Why might a bird with structural blue plumage appear greyer on a cloudy day?

Answer

The physics of light scattering are less intense, making the structural reflection fainter.

Structural color depends on light quality; when light is less intense, such as on a cloudy day, the reflection phenomenon weakens, making the underlying grey appearance of the keratin more dominant.

Why might a bird with structural blue plumage appear greyer on a cloudy day?
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