Monday, August 6, 2012
Solution by William von Herff and Tony Leukering
It was obvious to respondents that this week's quiz bird was a juvenile accipiter, and as is often the case with this genus, answers ran the gamut of possibilities. Quiz newcomer William von Herff provided an excellent analysis of the bird, so I'll start with his take and finish up with some minor additional notes. William?
"Ah, a perching accipiter! And a headless one at that! Oh joy! Anyway, you can get to accipiter because of the slim size, the streaks on the underside, and the barred tail. As well, the streaks mean that it's a juvenile.
"So, now we have to differentiate the three accipiters. Well, it's a juvenile, and you can notice that it has what appears to be a clean vent and undertail coverts. Northern Goshawk would have scattered teardrop-shaped splotches on the vent and undertail coverts and a much more broken and scattered under tail pattern, compared to that of the quiz bird.
"So, on to the Sharpie/Cooper's duo. Normally, we could just look at the head structure from this close up, but this bird is hiding its head shape, so we have to figure it out using other clues. So, let's look over some key ID points. The most well-known juvenile accipiter separator is the streaking: Cooper's has thin streaks that fade away around the belly area, while Sharpie has thicker streaks that go farther down. The quiz bird has thin streaks that fade away at the belly area, so this points to Cooper's. The trait is highly variable, though, so I'll give Cooper's 0.75 points (Sharpie: 0 Cooper's: 0.75). The second feature is shape: Cooper's has a very barrel-shaped chest, while Sharpie has a body shape that gets thinner as it goes down. The quiz bird, while it is at an awkward angle, looks very barrel-shaped. (Sharpie: 0 Cooper's: 1.75). Third of all, leg thickness: Cooper's have fairly thick legs, while Sharpies have pencil-thin legs. This bird seems to have thicker legs, so I'd give another point to Cooper's. (Sharpie: 0 Cooper's: 2.75). Next, there's the terminal band: This is very variable, so whichever gets this one, gets only a half-point. Cooper's has a fairly wide terminal band, while Sharpie has an extremely thin terminal band. The quiz bird seems to have a wide terminal band, so Cooper's gets a half-point (Sharpie: 0 Cooper's: 3.25). Then, there's the tail length: This is a very awkward angle, so I'm going to compare it to the length of the wing. On Sharpie, the wingtip ends at the end of the second band from the body, while in Cooper's, it ends at the very start of that band. In this bird, the wing ends at the start of that band, so another point for Cooper's. (Sharpie: 0 Cooper's: 4.25). Finally, tail shape: Cooper's tends to have a rounded tail, while Sharpie has a straight tail tip with two sharp corners. This bird seems to have a tail leaning more toward Sharpie, so a point for Sharpie. (Final score: Sharpie: 1 Cooper's: 4.25). Now, this duo has plenty of variation in every single aspect that I just mentioned, so that's why I thought it would be better to tally up the field marks, instead of doing elimination ID."
Thanks, William! Some other points that I wished to make on this one are that juvenile Northern Goshawk have very thin white bands bordering the dark tail bands that contrast paler both with the obviously dark bands, but also with the wide pale bands. My last comment is that I would probably have given Cooper's the whole point for tail shape because, though the tail is mostly squared-off, we can see the outermost right rectrix (it is misplaced to the middle of the tail) and it is obviously shorter and lacking a distinct corner at the outer edge of the tip (instead, the outer part of the tip is quite rounded). This is a feature that is diagnostic in differentiating Cooper's Hawk from Sharp-shinned Hawk. Steve Mlodinow took this picture of a juvenile Cooper's Hawk at the Shillapoo Wildlife Area, Douglas Co., Washington, in January 2011.
Incorrect species provided as answers:
Sharp-shinned Hawk - 2
Northern Goshawk - 1
Congratulations to the 17 of 20 respondents getting the quiz correct:
William von Herff
Answer: Cooper's Hawk