What crucial macronutrient source pivots the White-crowned Sparrow diet toward during the breeding season?

High levels of protein from insects and other arthropods

The feeding strategy of the White-crowned Sparrow demonstrates remarkable plasticity, characterized by a major dietary transition synchronized with its breeding cycle. While seeds and grains dominate the diet when nesting is not occurring, the shift to animal matter is sharp as spring approaches and breeding commences. This increased consumption of insects and arthropods is essential because the animal matter provides the necessary high levels of protein required for two primary biological demands: the female bird's need for energy during egg production, and the rapid tissue growth characteristic of developing nestlings. This reliance on protein for chick-rearing is a common evolutionary adaptation among sparrows, but the White-crowned Sparrow shows this reliance starkly, moving from a nearly entirely vegetarian diet in winter to one heavily supplemented by insects in summer to meet these intensive metabolic requirements.