Revisiting Perdeck's massive avian migration experiments debunks alternative social interpretations.

Whether avian migrants can adapt to their changing world depends on the relative importance of genetic and environmental variation for the timing and direction of migration. In the classic series of field experiments on avian migration, A. C. Perdeck discovered that translocated juveniles failed to reach goal areas, whereas translocated adults performed 'true-goal navigation'. His translocations of > 14 000 common starlings (Sturnus vulgaris) suggested that genetic mechanisms guide juveniles into a population-specific direction, i.e. 'vector navigation'. However, alternative explanations involving social learning after release in juveniles could not be excluded. By adding historical data from translocation sites, data that was unavailable in Perdeck's days, and by integrated analyses including the original data, we could not explain juvenile migrations from possible social information upon release. Despite their highly social behaviour, our findings are consistent with the idea that juvenile starlings follow inherited information and independently reach their winter quarters. Similar to more solitarily migrating songbirds, starlings would require genetic change to adjust the migration route in response to global change.

Tracing the introduction of the invasive common myna using population genomics.

The common myna (Acridotheres tristis) is one of the most invasive bird species in the world, yet its colonisation history is only partly understood. We identified the introduction history and population structure, and quantified the genetic diversity of myna populations from the native range in India and introduced populations in New Zealand, Australia, Fiji, Hawaii, and South Africa, based on thousands of single nucleotide polymorphism markers in 814 individuals. We were able to identify the source population of mynas in several invasive locations: mynas from Fiji and Melbourne, Australia, were likely founded by individuals from a subpopulation in Maharashtra, India, while mynas in Hawaii and South Africa were likely independently founded by individuals from other localities in India. Our findings suggest that New Zealand mynas were founded by individuals from Melbourne, which, in turn, were founded by individuals from Maharashtra. We identified two genetic clusters among New Zealand mynas, divided by New Zealand's North Island's axial mountain ranges, confirming previous observations that mountains and thick forests may form barriers to myna dispersal. Our study provides a foundation for other population and invasion genomic studies and provides useful information for the management of this invasive species.

Transcript- and annotation-guided genome assembly of the European starling.

The European starling, Sturnus vulgaris, is an ecologically significant, globally invasive avian species that is also suffering from a major decline in its native range. Here, we present the genome assembly and long-read transcriptome of an Australian-sourced European starling (S. vulgaris vAU), and a second, North American, short-read genome assembly (S. vulgaris vNA), as complementary reference genomes for population genetic and evolutionary characterization. S. vulgaris vAU combined 10× genomics linked-reads, low-coverage Nanopore sequencing, and PacBio Iso-Seq full-length transcript scaffolding to generate a 1050 Mb assembly on 6222 scaffolds (7.6 Mb scaffold N50, 94.6% busco completeness). Further scaffolding against the high-quality zebra finch (Taeniopygia guttata) genome assigned 98.6% of the assembly to 32 putative nuclear chromosome scaffolds. Species-specific transcript mapping and gene annotation revealed good gene-level assembly and high functional completeness. Using S. vulgaris vAU, we demonstrate how the multifunctional use of PacBio Iso-Seq transcript data and complementary homology-based annotation of sequential assembly steps (assessed using a new tool, saaga) can be used to assess, inform, and validate assembly workflow decisions. We also highlight some counterintuitive behaviour in traditional busco metrics, and present buscomp, a complementary tool for assembly comparison designed to be robust to differences in assembly size and base-calling quality. This work expands our knowledge of avian genomes and the available toolkit for assessing and improving genome quality. The new genomic resources presented will facilitate further global genomic and transcriptomic analysis on this ecologically important species.

Detection of a novel enterotropic Mycoplasma gallisepticum-like in European starling (Sturnus vulgaris) around poultry farms in France.

