Female begging calls reflect nutritional need of nestlings in the hen harrier Circus cyaneus

Background: Most birds exhibit bi-parental care with both sexes providing food for their young. Nestling signal food needs through begging. However, for some species, males rarely visit the nest, so have limited opportunity for gaining information directly from the chicks. Instead, females beg when males deliver food. We tested whether this calling signalled nutritional need and specifically the needs of the female (Breeder Need hypothesis) or that of their chicks (Offspring Need hypothesis).

Results: We observed begging and provisioning rates at 42 nests of hen harrier (Circus cyaneus) in Scotland, explored the factors associated with variation in begging rate and the relationship between begging and provisioning. We also tested the impact of food on begging and provisioning through a feeding experiment. Female begging rate increased up to a chick age of 3 weeks and then tailed off. In addition, begging increased when broods were large.

Conclusions: Our data provided support for the Offspring Need hypothesis. At nests where adlib food was provided females reduced their begging rate. These patterns suggested that female begging was an honest signal of need. However, begging continued even with adlib food and was only weakly associated with greater provisioning by males, suggesting that these calls may also play an additional role, possibly reflecting sexual or parent-offspring conflict.