Sampling parental cooperation: temporal fluctuations in starling provisioning and implications for methodology

Research on parental cooperation is an emerging subfield of behavioural ecology that often lacks standardized methodological framework. A common limitation is the failure to repeatedly sample parental activity throughout the entire nestling stage, potentially leading to misleading conclusions. Here, we show that the relative contributions of male and female European starlings Sturnus vulgaris to nestling provisioning fluctuate substantially across days. As a result, a single brief observation does not reliably reflect each parent’s share of provisioning. Our analyses indicate that while at least 7 hours of observation on different days is required, ideally 13 hours are needed to produce reliable estimates of parental division of labour. Whether similar temporal variation occurs in other bird species should be assessed when designing sampling protocols for studies of parental cooperation.