Recent outbreaks of highly pathogenic avian influenza in southwest France have raised questions regarding the role of commensal wild birds in the introduction and dissemination of pathogens between poultry farms. To assess possible infectious contacts at the wild-domestic bird interface, the presence of Mycoplasma gallisepticum (MG) was studied in the two sympatric compartments in southwest France. Among various peridomestic wild birds (n = 385), standard PCR primers targeting the 16S rRNA of MG showed a high apparent prevalence (up to 45%) in cloacal swabs of European starlings (Sturnus vulgaris, n = 108), while the MG-specific mgc2 gene was not detected. No tracheal swab of these birds tested positive, and no clinical sign was observed in positive birds, suggesting commensalism in the digestive tract of starlings. A mycoplasma strain was then isolated from a starling swab and its whole genome was sequenced using both Illumina and Nanopore technologies. Phylogenetic analysis showed that it was closely related to MG and M. tullyi, although it was a distinct species. A pair of specific PCR primers targeting the mgc2-like gene of this MG-like strain was designed and used to screen again the same avian populations and a wintering urban population of starlings (n = 50). Previous PCR results obtained in starlings were confirmed to be mostly due to this strain (20/22 positive pools). In contrast, the strain was not detected in fresh faeces of urban starlings. Furthermore, it was detected in one cloacal pool of white wagtails, suggesting infectious transmissions between synanthropic birds with similar feeding behaviour. As the new Starling mycoplasma was not detected in free-range ducks (n = 80) in close contact with positive starlings, nor in backyard (n = 320) and free-range commercial (n = 720) chickens of the area, it might not infect poultry. However, it could be involved in mycoplasma gene transfer in such multi-species contexts.

Environmental correlates of genetic variation in the invasive European starling in North America.

Populations of invasive species that colonize and spread in novel environments may differentiate both through demographic processes and local selection. European starlings (Sturnus vulgaris) were introduced to New York in 1890 and subsequently spread throughout North America, becoming one of the most widespread and numerous bird species on the continent. Genome-wide comparisons across starling individuals and populations can identify demographic and/or selective factors that facilitated this rapid and successful expansion. We investigated patterns of genomic diversity and differentiation using reduced-representation genome sequencing of 17 winter-season sampling sites. Consistent with this species' high dispersal rate and rapid expansion history, we found low geographical differentiation and few FST outliers even at a continental scale. Despite starting from a founding population of ~180 individuals, North American starlings show only a moderate genetic bottleneck, and models suggest a dramatic increase in effective population size since introduction. In genotype-environment associations we found that ~200 single-nucleotide polymorphisms are correlated with temperature and/or precipitation against a background of negligible genome- and range-wide divergence. Given this evidence, we suggest that local adaptation in North American starlings may have evolved rapidly even in this wide-ranging and evolutionarily young system. This survey of genomic signatures of expansion in North American starlings is the most comprehensive to date and complements ongoing studies of world-wide local adaptation in these highly dispersive and invasive birds.

Signatures of selection in a recent invasion reveal adaptive divergence in a highly vagile invasive species.

A detailed understanding of population genetics in invasive populations helps us to identify drivers of successful alien introductions. Here, we investigate putative signals of selection in Australian populations of invasive common starlings, Sturnus vulgaris, and seek to understand how these have been influenced by introduction history. We used reduced representation sequencing to determine population structure, and identify Single Nucleotide Polymorphisms (SNPs) that are putatively under selection. We found that since their introduction into Australia, starling populations have become genetically differentiated despite the potential for high levels of dispersal, and that starlings have responded to selective pressures imposed by a wide range of environmental conditions across their geographic range. Isolation by distance appears to have played a strong role in determining genetic substructure across the starling's Australian range. Analyses of candidate SNPs that are putatively under selection indicated that aridity, precipitation and temperature may be important factors driving adaptive variation across the starling's invasive range in Australia. However, we also noted that the historic introduction regime may leave footprints on sites flagged as being under adaptive selection, and encourage critical interpretation of selection analyses in non-native populations.

Flight training in a migratory bird drives metabolic gene expression in the flight muscle but not liver, and dietary fat quality influences select genes.

Training and diet are hypothesized to directly stimulate key molecular pathways that mediate animal performance, and flight training, dietary fats, and dietary antioxidants are likely important in modulating molecular metabolism in migratory birds. This study experimentally investigated how long-distance flight training, as well as diet composition, affected the expression of key metabolic genes in the pectoralis muscle and the liver of European starlings (Sturnus vulgaris, n = 95). Starlings were fed diets composed of either a high or low polyunsaturated fatty acid (PUFA; 18:2n-6) and supplemented with or without a water-soluble antioxidant, and one-half of these birds were flight trained in a wind-tunnel while the rest were untrained. We measured the expression of 7 (liver) or 10 (pectoralis) key metabolic genes in flight-trained and untrained birds. Fifty percent of genes involved in mitochondrial metabolism and fat utilization were upregulated by flight training in the pectoralis (P < 0.05), whereas flight training increased the expression of only one gene responsible for fatty acid hydrolysis [lipoprotein lipase (LPL)] in the liver (P = 0.04). Dietary PUFA influenced the gene expression of LPL and fat transporter fatty acid translocase (CD36) in the pectoralis and one metabolic transcription factor [peroxisome proliferator-activated receptor (PPAR)-α (PPARα)] in the liver, whereas dietary antioxidants had no effect on the metabolic genes measured in this study. Flight training initiated a simpler causal network between PPARγ coactivators, PPARs, and metabolic genes involved in mitochondrial metabolism and fat storage in the pectoralis. Molecular metabolism is modulated by flight training and dietary fat quality in a migratory songbird, indicating that these environmental factors will affect the migratory performance of birds in the wild.

