Why does the diet of the White-Eyed Vireo shift to incorporate more fruit as the weather cools and migration nears?

Answer

The birds incorporate fruit to build up necessary fat reserves crucial for fueling long migratory flights.

The transition in the White-Eyed Vireo's diet as the warm summer breeding season ends is directly linked to preparation for southward migration. While insects remain a component, the integration of carbohydrate-rich foods like fruits and berries becomes critical. This dietary supplementation is essential because the birds must rapidly accumulate substantial energy stores in the form of fat. These fat reserves serve as the primary fuel source required to sustain the bird throughout the physically demanding, long-distance travel associated with migration. Therefore, the shift toward high-energy berries is a vital physiological requirement driven by impending travel rather than a simple preference change.

Why does the diet of the White-Eyed Vireo shift to incorporate more fruit as the weather cools and migration nears?

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