The Function of Nest-Calls in the Chestnut-Crowned Babbler (Pomatostomus ruficeps)

Despite the predation risk, many birds emit conspicuous vocalisations at the nest. Five general hypotheses can explain nest-calling: stimulating nestling begging, conditioning offspring for fledging, signalling individual investment, coordinating provisioning events and broadcasting brood needs to other carers. Each have their own predictions for the timing of call emission and receiver response, but studies of call function rarely test them all together. I have investigated nest-call function in the chestnut-crowned babbler (Pomatostomus ruficeps), a cooperative passerine whose nest visits are commonly accompanied by ‘prompt’ calls and the structurally related flight call. Using videos from inside babbler nests, I generated a series of statistical models to determine the function of babbler nest-calls, including the specific function of prompt calls. Nest-calls were emitted in 66% of visits, with prompt calls in 45%. I first used binomial generalised linear mixed effects models to predict if carers would nest-call during a visit, and found them most likely in mid-length visits when the carer brings food for chicks that are already begging when the adult enters. Of visits that contained nest calls, I found carers emitted prompt calls in medium-sized groups and when the offspring were silent. During long visits, helpers and birds with food were more likely to prompt call than breeders and unladen birds. Considering the response of other birds to visits containing nest-calls, nestlings were more likely to beg during visits with nest-calls than without but showed no preference for the prompt call. In contrast, carers reduced their provisioning rate in response to prompt calls but not flight calls. Together with the observations of reduced prompt calling as correlates of brood hunger increase, I propose that chestnut- crowned babblers use prompt calls to inform co-carers of nestling satiation. More generally, nest-calling appears to play a role in stimulating brood begging and coordinating provisioning events