Data from: Sharing the burden: on the division of parental care and vocalizations during incubation

In species with biparental care, individuals only have to pay the costs
for their own parental investment, while the contribution of their partner
comes for free. Each parent hence benefits if its partner works harder,
creating an evolutionary conflict of interest. How parents resolve this
conflict and how they achieve the optimal division of parental tasks often
remains elusive. In this study, we investigated whether lesser
black-backed gulls (Larus fuscus) divide parental care during incubation
equally and whether this correlates with the extent of vocalizations
between pair-members during incubation. We then investigated whether pairs
showing more evenly distributed incubation behavior had a higher
reproductive success. To this end, we recorded incubation behavior and
vocalizations for 24h time periods. Subsequently, we experimentally
increased or decreased brood sizes in order to manipulate parental effort,
and followed offspring development from hatching till fledging. While
incubation bouts were, on average, slightly longer in females, patterns
varied strongly between pairs, ranging from primarily female incubation
over equal sex contributions to male-biased incubation. Pairs contributing
more equally to incubation vocalized more during nest relief, and had a
higher reproductive output when brood sizes were experimentally increased.
Thus, vocalizations and a more equal division of parental care during
incubation may facilitate higher levels of care during the nestling
period, as suggested by a greater reproductive success when facing high
brood demand, or they indicate pair quality.