Data from: Within-family parent-offspring co-adaptation in a wild bird: on static traits, behavioural reaction norms and sex differences

Parental care, a central component of reproduction in a wide range of
animal species, often involves elaborate behavioural interactions between
parents and their offspring. Due to the reciprocal nature of these
interactions, it has been hypothesized that parental and offspring
behaviours (e.g. parental food provisioning and offspring begging) are not
only target but also agent of selection. These traits are therefore
expected to co-evolve, ultimately leading to co-adaptation of parent and
offspring behaviours within families. However, empirical data on such
parent-offspring co-adaptation are limited, particularly for wild
populations. Furthermore, mean levels of behaviour (as measured in
previous studies) may not adequately describe the dynamic nature of the
reciprocal interplay between parents and their offspring, and instead
rather the behavioural reaction norms for provisioning and begging may be
co-adapted. We applied a large-scale cross-fostering study over 3
consecutive breeding seasons to investigate whether provisioning behaviour
of wild blue tit (Cyanistes caeruleus) parents co-varies with the begging
behaviour of their genetic, cross-fostered offspring. We simultaneously
analysed parent and offspring behaviours, both as static traits (mean
levels) and behavioural reaction norms (offspring begging as a function of
food deprivation and parental provisioning as a function of short-term
experimental changes in brood size). Neither maternal nor paternal
provisioning rates co-varied with the begging intensity of their genetic
offspring when analysed as mean levels of behaviour. However, the slopes
of the reaction norms for provisioning and begging were negatively
correlated between male, but not female, parents and their genetic
offspring. Thus, fathers that change their provisioning rate strongly with
brood size sire offspring whose level of begging only weakly increases
with hunger, and vice versa. The observed co-variation suggests the
existence of sex-specific optima for parent-offspring trait combinations.
Thus, our study not only highlights the importance of a behavioural
reaction norm approach when investigating parent-offspring interactions,
but also stresses the relevance of considering parents as separate units,
at least for biparental species.