Tuesday, December 23, 2008

Golden-cheeked Warbler!

Finally we had one in the flock this morning! This was our last flock at the Moxviquil site, and what a nice big juicy flock it was. It had 45 members from 18 different species, including 5 vireos of 4 different species. Vireos are everywhere in these oak forests; Blue-headed of course, and Warbling, and because this is Mexico also Hutton’s Vireo. But a bird I didn’t expect to see here was Cassin’s Vireo. Intermediate between Blue-headed and Plumbeous – both of which I’d seen in Honduran pine-oak forests – this was a first for me in these warbler flocks. But I saw it repeatedly, and every time it appeared subtly yet distinctly different from the two Blue-headed Vireos that were also part of this flock.** Other attractive flock members included a Red-faced Warbler, a female Rose-throated Becard, and a couple of MacGillivray’s Warblers. Most abundant flock member today was Bushtit – a species whose membership of insectivorous passerine flocks is perhaps debatable. They travel in dense flocks and stick close together, seemingly doing their own thing. Yet for the entire four hours that we observed this flock, we never saw them away from the other flock members. To me it seemed they totally paid their flock dues and deserved membership wholeheartedly. They were 17 strong, and formed a flock within a flock. Their behavior reminded me of Common Bush-Tanagers, who will also occasionally join mixed flocks but always stay close together.

Considering the size of the flock, it was remarkably light on Dendroicas. Besides the adult male goldencheek, there were five Townsend’s Warblers, but no other Dendroicas.

** Postscript: Oliver Komar suggests the Cassin's Vireo may have belonged to the Central American race of Plumbeous Vireo, or that it may have been a young Blue-headed Vireo. I don't have all my field guides here with me, so I cannot delve deeper into this. All I know is that the bird's head was less dark (less 'blue') than a regular Blue-headed Vireo, yet not as pale as a classic Plumbeous Vireo, a bird that is grey overall. This bird had a yellowish-greenish wash on the sides, while the edge between the dark hood and the white throat wasn't as sharply demarcated as in Blue-headed Vireo.

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