Saturday, March 19, 2011

The costs of mobbing


I went out to investigate a crow mob in the backyard today. It was a very raucous one with about 20 crows, so I was thinking "great-horned owl."

Nope, it was a hawk. I finally caught a glimpse of it when it flew away at my approach (this always happens), and it could have been a small red-tailed or a large Cooper's hawk...or maybe a red-shoulder, or (wishful thinking) a goshawk.

Anyway, I went over to the trees that the crows were in and was surprised to find a dead crow lying on the ground. I poked it with a stick, then touched it and it was flexible and warm with fresh blood on its face. It's eyes (usually the first to decay) looked relatively normal. Conclusion: the hawk killed it! This must have happened as I was just starting into the woods, when a big uproar of cawing erupted and the crows scattered for a minute, but I didn't see anything else fly.

Before this, I had always thought that mobbing in crows was more of a fun thing for them, or at least that they were really in control and safe. Otherwise they wouldn't be doing it. I've even seen the "mobee" attempt an attack on its annoyers, but I still didn't think they were in real danger. This makes me think that mobbing is serious business. Since it carries with it the risk of swift death, yet they still do it, it must be fairly important to them. There probably have been a million papers written on it, but my hunch is that its purpose is to drive potential nest predators (and predators of adults that are on nests) out of the area.

Maybe it was a young bird that got killed. It was regrowing the outermost primary on each wing, so I don't know if that helps age it at all. I'll have to look it up.