Nest mate size, but not short-term need, influences begging behavior of a generalist brood parasite

Hosts of generalist brood parasites often vary with regards to their life-history traits, and these differences have the potential to influence the competitive environment experienced by brood-parasitic nestlings. Although begging by brood parasites is more exaggerated than their hosts, it is unclear if generalist brood parasites modulate their begging behavior relative to host size. I examined the begging behavior of brown-headed cowbird (Molothrus ater) nestlings when competing against nest mates that differ in size and under different levels of short-term need. Cowbird nestlings begged on nearly all feeding visits, responded to adults as fast as (or faster than) their nest mates, and typically begged more intensively than their nest mates. Latency to beg, time spent begging, and maximum begging posture of cowbirds were similar during supplementation and deprivation treatments, indicating begging intensity was not influenced by short-term need. Time spent begging by cowbirds varied among hosts of 3 different sizes when short-term need was standardized, suggesting that nest mate size strongly influenced begging behavior. Cowbirds obtained more food when competing against an intermediate-sized host due to lower provisioning rates of small hosts or because of increased competitive ability of large host nestlings. Overall, cowbirds obtained the greatest volume of food per unit time spent begging when competing against intermediate hosts, but this value approached that of the small host when adjusted for modal brood size. These results demonstrate that cowbirds adjust their begging relative to the size of the hosts against which they compete but not to levels of short-term need.