Thursday, January 22, 2009

Last flock

That’s right: this morning we described flock number 25 in Coapilla, and at the moment of writing (early evening) have just returned to Moxviquil (San Cristóbal), our home away from home. We’ll be here for a day or two to wrap up the administrative part of the field season, and then for me it’s on to greener pastures. More about that tomorrow.

Fittingly, our last flock had a Golden-cheeked Warbler. It also had a Magnolia Warbler (pictured above). And that’s where things get tricky. The tree where we saw the goldencheek in was 734 m from another tree where two days earlier we had seen an immature female type goldencheek in. That flock also had a Magnolia Warbler… Today’s goldencheek also wasn’t an adult male…

We saw it only for a couple of minutes early in the game, well enough to know what it was, and to note plumage details. This bird appeared to have more dark on the throat than that other individual, and I thought I saw black streaking in the olive upperparts, which the earlier bird didn’t have. Yet, a morsel of doubt lingered and that Magnolia Warbler certainly didn’t help to eliminate that!

I thought we were far enough away from that earlier flock, and continued to observe this flock. Its composition was sufficiently different: it didn’t have a Greater Pewee, which the other flock had; it had two Warbling Vireos, while the other flock had none; and it only had two Black-throated Green Warblers, while the other flock had four.

And it had a Nashville Warbler, also not found in that other flock. In winter, superficially similar to Magnolia Warbler, but with a clean yellow breast and a very different tail pattern. In this photo, even the slight rufous markings on the head are visible.

When we were done describing the flock, I decided to simply walk back to the other flock location, and see which birds we would find over there. That strategy worked fine one time in La Granada, when a similar situation presented itself.

However, as we walked the 700+ meters back, we encountered small numbers of birds here and there. Mostly Wilson’s Warblers, which are practically everywhere anyway, but also a Greater Pewee and a Townsend’s Warbler. Back at the first flock location, there were some birds, but not very many. Mostly Townsend’s Warblers.

I thought this was most unusual. We tend to think of flocks as more or less stationary; go back the next day and you’ll find all the same birds there. But is that really the case?

I said to field assistant Hector that I thought this was a riddle, wrapped in a mystery, inside an enigma.

But perhaps there is a key, I added.

What if these two flocks are siamese twins, a double star? What if they are really only one big flock with two centers, and a corridor between them – the trees planted along the road that connects the two locations – through which osmosis takes place?

As we walked back towards the village, for lunch along the Laguna Verde, we found another Magnolia Warbler. This was maybe 400 m from where I had seen the very first one on an afternoon stroll, and perhaps also 400 m from flock 1, but more than a kilometer from where we saw a Magnolia today, in flock 2. Hey, I said, I’m willing to believe there are two Magnolias, but three Magnolias – at this elevation – seems a stretch. Which one is this?

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