Black-throated Blue Warbler nest with 3 nestlings (photo by N. Desnoyers).

Breeding birds have a lot of mouths to feed, and successfully raising young depends on being able to find enough food. All songbirds feed their nestlings insects, which are full of the protein growing birds need. Insect availability can affect the number of young that survive, as nestlings can starve when food is limited. Also, the weight of the nestlings when they leave the nest can have an important effect on their future survival. In some species, heavier nestlings have been found to be more likely to survive and reproduce.
Yet the effects of food can also be much less obvious. For example, the strategies of parents can differ depending on how much food is available. Birds can change the number of eggs they lay in each nest depending on how much food they have or expect to have (scientists are still working to figure out what environmental cues birds use to estimate how much food they will have several weeks later).

Also, when birds’ eggs or nestlings get eaten by predators, birds often start over; they build a new nest and lay new eggs. Migratory birds can re-nest after predation several times, depending on when in the breeding season their first nest fails and how much more time they have before they have to leave on migration. Field researchers think that food availability is one of the most important factors affecting how many nests a bird will build in a season. After all, each egg is full of protein and fat, and each nestling has to be fed hundreds or even thousands of insects before it will be big enough and old enough to be independent.

Learning module developed by K. Langin, H. Sofaer and S. Sillett for the Hubbard Brook Research Foundation (2009).