DNA Integrity Estimated via the Comet Assay Reflects Oxidative Stress and Competitive Disadvantage in Developing Birds.

AbstractIncreases in DNA degradation have been detected in numerous situations in which organisms are exposed to pollutants. However, outside of the ecotoxicological literature, few studies have investigated whether there exists important variation in DNA integrity in free-living, healthy animals. Using the alkaline version of the comet assay to estimate DNA integrity in blood samples, we aimed to evaluate whether DNA integrity during early life is associated with nestlings' age, body mass, within-brood status, and oxidative stress using nestlings from a wild population of spotless starlings (Sturnus unicolor) as a model. We found important levels of variation in DNA integrity, suggesting the possibility that DNA integrity may have implications for offspring fitness. DNA integrity was dependent on the developmental stage, being lower at hatching than at the end of the nestling period. DNA integrity was also negatively related to the levels of oxidative damage at hatching and positively associated with wing length at fledging. In addition, position within the size hierarchy of the brood at fledging explained differences in DNA integrity, with higher levels in core than in marginal nestlings. Finally, despite extensive within-individual variation along nestling's age, we found DNA integrity during early life to be moderately repeatable within broods. Hence, DNA integrity in early life appears to be mainly affected by environmental factors, such as natural stressors. Our results suggest that measuring the variation in DNA integrity may be a fruitful approach for the assessment of individual fitness in natural populations and can be applied to studies in developmental biology and ecology.

Conditioned place preferences induced by hearing song outside the breeding season relate to neural dopamine D1 and cannabinoid CB1 receptor gene expression in female European starlings (Sturnus vulgaris).

The affective state induced by sensory stimuli changes to adaptively modify behaviors that are critical for survival and reproduction. In European starlings, during the spring breeding season, male courtship song is rewarding to females, but only to those that possess resources that are necessary for reproduction (i.e., nesting sites). In fall, starling song is non-sexual and proposed to maintain flocks. This suggests that in fall it may be adaptive for females to be rewarded by fall rather than spring, courtship song. We used a conditioned place preference (CPP) test to evaluate song-induced affective state in fall condition females and quantitative real-time PCR to measure expression of genes that modulate affective state (CB1 endocannabinoid and D1 dopamine receptors) in brain regions that were previously implicated in song-induced reward (i.e., the medial preoptic nucleus (mPOA) and ventromedial hypothalamus (VMH)). Fall condition females developed an aversion to a place that had been paired with playback of both male fall and courtship song, indicating that in general male song induces a negative affective state outside the breeding season. Song-induced aversion was stronger in birds conditioned towards an initial place preference. For mPOA, CB1 receptor expression correlated positively with fall and spring song-induced CPP. D1 receptor expression correlated negatively with fall (but not spring) song-induced CPP, and the ratio of CB1 to D1 receptor expression correlated positively with fall (but not spring) song-induced CPP. These correlations suggest that interactions between D1 and CB1 receptors in mPOA may play a role in modifying affective responses to song.

Nest box exploration may stimulate breeding physiology and alter mRNA expression in the medial preoptic area of female European starlings.

