I can't name species, but I've long had the impression that Barn Swallows
chased other birds. Maybe it's just more practice to develop effective
bug-catching techniques, maybe it is indeed to chase them out of a feeding
territory, even though they aren't really competitors. It may be easier to
program an animal to be aggressive toward anything of a certain size rather
than fine-tuning it for recognition of just those species with which it is
most directly competing. Just gleaning insectivores (like warblers and
chickadees) come in a great range of sizes and colors, and I doubt if a
bird could be programmed to distinguish an insectivorous from a granivorous
bill shape, for example. Same with swallows. They compete directly with
flycatchers but not with the similar-looking vireos, so they should just
chase any small bird that is flying in "their" air space. Not that a
kingfisher or a spotted sandpiper is so small, but birds with agile flight
capabilities seem more than able to chase larger birds.

I think if we watched birds still more closely than we usually do, we'd
probably see an amazingly diverse set of interactions, species x with
species y to the nth degree.

Dennis Paulson, Director phone: (206) 756-3798
Slater Museum of Natural History fax: (206) 756-3352
University of Puget Sound e-mail: dpaulson at ups.edu
Tacoma, WA 98416