Why do adult Willow Flycatchers avoid feeding large, heavily armored beetles to chicks?
Answer
Time is better spent securing multiple manageable, high-protein caterpillars.
The intense dietary demands of rapidly growing offspring necessitate extreme efficiency for the adult bird. Attempting to process a large, heavily armored beetle for a chick represents a poor use of foraging time because the energy expenditure versus nutritional return is low, and the hard structure is difficult for the developing digestive system to handle. The selective pressure during breeding favors securing several smaller, high-protein, easily digestible items, such as soft-bodied caterpillars, instead of wasting precious time on a single, difficult-to-process beetle.
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