Why do some seed-eating passerines feed insects to their nestlings?

Even seed-eating passerines, such as finches, often rely on insects to feed their nestlings because growing young require high levels of protein for rapid development. While adult birds may specialize in a diet of seeds, insects provide the essential nutrients necessary for the structural growth of nestlings that seeds alone may not sufficiently supply.1

This behavior is part of a broader pattern where songbirds prioritize the nutritional needs of their offspring, often at the expense of their own body condition, sometimes experiencing marked weight loss while actively foraging to provide prey for their young. Although the majority of passerines are insectivorous at some stage of their lives, this dietary shift during the nesting period highlights the specific physiological demands of early development.21