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Conflicts are inherent to family systems and may occur at three levels. First, each parent benefits if its mate takes the greater share of parental investment. Second, offspring try to manipulate their parents into devolving more resources than it is optimal for them. Third, siblings compete for resources. Food availability can affect the dynamics of each level of interaction. By means of a food supplementation experiment, we assessed how the initial availability of extra food during breeding affects later parental effort, sibling competition, and parent-offspring interactions in a small dimorphic raptor, the lesser kestrel (Falco naumanni). Being the larger sex, female nestlings are likely to have higher energy requirements. Female-biased broods had a higher rate of aggressive interactions and were fed more frequently in control nests. Food was not shared evenly among broodmates, and daughters tended to receive more feedings than sons. In young broods, parents controlled food allocation by entering the nest, whereas in older broods, offspring controlled food allocation by monopolizing the nest site entrance. Extra food induced male parents to reduce the rate of feedings delivered by entering the nest. Additionally, extra food improved nestling growth in male-biased broods, leading to an increase in the frequency of parental feedings and aggressive interactions, likely due to faster growth rates. These findings reveal a key effect of brood sex ratio in determining family interactions in a species with reverse sexual size dimorphism and suggest that all levels of conflict between family members should be considered simultaneously when investigating the evolution of parental care. Breeding poses energetic costs on parents, and environmental resources are usually limited. To successfully breed, parents coordinate their efforts and manage the allocation of resources to offspring, while offspring communicate their needs. Conflict can arise among family members: each parent benefits if the other takes on most of the workload, offspring ask for more care than what parents can provide, and siblings compete for food. Food availability affects these interaction levels separately, but they have rarely been integrated. By providing extra food to lesser kestrel (Falco naumanni) nests, we show that parental provisioning behaviour and sibling competition are simultaneously affected by initial resource availability during breeding. Although lesser kestrel nestlings show only moderate sex differences in body size (and therefore energy requirements), the effect of extra food on family interactions primarily depended on the sex ratio of the brood.

Nocturnal birds rely on achromatic visual signals to assess rivals and potential mates, but variation in the expression of these displays has been understudied. Here we use UV-visible reflectance spectrometry to study colour variation and the potential signalling function of the dark brown chest and white rump plumage – a colour pattern conspicuously exhibited during twilight courtship displays – in nocturnal Mediterranean Storm Petrels Hydrobates pelagicus melitensis from three islands with different exposures to predation and human disturbance. Human activity may increase perceived predation risk and thus affect activity budgets and the physiological stress response, with possible consequences on the relationship between individual body condition and plumage coloration. We found that chest and especially rump feathers reflected in the ultraviolet (UV) but there was no evidence of sexual dichromatism. However, we found support for a male-specific signalling role of plumage coloration. Indeed, chest yellow–red chroma and rump UV chroma were positively related to body condition in males, but only in the two islands representing the poorer environmental conditions. Therefore, environmental variability can potentially modify the reliability of achromatic displays, with possible consequences on sexual selection patterns in a nocturnal bird., European Fisheries Fund (EFF 2007‐2013) ‐ Fundación Biodiversidad (Spanish Ministry of Food, Agriculture and Environment). Grant Number: AQUA&PAIÑO (FEP‐FB 20122054) European Commission ‐ University of Padova. Grant Number: ACCORDO N. 2017/2018