Behavioral Domination of Food Delivery by Tree Swallow Nestlings

Abstract

In altricial birds, scramble competition for favorable feeding positions in the nest may cause younger chicks to starve. We studied nestling survival in a population of Tree Swallows (Tachycineta bicolor) breeding in nestboxes near Seattle, Washington, in a year of bad weather. Only a mean of 0.5 chicks fledged per nest (range 0–2, 22 nests). Nestling mortality appeared to be caused by starvation. Video filming showed that when chicks were old enough (largest chick within the brood had reached a wing length of about 55–60 mm), they spent much time in the nestbox opening, calling loudly. This behavior seemed to be driven by hunger because it occurred mainly in broods where chicks had low body masses. Parents fed the chick in the opening and did not try to push it back into the nest cavity or go around it. We suspect that blocking of the nestbox opening accelerated mortality of other young in the nest. Such behavior by dominant chicks may be more readily achieved in hole nesting birds with narrow entrances to their nest site than by open nesting birds.