Friday, January 2, 2009

Another big flock

We took the day off on New Year’s Day, but today it was back to the harsh reality of looking for birds in the mountains of Chiapas, Mexico. Day two at the Laguna del Cochi site.

Today’s flock was almost a repeat performance of the previous one, with many of the same species. Including another Golden-cheeked Warbler! I have a photo with pine needle clusters in focus, and a blurry blob underneath that is the bird. So it goes.

The bird pictured above is either an adult female or first winter male Townsend’s Warbler. Like many Dendroica warblers – and indeed our study bird, the Golden-cheeked Warbler – young males resemble adult females in plumage.

Today’s Golden-cheeked Warbler was a female – or, I should say, a female-type. Adult males are distinctive, with all-black throats and heavy black breast streaking, blackish also on the upperparts. Today’s bird had whitish fringes on the black throat, especially evident in the center of the throat, and dark olive upperparts. It may have been a first winter male, or an adult female – those plumages are practically indistinguishable in the field. It wasn’t an adult male, like all three previous birds we've seen here.

To get a sense of just how difficult it is to separate males from females, take a look at these museum specimens, from the collection in the American Museum of Natural History in New York City. Left row females, right row males, both arranged from darkest to lightest marked birds. This photo illustrates very well the overlap found between heavily marked females and lightly marked males. Quite likely, the higher birds in both rows are adults and the lower birds immatures.

Again Hermit Warblers in this flock, though not as many as in the previous flock. I got to five today. This bird is also either an adult female or a first winter male. The bib is more pronounced in adult males and practically absent in first winter females.

The entire flock was 60 birds strong. Species with more than one individual in this flock were Blue-headed Vireo, Townsend’s Warbler, Olive Warbler, Brown Creeper, Red-faced Warbler, Crescent-chested Warbler, Wilson’s Warbler, MacGillivray’s Warbler, Bushtit, and Ruby-crowned Kinglet.

Dunn & Garrett (1997) note that Red-faced Warblers join mixed-species flocks in winter, “often with only one Red-faced per flock”. (1) Today’s flock had two, which we were able to separate by the color of the red in the face. The first bird we saw had a relatively dullish red color in the face, while the red on the second bird was much more intense. We saw both birds every now and again during the four hours of flock observation, though never together.

The Birds of North America again has intriguing information regarding this color difference:

Although current field guides suggest that this species is monomorphic, it is dimorphic in face and upper-breast color (TEM, PMB): scarlet (color names from Smithe 1975) in male and flame scarlet to chrome orange in female (TEM, PMB). Colors vary among individuals and may reflect age effects. Juveniles are duller than adults, with head and upperparts sooty brown and nuchal collar and underparts tinged with buff (Curson et al. 1994). Further study of color variation with respect to age or other causes is needed. (2)

Based on this, I’m inclined to think we saw an adult and an immature bird. The BNA account calls this species one of the least studied North American warblers, and it seems even something as basic as plumage hasn’t been described in great detail.

We encounter this species practically every day. I’m going to pay more attention to plumage details from now on.

Finally, here in Chiapas, spring is around the corner: today we saw a female Red Crossbill with nesting material (a small twig), while nearby a male was singing his little heart out. Both in the Old and New World, crossbills are known as early breeders. And since a few days, I hear a Whip-poor-will calling around the house every morning at 5:30 AM, when yours truly is enjoying his breakfast. I'm assuming that bird is also setting up shop.

(1) Dunn & Garrett (1997) A field guide to the warblers of North America; Peterson Field Guide Series.

(2) Martin, Thomas E. and Patricia M. Barber. 1995. Red-faced Warbler (Cardellina rubrifrons), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:

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