Begging to differ: Scrubwren nestlings beg to alarm calls and vocalize when parents are absent

Nestling birds face a dilemma: they can increase parental provisioning by begging more intensively, but by doing so may also increase their risk of predation. Nestlings could deal with this dilemma by reducing begging intensity after parents have warned them of a nearby predator. We therefore tested experimentally whether nestling scrubwrens, Sericornis frontalis, increase begging intensity with hunger but reduce it after adult alarm calls. Single 5- and 8-day-old nestlings were temporarily taken into the laboratory for playback experiments. Over a 90-min period of food deprivation we simulated parental visits every 10 min by playing back adult feeding calls. Hungrier nestlings begged louder and longer to simulated parental visits, but contrary to expectation did not beg less if they had previously heard playback of alarm calls, and even begged to the alarm calls themselves. The results were similar for both 'mobbing' and 'flee' alarm calls. Nestlings also gave distinctive calls in the 10-min interval between simulated parental visits, and the number of these calls increased with hunger and after playback of alarm calls. We suggest that nestlings acquire the ability to respond appropriately to alarm calls late in the nestling period and that therefore parents might be selected to avoid alarm calling when defending young nestlings.