Sunday, February 03, 2008

Mixed-Species Flock

I've been seeing a lot of cedar waxwings and robins recently, and I'm used to seeing them together, but Saturday in our backyard was the biggest mixed flock of these two species Ive ever seen. Forty birds shooting this way and fifty coming back that way, a clamor of robin whinnies and waxwing whistles from ever tree and bush. The robins numbered in the hundreds and waxwings outnumbered them, but in the whirling bickering flock, I couldn't really tell how many hundreds there were. There were American and Lesser Goldfinches, Chestnut-backed Chickadees, Oak Titmice and at least one Bewick's Wren feeding with the flock. Anything with berries on it was covered in birds, coming, filling their bellies and quickly being replaced by the next wave. I took the opportunity to take a couple of hundred photos, although most of them ended up with streaks as other birds flew into, out of, or through the frame. Some of the best shots are in this album.

I also took the opportunity to teach Iris how to identify these species. She asked the excellent question of why all there birds were moving around together, when it seemed like they were all squabbling over the same food. This is actually a questions several scientists have looked into, and the answer seems to be that like most things, one has to consider the costs and benefits. When the costs of participating in the flock (competition for food, time lost arguing) are outweighed by the benefits (protection in numbers from predators, more guides looking for food) then the birds flock. In the case of waxwings and robins, they are looking for a very patchy resource. While a bush is making berries, it is hard for even a huge flock to use up all the food, but the good berry bushes may be rare. So the cost of competition is low, because there is so much food on each bush, but the benefit is high, because by following the flock one doesn't have to waste time finding the food. Go where the flock is loudest and there is sure to be a bush covered in berries. And with so many eyes around, no predator could possibly sneak up. I saw one scrub-jay (which will occasionally kill and eat smaller birds) being chased by about 40 waxwings and a couple of chickadees. Once the jay was well away, they all went back to feeding.

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