Friday, February 5, 2010

Plumage details

Our Golden-cheeked Warbler field work in Honduras is done. We described 25 insectivorous mixed-species flocks at 5 different pine-oak forest locations throughout the country, and encountered 32 individual Golden-cheeked Warblers. Some of them were outside the flocks we described. A scientific paper describing the results of our field work and that from four other Central American countries is in preparation. Without going into too much detail, I can report for Honduras that the vast majority of the birds we saw this year were adult males. Most were first seen in a thin-leaved oak species called Quercus sapotifolia - clearly a favorite foraging tree species for Golden-cheeked Warblers on the wintering grounds. Practically every photo I've posted here of goldencheeks shows them in this oak species.

Throughout this season and the previous winter, I have talked here in this blog about plumage details of Golden-cheeked Warblers: how to separate this species from closely related warblers, and within the species how to distinguish the various sex and age classes.

Let's take that last subject to another level. I believe the bird pictured above is an adult male, most likely a second-year male.

In this photo of the same bird, the throat and chin appear solid black, while the eye stripe also appears black or blackish. These are field marks for an adult male Golden-cheeked Warbler. Females have a yellowish or whitish upper chin, while the chins and throats of immature males tend to be black with whitish feather tips.

But look more carefully at the top photo. One of things that's notable in that photo is that the mantle or upperparts are not entirely black. The crown and eye stripe appear black, but the mantle seems dark olive with thick black streaks. That last field mark is better for adult female or a younger male. Adult females and first winter males, however, don't have black chins.

Here's another photo that shows the mantle. Again, the mantle appears dark olive with broad black streaks - not black.

Also, in the lower white wing bar on the median coverts we see dark shaft streaks. These wing bars are completely white on adult males only.

So what we have here is a bird that shows some field marks for an adult male, and some for a younger bird.

Now take a look at this one. Here the sunlight hits the bird sideways, and as we saw earlier, the black on the throat goes up all the way to the chin. However, now we also see some yellow feathers in there that were practically invisible from other angles, for example in the second photo from the top. All these photos are of the same individual. (Go back to that second photo, click on it for a larger view, and you will see a hint of yellowish on the throat...)

I photographed this bird in the morning of January 31, 2010 near Monte Uyuca in Honduras. Over lunch that day, I looked at these photos with my field assistants Kashmir and Fabiola, and with Salvadoran banders Roselvy and Lya. For Roselvy, one of the most experienced banders in Central America and someone well versed in the subtle art of looking at plumage details on warblers, it was clear that this bird had to be a second year male.

During previous field seasons, I sometimes wondered whether it's really possible to separate immature males from adult females in the field. This year, with more field experience with the species, I felt more comfortable assigning sex and age classes to the birds we saw. In some cases, we saw the bird only briefly or from a distance, but in most cases we were able to observe the birds for extended periods of time and from various angles. Paying close attention to plumage details helped us separate different individuals in the same warbler flock, even if we didn't get the opportunity to see them feeding side by side.

This weekend I'm set to count shorebirds in western El Salvador. This will be the fourth year of this count, and the second year of my participation in it. This shorebird count is organized by the Salvadoran Ministry of Environmental Affairs, and each year is repeated in April. I'm hoping our team - Roselvy, Karla and me - will find some interesting species! We're going to do this without a telescope, only binoculars, so finding the rarer species will be quite a challenge...

Shorebirding is very different from following a mixed warbler flock in a forest, yet here too it pays to look for plumage details, as well as structure and behavior. Monday I'll be traveling, but I should be able to post some shorebird photos around mid-week.

No comments: