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Eastern Buzzards: what determines the width of the dark trailing edge of underwing? (1 Viewer)


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I am interested in this question, largely because I am researching the difference between Buteo japonicus and Buteo refectus.

Ebird mentions the more prominent dark trailing edge of the underwing of refectus as one separating feature. (The other feature is marginally darker trousers. I don't think that that can be verified confidently on a photo.)

But as I look through photos of japonicus on eBird, some individuals have a thicker dark trailing edge, some have a thinner dark one, and others have a trailing edge that looks dim but not actually dark.

As for photos of refectus, indeed most of these have a slightly thicker trailing edge than the japonicus with the thinner-looking dark trailing edge, but some other photos of refectus show a dim trailing edge.

What determines this? Is it age? Is it the moult within a year? Is it the plumage morph? Is it simply the light that goes through the feathers at the trailing edge, making them brighter on the photo?

In particular, can there be high variability in both the darkness and the width of the trailing edge of light-morph refectus? If so, this would mean that a vagrant japonicus that happened to turn up in the Himalayas might not be identifiable with good-quality photos, even if it had a thin dark trailing edge.

Thanks and cheers!
Upland Buzzard? Gangneung, South Korea (9/2/21) (old) + Buteo identification north Thailand (among the threads linked in the former)
If it doesn't help much, you'll at least know whom to ask.

EDIT: Some more threads:
Note that the taxonomic terminology (both Latin and common names), understanding of ranges, etc., in most--if not all--of the threads may (or may not) be different from the current nomenclature, so you should pay attention and check it first. For broken links try Wayback Machine. Good luck!
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Darker individuals have broader trailing bands on average, but more importantly, adults have those broader than first years. And both refectus and japonicus have those narrower than vulpinus, when comparing the corresponding age classes. Ageing is very important in this case.

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