I contacted Cornell Lab of Ornithology regarding my photo of the Black-billed
Magpie. Below is their response:
Hi, Terry,
I contacted several of our staff concerning these photos including two who
have studied the Black-billed Magpie in the field. I've left their input as
they gave it rather than putting it into my own words. Hope this helps.
My impression is that it is a real Black-billed Magpie.
1. The two species are not that different in size; their tarsi are virtually
identical. It's mostly tail and wings that are longer in black-billed.
2. As far as I can tell from the one photo, yes that is indeed a Black-billed
Magpie. I handled a bunch of them years ago (did my MSc research on them), and
I can't see anything about the proportions and coloration that would suggest to
me that the dark-billed bird was anything other than Black-billed. Regarding
being the same size as the Yellow-billed Magpies around it, I personally have
handled some very small Black-billed Magpies, so the size per se doesn't look
unusual to me. I also would wonder why a melanistic Yellow-billed Magpie could
have a black beak but not show signs of darker elsewhere (i.e. in its plumage).
I also wandered across the hall to ask Walt (I believe he's actually written
papers about Yellow-billed Magpies) his thoughts, and he said that if anything
he things that I remembers reading or hearing people suggest that some
Black-billed Magpies could have lighter bills, but never the other way around.
As for how the bird got there, Walt's thought was that it might have been a
captive bird that we released in the area. If it was a captive-raised bird,
that could even explain the small size. I raised a bunch of nestling
Black-billed Magpies and the captive raised birds were generally smaller than
wild birds.
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