Data from: Forecasting the future? Differential allocation of maternal hormones in function of experimentally manipulated social contexts in blue tits Cyanistes caeruleus

Across the tree of life, there is a significant variability in sociability and, interestingly, this also holds at the intra-specific level, with some individuals being consistently more sociable than others. This variation may be maintained because in some contexts, e.g., more socially dense conditions, individuals with a higher sociability are likely to thrive better, while it might be more beneficial to be less social if there is a high prevalence of socially transmitted diseases. As mothers know about their offspring’s postnatal social environment, they might benefit from conveying that information by transferring non-genetic compounds (such as hormones) via the placenta or via allocation into the eggs. Such maternal hormones could prepare the offspring to match the social context that they will experience after birth. 

To test this hypothesis, we experimentally manipulated the social density as perceived by blue tit (Cyanistes caeruleus) females before egg laying. We then collected one egg per clutch to measure the maternal yolk testosterone concentrations. Subsequently, we investigated the social interactions of the nestlings with their siblings (by means of social network analyses) and parents (i.e., begging intensity, feeding rates).