No evidence for olfactory kin discrimination in begging blue tit nestlings

Olfactory kin discrimination occurs in many animal taxa, but its potential contribution to commonly observed kin-biased behaviours in birds has rarely been tested. In a previous odour discrimination experiment, 7-day-old blue tit, Cyanistes caeruleus, nestlings showed stronger begging responses to olfactory cues from conspecific nestlings from other nests than from their own. The authors hypothesized olfaction to mediate kin-biased sibling competition in nests with varying relatedness due to extrapair paternity. In the present study, we aimed to test this hypothesis. We therefore replicated the previous experiment with a crucial modification: we cross-fostered two nestlings of each brood the day after hatching. This allowed us to test for olfactory kin discrimination when nestmates differed in relatedness (due to being cross-fostered) but not in familiarity. We ascertained the relatedness of nestlings using genetic parentage assignment. We preregistered our research plan with the Open Science Framework (OSF) to increase research transparency and reduce researcher degrees of freedom. We found that nestlings did not differ in their begging responses to related versus unrelated (cross-fostered) nestmates’ odours, indicating that nestlings do not discriminate kin from nonkin odours when these are both familiar. Moreover, in an exploratory analysis, cross-fostered nestlings did not differ in survival or size from their non-cross-fostered nestmates shortly before fledging, indicating that the presence of unrelated individuals did not affect the distribution of parental care in the nest. In conclusion, we found no evidence for olfactory kin discrimination in begging blue tit nestlings.