Old red eyes is back

Coots have extraordinary eyes. The irises of this one are red:

Coot - The Hall of Einar - photograph (c) David Bailey (not the)

Sometimes, however, their irises are dark red or even brown. I’d love to know why. Sometimes eye colour in birds is a hormonal signal and changes with age or breeding plumage. It’s likely to be a very subtle but important signal of sexual maturity; for a Coot.

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