Animal response to stressors such as harsh environmental conditions and demanding biological processes requires energy generated through increased mitochondrial activity. This results in the production of reactive oxygen species (ROS). In vitro and some in vivo studies suggest that oxidative damage of DNA caused by ROS is responsible for telomere shortening. Since telomere length is correlated with survival in many vertebrates, telomere loss is hypothesised to trigger cellular ageing and/ or to reflect the harshness of the environment an individual has experienced. To improve our understanding of stress‐induced telomere dynamics in non‐human vertebrates, we analysed 109 relevant studies in a meta‐analytical framework. Overall, the exposure to possible stressors was associated with shorter telomeres or higher telomere shortening rate (average effect size = −0.16 ± 0.03). This relationship was consistent for all phylogenetic classes and for all a priori‐selected stressor categories. It was stronger in the case of pathogen infection, competition, reproductive effort and high activity level, which emphasises their importance in explaining intraspecific telomere length variability and, potentially, lifespan variability. Interestingly, the association between stressor exposure and telomeres in one hand, and oxidative stress in the other hand, covaried, suggesting the implication of oxidative stress in telomere dynamics.

Trace metals produced by anthropogenic activities are of major importance in urban areas and might constitute a new evolutionary force selecting for the ability to cope with their deleterious effects. Interestingly, melanin pigments are known to bind metal ions, thereby potentially sequestering them in inert body parts such as coat and feathers, and facilitating body detoxification. Thus, a more melanic plumage or coat coloration could bring a selective advantage for animals living in polluted areas. We tested this hypothesis by investigating the link between melanin-based coloration and zinc and lead concentrations in feathers of urban feral pigeons, both at capture time and after one year of captivity in standardized conditions. Results show that differently coloured pigeons had similar metal concentrations at capture time. Metal concentrations strongly decreased after one year in standardized conditions, and more melanic pigeons had higher concentrations of zinc (but not lead) in their feathers. This suggests that more melanic pigeons have a higher ability to store some metals in their feathers compared with their paler counterparts, which could explain their higher success in urbanized areas. Overall, this work suggests that trace metal pollution may exert new selective forces favouring more melanic phenotypes in polluted environments.

Feathers are inhabited by numerous bacteria, some of them being able to degrade feathers, and thus potentially alter thermoregulation and visual communication. To limit the negative effects of feather bacteria on fitness, birds have therefore evolved antimicrobial defense mechanisms, including preening feathers with secretions of the preen gland. However, whether feather bacteria can alter feather condition and color signaling in vivo, and thus whether birds adjust their investment in preening according to feather bacterial load, has barely been investigated. Here, we experimentally decreased and increased feather bacterial load on captive feral pigeons Columba livia and investigated the effects on plumage characteristics and investment in preening. We found that birds of both sexes had a plumage in higher condition and invested less in preen secretion quantity and preening behavior when feather bacterial load was lower. It suggests that preen secretions may be used by pigeons to limit feather degradation by bacteria, but as they are probably costly to produce, their quantity is adjusted depending on feather bacteria load. Birds with lower bacteria load on feathers had brighter iridescent neck feathers, suggesting that feather bacteria may play an important role in the evolution of the signaling function of iridescent color in pigeons. Altogether, our study provides the first experimental evidence for in vivo effects of feather bacteria on plumage degradation and coloration and suggests that preening is an inducible antibacterial defense.

Trace metals from anthropogenic activities are involved in numerous health impairments and may therefore select for detoxification mechanisms or a higher tolerance. Melanin, responsible for the black and red colourations of teguments, plays a role in metal ion chelation and its synthesis is positively linked to immunity, antioxidant capacity and stress resistance due to pleiotropic effects. Therefore, we expected darker birds to (1) store higher amounts of metals in their feathers, (2) maintain lower metal concentrations in blood and (3) suffer less from metal exposure. We exposed feral pigeons (Columba livia) exhibiting various plumage darkness levels to low, but chronic, concentrations of zinc and/or lead, two of the most abundant metals in urban areas. First, we found negative and positive effects of lead and zinc, respectively, on birds' condition and reproductive parameters. Then, we observed positive relationships between plumage darkness and both zinc and lead concentrations in feathers. Interestingly, though darker adults did not maintain lower metal concentrations in blood and did not have higher fitness parameters, darker juveniles exhibited a higher survival rate than paler ones when exposed to lead. Our results show that melanin-based plumage colouration does modulate lead effects on birds' fitness parameters but that the relationship between metals, melanin, and fitness is more complex than expected and thus stress the need for more studies.