Environmental resources are proposed to fine-tune the timing of breeding, yet how they may do so remains unclear. In female European starlings (Sturnus vulgaris), nest cavities are limited resources that are necessary for breeding. Females that explore nest cavities, compared with those that do not, readily perform sexually motivated behaviors. We assigned female starlings to aviaries with: (1) no nest boxes, (2) nest boxes, or (3) nest boxes, plants, flowing water, insects and berries to test the hypothesis that environmental resources alter neural systems to stimulate mating behavior. Compared with other females, females that were housed with and explored nest boxes had higher estradiol, higher preproenkephalin (PENK) mRNA and lower levels of D1 and D2 dopamine receptor mRNA in the medial preoptic area (mPOA); a region in which opioids and dopamine modify female sexual behaviors and sexual motivation. Additionally, in the mPOA, PENK and tyrosine hydroxylase mRNA positively predicted, whereas estrogen receptor beta mRNA negatively predicted, nest box exploration. In the ventromedial hypothalamus (a region in which estradiol acts to stimulate sexual behavior), estrogen receptor alpha mRNA was highest in females that had access to but did not explore nest cavities. It is likely that seasonal increases in estradiol modify mRNA in the mPOA to facilitate nest cavity exploration. It is also possible that nest cavity exploration further alters gene expression in the mPOA, functioning to coordinate mating with resource availability. Thus, nest cavity exploration may be a form of self-stimulation that alters neural systems to fine-tune sexual behavior.

Neocortical Association Cell Types in the Forebrain of Birds and Alligators.

The avian dorsal telencephalon has two vast territories, the nidopallium and the mesopallium, both of which have been shown to contribute substantially to higher cognitive functions. From their connections, these territories have been proposed as equivalent to mammalian neocortical layers 2 and 3, various neocortical association areas, or the amygdala, but whether these are analogies or homologies by descent is unknown. We investigated the molecular profiles of the mesopallium and the nidopallium with RNA-seq. Gene expression experiments established that the mesopallium, but not the nidopallium, shares a transcription factor network with the intratelencephalic class of neocortical neurons, which are found in neocortical layers 2, 3, 5, and 6. Experiments in alligators demonstrated that these neurons are also abundant in the crocodilian cortex and form a large mesopallium-like structure in the dorsal ventricular ridge. Together with previous work, these molecular findings indicate a homology by descent for neuronal cell types of the avian dorsal telencephalon with the major excitatory cell types of mammalian neocortical circuits: the layer 4 input neurons, the deep layer output neurons, and the multi-layer intratelencephalic association neurons. These data raise the interesting possibility that avian and primate lineages evolved higher cognitive abilities independently through parallel expansions of homologous cell populations.

Contributions of testosterone and territory ownership to sexually-motivated behaviors and mRNA expression in the medial preoptic area of male European starlings.

Animals integrate social information with their internal endocrine state to control the timing of behavior, but how these signals are integrated in the brain is not understood. The medial preoptic area (mPOA) may play an integrative role in the control of courtship behavior, as it receives projections from multiple sensory systems, and is central to the hormonal control of courtship behavior across vertebrates. Additionally, data from many species implicate opioid and dopaminergic systems in the mPOA in the control of male courtship behavior. We used European starlings to test the hypothesis that testosterone (T) and social status (in the form of territory possession) interact to control the timing of courtship behavior by modulating steroid hormone-, opioid- and dopaminergic-related gene expression in the mPOA. We found that only males given both T and a nesting territory produced high rates of courtship behavior in response to a female. T treatment altered patterns of gene expression in the mPOA by increasing androgen receptor, aromatase, mu-opioid receptor and preproenkephalin mRNA and decreasing tyrosine hydroxylase mRNA expression. Territory possession did not alter mRNA expression in the mPOA, despite the finding that only birds with both T and a nesting territory produced courtship behavior. We propose that T prepares the mPOA to respond to the presence of a female with high rates of courtship song by altering gene expression, but that activity in the mPOA is under a continuous (i.e. tonic) inhibition until a male starling obtains a nesting territory.

Selection, constraint, and the evolution of coloration in African starlings.

Colorful plumage plays a prominent role in the evolution of birds, influencing communication (sexual/social selection), and crypsis (natural selection). Comparative studies have focused primarily on these selective pressures, but the mechanisms underlying color production can also be important by constraining the color gamut upon which selection acts. Iridescence is particularly interesting to study the interaction between selection and color-producing mechanisms because a broad range of colors can be produced with a shared template, and innovations to this template further expand this by increasing the parameters interacting to produce colors. We examine the patterns of ornamentation and dichromatism evolution in African starlings, a group remarkably diverse in color production mechanisms, social systems, and ecologies. We find that the presence of iridescence is ancestral to the group, being predominantly lost in females and cooperative breeders, as well as species with less labile templates. Color-producing mechanisms interact and are the main predictors of plumage ornamentation and elaboration, with little influence of selective pressures in their evolution. Dichromatism, however is influenced by social system and the loss of iridescence. Our results show the importance of considering both selection and constraints, and the different roles that they may have, in the evolution of ornamentation and dimorphism.

