Olfactory kin discrimination in begging blue tit nestlings?

Animals rely on their senses to adaptively respond to their social and non-social environment. In many animals, olfaction plays a key role in modulating social behaviour. Olfactory cues are, for example, used to discriminate kin when expressing kin-biased behaviours. Surprisingly, however, in birds the importance of olfaction in mediating social behaviour remains largely unknown, even though they are a key model in sensory ecology. Recently, there has been growing evidence suggesting that birds use olfactory cues in a range of social and non-social contexts. Furthermore, first empirical evidence suggests that competing young in the nest can recognise kin using olfactory cues. A recent experiment on wild blue tit (Cyanistes caeruleus) nestlings has shown that they beg more intensely when presented with olfactory cues from unfamiliar nestlings from a different nest compared to familiar nestlings from the same nest. Such modulation of begging behaviour in response to olfactory cues could potentially cause kin-biased begging behaviour when relatedness among competing nestlings varies due to the frequent occurrence of extra-pair paternity. However, while in natural broods nestmates may differ in relatedness due to extra-pair paternity, they are always equally familiar. Therefore, we repeated the odour discrimination experiment using cross-fostered nestlings which allowed us to test whether blue tit nestlings can also discriminate between odours of related versus unrelated conspecific nestmates which are both familiar. Focal nestlings did not beg longer to the odours of unrelated nestmates than those of related nestmates.104Hence, we found no evidence for olfactory kin discrimination. Before the start of the experiment, we preregistered our hypothesis, study design, and analysis with the Open Science Framework (OSF;https://osf.io/mghr8).