Introduction Rapid environmental change driven by urbanization offers a unique insight into the adaptive potential of urban‐dwelling organisms. Urban‐driven phenotypic differentiation is increasingly often demonstrated, but the impact of urbanization (here modelled as the percentage of impervious surface (ISA) around each nestbox) on offspring developmental rates and subsequent survival remains poorly understood. Furthermore, the role of selection on urban‐driven phenotypic divergence was rarely investigated to date. Methods and Results Data on nestling development and body mass were analysed in a gradient of urbanization set in Warsaw, Poland, in two passerine species: great tits (Parus major) and blue tits (Cyanistes caeruleus). Increasing levels of impervious surface area (ISA) delayed the age of fastest growth in blue tits. Nestling body mass was also negatively affected by increasing ISA 5 and 10 days after hatching in great tits, and 10 and 15 days in blue tits, respectively. High levels of ISA also increased nestling mortality 5 and 10 days after hatching in both species. An analysis of selection differentials performed for two levels of urbanization (low and high ISA) revealed a positive association between mass at day 2 and survival at fledging. Discussion This study confirms the considerable negative impact of imperviousness—a proxy for urbanization level—on offspring development, body mass and survival, and highlights increased selection on avian mass at hatching in a high ISA environment.

1 2Trace metals are chemical pollutants of prime concern nowadays given their implication in 3 several human diseases and their noxious effects on wildlife. Previous studies demonstrated 4 their negative (e.g. lead, cadmium) or positive (e.g. zinc) effects on body condition, immunity 5 and reproductive success in birds. Because of their effects on bird condition, trace metals are 6 likely to influence the production of condition-dependent plumage colours, that may be used 7 in mate choice. In the feral pigeon (Columba livia), we investigated iridescent colouration in 8 response to lead and zinc experimental (i.e. metal supplementation in standardized 9 conditions) and natural exposure (i.e. metal concentrations in feathers of wild urban 10 pigeons), and melanic feather colouration in response to experimental lead and zinc 11 exposure. Both studies (i.e. experimental and correlative) consistently showed that lead 12 exposure decreased iridescent neck feather brightness independently of colour morph. 13Moreover, lead, when provided alone, decreased melanic feather reflectance in the middle 14 wavelengths while zinc supplementation increased melanic feather reflectance in the violet-15 wavelength. In conclusion, our study suggests that the colouration of iridescent and melanic 16 feathers depends on the exposure to pollutants. Whether trace metal exposure affected the 17 ability of birds to produce melanin pigments, to grow the microstructural feather elements 18 required for maximum colour display, or to cope with bacteria that degrade feather 19 microstuctures remains unclear. Future studies should investigate whether these metal-20 induced modifications of plumage colouration affect behaviours involved in sexual selection. 21 22 23

Aposematic prey warn predators of their toxicity using conspicuous signals. However, predators regularly include aposematic prey in their diets, particularly when they are in a poor energetic state and in need of nutrients. We investigated whether or not an environmental factor, ambient temperature, could change the energetic state of predators and lead to an increased intake of prey that they know to contain toxins. We found that European starlings, Sturnus vulgaris, increased their consumption of mealworm, Tenebrio molitor, prey containing quinine (a mild toxin) when the ambient temperature was reduced below their thermoneutral zone from 20 °C to 6 °C. The birds differed in their sensitivity to changes in ambient temperature, with heavier birds increasing the number of toxic prey they ate more rapidly with decreasing temperature compared to birds with lower body mass. This could have been the result of their requiring more nutrients at lower temperatures or being better able to detoxify quinine. Taken together, our results suggest that conspicuous coloration may be more costly at lower temperatures, and that aposematic prey may need to invest more in chemical defences as temperatures decline. Our study also provides novel insights into what factors affect birds' decisions to eat toxic prey, and demonstrates that selection pressures acting on prey defences can vary with changing temperature across days, seasons, climes, and potentially in response to climate change.

No abstract