Alternation of nest visits varies with experimentally manipulated workload in brood-provisioning great tits

In species with biparental care, the amount of care devoted to offspring is affected by the negotiation rules that the parents adopt. Recently, turn taking in provisioning visits has been proposed as a negotiation rule by which parents respond to their partner's behaviour, which results in a perfect alternation of the nest visits by the parents. Empirical evidence suggests that parents do not strictly alternate their visits, and, so far, this imperfect alternation has received no experimental investigation. In this study, we tested whether alternation of nest visits might be subject to time constraints affecting the ability of parents to strictly take turns. We manipulated the workload of 15 great tit, Parus major, pairs using a short-term brood size manipulation. Parental nest visits alternated more in reduced than control and enlarged broods. To understand whether this variability could be caused by changes in turn taking, we explored the rate and regularity of the parents' intervisit intervals. Treatment differences in alternation were still present when controlling for the rate and regularity of the visits by each of the two parents, suggesting that workload also affected alternation via the temporal sequence of the intervals (e.g. via turn taking). Our results show that alternation of nest visits varies in response to workload and is not merely a by-product of variation in visit rate or regularity.