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In searching for info to answer my question, I’ve found a fascinating 1998 article by Owens and Hartley from Proc. R. Soc. Lond. B. It reports such cool findings as that

When we break up plumage-colour dimorphism according to whether it is due to melanins, carotenoids or structural colours, we find that each category of plumage-colour dimorphism shows a different pattern of covariation. The correlation between overall plumage-colour dimorphism and the rate of extra-bond paternity is due to structural colours, whereas melanin-based dimorphism is associated with sex differences in parental care.

Intriguing, but not on point for my question; so I turn to StackExchange.

Is it known why, for example, northern cardinals (Cardinalis cardinalis) are essentially monomorphic in size but dimorphic in plumage color, American robins (Turdus migratorius) and blue jays (Cyanocitta cristata) are pretty much monomorphic for both traits, and the sexes in the brown-headed cowbirds (Molothrus ater) vary so in size and coloration that they could easily be mistaken for separate species?

Whether it involves caring for young or pair monogamy or whatever, how does one species arrive at a particular morphological-behavioral complex while another ends up someplace entirely different? Is that entirely a matter of evolutionary contingency?

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