Fitness benefits of alternated chick provisioning in cooperatively breeding carrion crows

In most bird species, parents raise offspring cooperatively. In some cases, this cooperation extends to helpers-at-the-nest who assist the breeders with a range of tasks. While cooperative food provisioning might merely arise incidentally, as a result of the efforts of carers that act independently from each other, recent studies suggest that birds may coordinate by taking turns in visiting the nest. However, evidence that such coordination emerges because individuals actively respond to each other’s behaviour is controversial, and the potential benefits of carers’ alternation remain unknown. We addressed this knowledge gap by analysing a multi-year dataset for cooperatively breeding carrion crows, Corvus corone, comprising 8,693 nest visits across 50 groups. Our results reveal that turn taking does occur in this species and that all group members, regardless of their sex and social role (breeder/helper), tend to alternate at the nest with other carers rather than to make repeat visits. Importantly, we found that the body mass of nestlings increased significantly with the degree of carers’ alternation, possibly because well-coordinated groups provided food at more regular intervals. Using earlier monitoring data, the observed increase in body mass is predicted to substantially boost post-fledging survival rates. Our analyses demonstrate that alternation in nestling provisioning has measurable fitness benefits in this study system. This raises the possibility that cooperatively breeding carrion crows, as well as other bird species with similarly coordinated brood provisioning, exhibit specialized behavioural strategies that enable effective alternation.

Methods
During the breeding seasons of 1999–2000, 2003–2007, 2015, 2018 and 2019, we monitored a total of 50 nests of cooperatively breeding carrion crows Corvus corone with camouflaged small video-cameras. First, we investigated if crow carers took turns when visiting the nest to feed the brood and if the tendency to alternate varied among types of carers (breeders/helper). To do so, we applied the k-categories runs tests, a randomization test that accounts for the “refractory time” between successive visits of the same carer and Continuous Time Random Models. Second, we investigate if better nest visit alternation among carers was linked with higher reproductive output (proportion of chicks fledged) and/or higher nestling quality, using Generalized Lineal Mixed Models. We also performed additional analyses to elucidate whether the correlations found were causal. For details, see Trapote et al. Fitness benefits of alternated chick provisioning in cooperatively breeding carrion crows.