Monday, January 26, 2009
Quiz #284 (2009-1-05) Answer
Click the picture for a larger view.
Answer by Tony Leukering and Mark Gabriel
A fairly new respondent, Mark Gabriel, provided a reasonable answer with which we'll start this shindig.
"These birds are either Brewer's Blackbirds or Rusty Blackbirds. Cowbirds would be stubbier with smaller, more conical bills and no cowbird has the bright yellow irides that these birds show. Grackles would be much longer-tailed than these, with bigger, stronger bills. This leaves the two species in question. I see nothiing in the photo to say that these are not all of the same species, whichever of the two species they are.
"The top three birds, anyway, are black with bright yellow irides, thus are males. The colors of these birds point to Brewer's, which show a much more glossy look, with a purplish or bluish sheen on the head and greenish sheen on the body -- which is what these birds show. Even given the potential difficulties of seeing this in a photo -- since iridescence is dependent on light angle and such -- it seems so clear on my viewing that I think it precludes these birds being Rusties. Also, Rusties would be much less iridescent, especially about the head, than are these birds."
There are other characters that we can use to separate these two species, though one of them was not usable here: Brewer's Blackbirds walk funny! Another one isn't particularly usable in this picture, as it is not shown to great advantage: Brewer's tend to have rounder, smaller heads than do Rusties, a feature that is most apparent when they are walking; the angles of these birds and their scrunched-necked appearance while perched does not provide the proper view. But, the final aspect that I wished to discuss is visible and usable on our quiz birds: bill shape. A (small) number of respondents mentioned this character, which can take a bit of experience to use, but Brewer's Blackbirds have deeper-based, straighter, and shorter bills than do Rusties, which usually show a bit of a droop to the tips of their longer, thinner bills.
Finally, I wanted to talk about plumage states. The non-oriole blackbirds, in general, have a very limited pre-alternate molt or essentially lack it. Thus, plumage change from their winter aspect to their summer aspect is due mostly or entirely to feather wear. As I took the picture in December, the birds are in basic plumage and as Brewer's Blackbirds have little or no pre-alternate molt, they are still in basic plumage come June. We can define the two appearances, however, as winter and summer aspects of basic plumage. Of course, most Brewer's Blackbirds don't even have much of change between those two aspects. However, a minority of male Brewer's Blackbirds approach the plumage appearance of male Rusty Blackbirds, with rusty fringes to head and upperparts (sometimes some underparts). However, Rusty Blackbird in winter aspect almost always show rusty-fringed tertials, something that even the most Rusty-appearing male Brewer's lack. Hopefully. Finally, black blackbirds are very difficult or impossible to age in the field (except Red-wingeds), as most have a nearly complete first pre-basic molt that makes youngsters essentially adult-like in appearance. For some of these species, the underside median coverts are a clue as to age, but correct assessment really requires having the bird in hand, and even there it's not all that easy.
I took this picture of four male Brewer's Blackbirds near Rocky Ford, Otero Co., CO, on 19 December 2002. Two respondents' answers were precluded from being correct for the competition as they omitted the apostrophe in the first word of the species' name.
Tallies of incorrect answers for quiz species:
Rusty Blackbird - 6
The 28 of 34 providing the correct answer:
Ben & Noel Kelly
Answer: Brewer's Blackbird