Sex-specific fitness effects of unpredictable early life conditions are associated with DNA methylation in the avian glucocorticoid receptor.

Organisms can adapt to variable environments by using environmental cues to modulate developmental gene expression. In principle, maternal influences can adaptively adjust offspring phenotype when early life and adult environments match, but they may be maladaptive when future environments are not predictable. One of the best-studied 'maternal effects' is through modification of the offspring's hypothalamic-pituitary-adrenal (HPA) axis, the neuroendocrine system that controls responses to stress. In addition to the direct transfer of glucocorticoids from mother to offspring, offspring HPA function and other phenotypes can also be affected by epigenetic modifications like DNA methylation of the glucocorticoid receptor promoter. Here we examine how among-year variation in rainfall is related to DNA methylation during development and fitness in adulthood in the superb starling (Lamprotornis superbus), which lives in a climatically unpredictable environment where early life and adult environments are unlikely to match. We found that DNA methylation in the putative promoter of the glucocorticoid receptor gene is reduced in chicks - particularly in males - born following drier prebreeding periods. Additionally, DNA methylation is lower in males that become breeders than those that never breed. However, there is no relationship in females between DNA methylation and the likelihood of dispersing from the natal group to breed elsewhere. These results suggest that early life conditions may positively affect fitness in a sex-specific manner through chemical modification of an HPA-associated gene. This study is the first to show that epigenetic modifications during early life may influence the fitness of free-living organisms adapted to unpredictable environments.

The fitness consequences of kin-biased dispersal in a cooperatively breeding bird.

Cooperative alliances among kin may not only lead to indirect fitness benefits for group-living species, but can also provide direct benefits through access to mates or higher social rank. However, the immigrant sex in most species loses any potential benefits of living with kin unless immigrants disperse together or recruit relatives into the group in subsequent years. To look for evidence of small subgroups of related immigrants within social groups (kin substructure), we used microsatellites to assess relatedness between immigrant females of the cooperatively breeding superb starling, Lamprotornis superbus. We determined how timing of immigration led to kin subgroup formation and if being part of one influenced female fitness. Although mean relatedness in groups was higher for males than females, 26% of immigrant females were part of a kin subgroup with a sister. These immigrant sibships formed through kin recruitment across years more often than through coalitions immigrating together in the same year. Furthermore, females were more likely to breed when part of a kin subgroup than when alone, suggesting that female siblings form alliances that may positively influence their fitness. Ultimately, kin substructure should be considered when determining the role of relatedness in the evolution of animal societies.

Expected Shannon Entropy and Shannon Differentiation between Subpopulations for Neutral Genes under the Finite Island Model.

Shannon entropy H and related measures are increasingly used in molecular ecology and population genetics because (1) unlike measures based on heterozygosity or allele number, these measures weigh alleles in proportion to their population fraction, thus capturing a previously-ignored aspect of allele frequency distributions that may be important in many applications; (2) these measures connect directly to the rich predictive mathematics of information theory; (3) Shannon entropy is completely additive and has an explicitly hierarchical nature; and (4) Shannon entropy-based differentiation measures obey strong monotonicity properties that heterozygosity-based measures lack. We derive simple new expressions for the expected values of the Shannon entropy of the equilibrium allele distribution at a neutral locus in a single isolated population under two models of mutation: the infinite allele model and the stepwise mutation model. Surprisingly, this complex stochastic system for each model has an entropy expressable as a simple combination of well-known mathematical functions. Moreover, entropy- and heterozygosity-based measures for each model are linked by simple relationships that are shown by simulations to be approximately valid even far from equilibrium. We also identify a bridge between the two models of mutation. We apply our approach to subdivided populations which follow the finite island model, obtaining the Shannon entropy of the equilibrium allele distributions of the subpopulations and of the total population. We also derive the expected mutual information and normalized mutual information ("Shannon differentiation") between subpopulations at equilibrium, and identify the model parameters that determine them. We apply our measures to data from the common starling (Sturnus vulgaris) in Australia. Our measures provide a test for neutrality that is robust to violations of equilibrium assumptions, as verified on real world data from starlings.

Bateman's principle is reversed in a cooperatively breeding bird.

