[cobirds] Rare warbler diets

On Wednesday afternoon, Curtis Beimborn discovered a Yellow-throated Warbler and a Northern Parula along a section of Boulder Creek just below the university, about 100 yards east of 17th Street in Boulder. Both birds were still present this evening at about 6:00 PM. With the exception of a 24-hour disappearance by the Yellow-throated, both birds have been consistently sticking to the same 50-yard stretch of creek.

This same stretch of creek has also been the center of activity for juncos, goldfinches, nuthatches, Ruby-crowned Kinglets, and Yellow-rumped Warblers. But if you walk more than a few dozen yards away from this spot in either direction, bird numbers drop precipitously. This situation has remained stable for the past three days, and so, Dave Leatherman-style, I decided that the birdy spot must have some kind of food that is keeping all these birds around.

I tried this evening to observe food habits. Interestingly, the two rare warblers are engaging almost exclusively in flycatching -- especially the parula, which sallies constantly. The Yellow-throated's sallies are less frequent and possibly less successful, and it also gleaned some branches while I was there. Whatever they are catching is too small for me to see in the air. And most of the other birds present seem to be following more typical foraging strategies for their species -- i.e. few of them are flycatching.

The rare warblers are frequenting some dead branches over the creek as well as some barely leafed-out elms, cottonwoods, and buckthorns. Dave, what do you think they are going for? Flying adult aphids?

Why would the flying insects be so faithful to this one short stretch of creek? And how long might this situation last? Can we expect other migrants to join the fun in the coming days?

Nathan Pieplow

Boulder

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