Of these, 695 samples were from 170 individuals sampled ≥ 3 times across seasons or years, allowing us to assess changes in dietary breadth at the population and individual levels across a boom–bust population cycle. In 2014, the P. flavus population increased to 412 captures compared to 8 captures in prior and subsequent years, while populations of co-occurring small mammals remained stable. As intraspecific competition increased, the population-wide dietary niche of P. flavus did not change, but individual specialization increased significantly. During this period, ~ 27% (41/151) of individuals sampled specialized on C₃ resources, which were abundant during the spring and previous fall seasons. Most of the remaining individuals were C₃–C₄ generalists (64%) (96/151), and only 9% (14/151) specialized on C₄ resources. In 2015, P. flavus population density and resource availability declined, individual dietary breadth expanded (84% generalists), no C₃ specialists were found, and specialization on C₄ resources increased (16%). Our results demonstrate a high degree of inter-individual plasticity in P. flavus foraging strategies, which has implications for how this species will respond to environmental change that is predicted to decrease C₃ resources in the future.

A long-standing debate in evo-devo research concerns the relative role of protein-coding and cis-regulatory regions in adaptation.Recent studies of genetic adaptation have revealed that the number of substitutions contributing to phenotypic variation is lower in cis-regulatory than in structural regions, which has led to the idea that cis-regulatory regions are less important in phenotypic adaptation. However, the number of substitutions is not the only important factor, the \"size\" of the adaptive contribution of these substitutions is important too. A geometrical reasoning predicts that, given their lesser pleiotropic effects, cis-regulatory substitutions should have a larger average adaptive contribution than protein-coding substitutions. Thus it is possible that even with a lower number of adaptive mutations, cis-regulatory regions may contribute at the same level or even more than proteincoding regions.

A large number of physiological acclimation studies assume that flexibility in a certain trait is both adaptive and functionally important for organisms in their natural environment; however, it is not clear how an organism's capacity for temperature acclimation translates to the seasonal acclimatization that these organisms must accomplish. To elucidate this relationship, we measured BMR and TEWL rates in both field-acclimatized and laboratory-acclimated adult rufous-collared sparrows (Zonotrichia capensis). Measurements in field-acclimatized birds were taken during the winter and summer seasons; in the laboratory-acclimated birds, we took our measurements following 4 weeks at either 15 or 30°C. Although BMR and TEWL rates did not differ between winter and summer in the field-acclimatized birds, laboratory-acclimated birds exposed to 15°C exhibited both a higher BMR and TEWL rate when compared to the birds acclimated to 30°C and the field-acclimatized birds. Because organ masses seem to be similar between field and cold-acclimated birds whereas BMR is higher in cold-acclimated birds, the variability in BMR cannot be explained completely by adjustments in organ masses. Our findings suggest that, although rufous-collared sparrows can exhibit thermal acclimation of physiological traits, sparrows do not use this capacity to cope with minor to moderate fluctuations in environmental conditions. Our data support the hypothesis that physiological flexibility in energetic traits is a common feature of avian metabolism.

Seed-eating birds have a diet of high nutritional value; however, they must cope with plant secondary metabolites (PSM). We postulated that the detoxification capacity of birds is associated with a metabolic cost, given that the organs responsible for detoxification significantly contribute to energetic metabolism. We used an experimental approach to assess the effects of phenol-enriched diets on two passerines with different feeding habits: the omnivorous rufous-collared sparrow ( Zonotrichia capensis ) and the granivorous common diuca-finch ( Diuca diuca ). The birds were fed with one of three diets: control diet, supplemented with tannic acid, or supplemented with Opuntia ficus - indica phenolic extract (a common food of the sparrow but not the finch). After 5 weeks of exposure to the diets, we measured basal metabolic rates (BMR), energy intake, glucuronic acid output and digestive and kidney structure. In both species, detoxification capacity expressed as glucuronic acid output was higher in individuals consuming phenol-enriched diets compared to the control diet. However, whereas sparrows increase energy intake and intestinal mass when feeding on phenol-enriched diets, finches had lower intestinal mass and energy intake remains stable. Furthermore, sparrows had higher BMR on phenol-enriched diets compared to the control group, whereas in the finches BMR remains unchanged. Interspecific differences in response to phenols intake may be determined by the dietary habits of these species. While both species can feed on moderate phenolic diets for 5 weeks, energy costs may differ due to different responses in food intake and organ structure to counteract the effects of PSM intake.