Bateman's principle is not only used to explain sex differences in mating behaviour, but also to determine which sex has the greater opportunity for sexual selection. It predicts that the relationship between the number of mates and the number of offspring produced should be stronger for males than for females. Yet, it is unclear whether Bateman's principle holds in cooperatively breeding systems where the strength of selection on traits used in intrasexual competition is high in both sexes. We tested Bateman's principle in the cooperatively breeding superb starling (Lamprotornis superbus), finding that only females showed a significant, positive Bateman gradient. We also found that the opportunity for selection was on average higher in females, but that its strength and direction oscillated through time. These data are consistent with the hypothesis that sexual selection underlies the female trait elaboration observed in superb starlings and other cooperative breeders. Even though the Bateman gradient was steeper for females than for males, the year-to-year oscillation in the strength and direction of the opportunity for selection likely explains why cooperative breeders do not exhibit sexual role reversal. Thus, Bateman's principle may not hold in cooperative breeders where both sexes appear to be under mutually strong sexual selection.

Developmental telomere attrition predicts impulsive decision-making in adult starlings.

Animals in a poor biological state face reduced life expectancy, and as a consequence should make decisions that prioritize immediate survival and reproduction over long-term benefits. We tested the prediction that if, as has been suggested, developmental telomere attrition is a biomarker of state and future life expectancy, then individuals who have undergone greater developmental telomere attrition should display greater choice impulsivity as adults. We measured impulsive decision-making in a cohort of European starlings (Sturnus vulgaris) in which we had previously manipulated developmental telomere attrition by cross-fostering sibling chicks into broods of different sizes. We show that as predicted by state-dependent optimality models, individuals who had sustained greater developmental telomere attrition and who had shorter current telomeres made more impulsive foraging decisions as adults, valuing smaller, sooner food rewards more highly than birds with less attrition and longer telomeres. Our findings shed light on the biological embedding of early adversity and support a functional explanation for its consequences that could be applicable to other species, including humans.

A comparison of single nucleotide polymorphism and microsatellite markers for analysis of parentage and kinship in a cooperatively breeding bird.

The development of genetic markers has revolutionized molecular studies within and among populations. Although poly-allelic microsatellites are the most commonly used genetic marker for within-population studies of free-living animals, biallelic single nucleotide polymorphisms, or SNPs, have also emerged as a viable option for use in nonmodel systems. We describe a robust method of SNP discovery from the transcriptome of a nonmodel organism that resulted in more than 99% of the markers working successfully during genotyping. We then compare the use of 102 novel SNPs with 15 previously developed microsatellites for studies of parentage and kinship in cooperatively breeding superb starlings (Lamprotornis superbus) that live in highly kin-structured groups. For 95% of the offspring surveyed, SNPs and microsatellites identified the same genetic father, but only when behavioural information about the likely parents at a nest was included to aid in assignment. Moreover, when such behavioural information was available, the number of SNPs necessary for successful parentage assignment was reduced by half. However, in a few cases where candidate fathers were highly related, SNPs did a better job at assigning fathers than microsatellites. Despite high variation between individual pairwise relatedness values, microsatellites and SNPs performed equally well in kinship analyses. This study is the first to compare SNPs and microsatellites for analyses of parentage and relatedness in a species that lives in groups with a complex social and kin structure. It should also prove informative for those interested in developing SNP loci from transcriptome data when published genomes are unavailable.

An experimental demonstration that early-life competitive disadvantage accelerates telomere loss.

Adverse experiences in early life can exert powerful delayed effects on adult survival and health. Telomere attrition is a potentially important mechanism in such effects. One source of early-life adversity is the stress caused by competitive disadvantage. Although previous avian experiments suggest that competitive disadvantage may accelerate telomere attrition, they do not clearly isolate the effects of competitive disadvantage from other sources of variation. Here, we present data from an experiment in European starlings (Sturnus vulgaris) that used cross-fostering to expose siblings to divergent early experience. Birds were assigned either to competitive advantage (being larger than their brood competitors) or competitive disadvantage (being smaller than their brood competitors) between days 3 and 12 post-hatching. Disadvantage did not affect weight gain, but it increased telomere attrition, leading to shorter telomere length in disadvantaged birds by day 12. There were no effects of disadvantage on oxidative damage as measured by plasma lipid peroxidation. We thus found strong evidence that early-life competitive disadvantage can accelerate telomere loss. This could lead to faster age-related deterioration and poorer health in later life.