Reid, Mary L. 1987. Costliness and reliability in the singing vigour of Ipswich sparrows. Animal Behaviour, 35: 1735-1743.
Summary (AGH): Birds sing mainly to defend territories and attract mates, but why should singing be an effective way to do either of these tasks? Many studies now suggest that singing can be a reliable indicator of a bird’s energy reserves; a male that sings a lot has the energy to be both a formidable opponent that males should avoid, and a healthy mate that females should approach. One of the first such studies was done on Sable Island’s Ipswich Sparrows. It showed that males that have to forage more in the morning, presumably because they are inefficient foragers and/or in poor condition, cannot sing as much, and that providing extra food (mealworms and birdseed) on a male’s territory allowed him to sing more. Both patterns were especially apparent after cold nights and in colder parts of the breeding season, suggesting that singing rate is an especially reliable signal of a male’s energy reserves when conditions are harsh.