Thirteen ways of looking at a warbler flock

Contrary to where I said we would be in the previous post, we’re still in San Cristóbal. It now looks like we’ll be traveling to our last field site tomorrow, so we can start with field work there Saturday morning.

This extra day off gave me an opportunity for some casual birding around the house. I decided to look for the flock that a couple of days ago had three Golden-cheeked Warblers in it, in hopes of getting some halfway decent photos of these birds. That, alas, did not transpire. I was able to relocate the flock, and I did see one goldencheek in there (again the ‘white-throated’ individual), but photo ops were hardly better this time around and I was OK with getting a couple of shots of other flock members.

This is a Townsend’s Warbler of course, while the bird at the top is a Crescent-chested Warbler.

I found the flock just north of Pronatura’s Moxviquil visitor center, in exactly the same place as a couple of days ago. And again, they went up the hill to Dead Dog Hole. But as I casually looked through the flock, I ruminated on what constitutes a flock, who are its members and who are just passers-by, and how do you become a member? In this flock, which I assumed to be the exact same flock from two days ago, I found several birds not encountered previously. There was a Nashville Warbler, for example, an uncommon species around here; a Yellow-bellied Sapsucker (also not common); a Guatemalan (Northern) Flicker; and finally a Western Tanager, which I didn’t see but heard only. I certainly did not stay long enough with this flock to identify all of its members – after all, this was my day off – but I assumed the Townsend’s, Crescent-chested and Wilson’s Warblers, and the Blue-headed, Warbling and Hutton’s Vireos that I saw to all be the same individuals we observed two days ago. There were Bushtits also, dubious flock members. Previously, in earlier flocks, I had counted them in, because they seemed to go where everybody else was going. Lately, I’m not so sure. A notable absentee today was Greater Pewee, a vocal species that is among the easiest birds to identify in a flock. Where was it today? Did it perhaps also have a day off? Or take Dusky-capped Flycatcher, another vocal bird. Rarely seen but often heard at the edge of the flock, giving its wheezy call. Sometimes they seem to move with the flock, and sometimes they don’t.

Each morning, we look for flocks and when we find one, we describe it by noting its members. The next day, we move on to a different flock. With this approach, it is easy to get the impression that every flock you find is by and large composed of the birds you described as its members, and that the next day or the next week, that flock will be there in the same location, with the same members.

But I’ve wondered before just exactly how tight or loose these mixed species associations are anyway. Do the same flock members associate with each other throughout the winter in that same spot? Or is there some kind of osmosis going on, with birds casually entering and leaving the flock over the season? Do individual birds switch flocks occasionally; do they sometimes feed just by themselves?

Finally, more time also today to look for butterflies here, but all I found were very common species, like the Mexican Silverspot pictured above. Still, what a knockout!

The same can be said for this American Lady, a familiar species in the US also.