Arnaud Da Silva

The disruption of daily rhythms is one of the most studied ecological consequences of light pollution. Previous work showed that several songbird species initiated dawn song earlier in areas with light pollution. However, the mechanisms underlying this shift are still unknown. Individuals may immediately adjust their timing of singing to the presence of artificial light (behavioural plasticity), but the observed effect may also be due to phenotype-dependent habitat choice, effects of conditions during early life or micro-evolution. The main aim of this study was to experimentally investigate how males of four common passerine species respond to day-today variation in the presence of artificial night lighting in terms of the timing of singing. During two consecutive breeding seasons, we manipulated the presence of light throughout the night in a cyclic fashion in several naturally undisturbed forest patches. We show that individuals of all four species immediately and reversibly adjusted their onset of dawn singing in response to artificial light. The effect was strongest in the European robin, but relatively small in the blue tit, the great tit and the blackbird. The effect in the latter two species was smaller than expected from the correlational studies. This may be coincidence (small sample size of this study), but it could also indicate that there are longer-term effects of living in light-polluted urban areas on timing of dawn singing, or that birds use compensatory behaviours such as light avoidance. We found no evidence that our light treatment had carryover effects into the subsequent dark period, but robins progressively advanced their dawn singing during the light treatment. The singing behaviour of many songbird species is characterized by a peak of song production around sunrise, i.e. the dawn chorus, and previous work suggests that the timing of dawn song initiation may be regarded as a reliable predictor of male quality (Grava, & Kempenaers, 2006). Several studies suggest that artificial night lighting advances the start of the dawn chorus in songbirds (Da

Barn owl (Tyto alba) siblings preen and offer food items to one another, behaviours that can be considered prosocial because they benefit a conspeci-fic by relieving distress or need. In experimental broods, we analysed whether such behaviours were reciprocated, preferentially exchanged between specific phenotypes, performed to avoid harassment and food theft or signals of hierarchy status. Three of the results are consistent with the hypothesis of direct reciprocity. First, food sharing was reciprocated in three-chick broods but not in pairs of siblings, that is when nestlings could choose a partner with whom to develop a reciprocating interaction. Second, a nestling was more likely to give a prey item to its sibling if the latter individual had preened the former. Third, siblings matched their investment in preening each other. Manipulation of age hierarchy showed that food stealing was directed towards older siblings but was not performed to compensate for a low level of cooperation received. Social behaviours were related to melanin-based coloration, suggesting that animals may signal their propensity to interact socially. The most prosocial phenotype (darker reddish) was also the phenotype that stole more food, and the effect of col-oration on prosocial behaviour depended upon rank and sex, suggesting that colour-related prosociality is state dependent.

ABSTRACT When competing over parental resources, young animals may be typically selfish to the point of siblicide. This suggests that limited parental resources promote the evolution of sibling competition rather than altruistic or cooperative behaviours. In striking contrast, we show here that in 71% of experimental three chick broods, nestling barn owls, Tyto alba, gave food to their siblings on average twice per night. This behaviour prevailed in the first-born dominant nestlings rather than the last-born subordinate nestlings. It was also more prevalent in individuals displaying a heritable dark phaeomelanin-based coloration, a typical female-specific plumage trait (owls vary from dark reddish to white, females being on average darker reddish than males). Stealing food items from siblings, which occurred in 81% of the nests, was more frequent in light than dark phaeomelanic dominant nestlings. We suggest that food sharing has evolved in the barn owl because parents store prey items in their nest that can be used by the offspring to feed their nestmates to derive indirect (kin selection) or direct benefits (pseudoreciprocity or by-product mutualism). The cost of feeding siblings may be relatively low for dominant individuals while the indirect genetic benefits could be high given that extrapair paternity is infrequent in this species. Thus, in situations in which young animals have access to more food resources than they currently need, they can altruistically share them with their siblings.