Q. Why don’t hawks fight back when crows mob them? They seem almost indifferent to the attacks, like one I saw yesterday. He just sat at the top of the tree as the crows dove at him. He didn’t budge. Why would he do that?

A. Mobbing is a commonly observed action in which several smaller birds harass a larger predatory bird, a mammal or a snake. I think the simplest way to view the hawk’s behavior is from a cost-benefit perspective. Crows are not really a threat to hawks in most instances; they are only pests, like a fly buzzing around would be to us. Under normal mobbing situations, the hawks probably do not get physically attacked and they determine that retaliation is simply not worth the effort. No doubt there are plenty of exceptions to this scenario, but that is the basic model.

Asking why a hawk would let a bunch of smaller birds, such as crows, pester it is one question. A counter question is why would a smaller bird risk attacking a larger predator like a red-tailed hawk, bald eagle or great horned owl? The answer, as is usually the case when animal behavior and ecology are involved, is complex. I have spoken to several ornithologists over the years about this phenomenon. Some explanations seem quite reasonable and relatively straightforward, whereas others are more speculative.

From the crows’ perspective, mobbing behavior may have adaptive significance in terms of survival: a large predatory animal may be driven from an area where crows raise their young. The babies could become prey for some raptors. In a situation in which a predator such as a large hawk is simply in between meals, either sitting or flying, and has no special stake in a particular location, mobbing behavior by crows could be effective in driving the hawk away. For its part, the hawk would presumably find the pesky crows not worth the effort of staying around and would move on to another area to hunt. In other words, the crows do not want the predator in the area, and the hawk itself does not particularly care whether it is there or somewhere else.

What might a pair of hawks with a couple of babies in a nest do if harassed by crows? One ornithologist said, “I have never seen crows approach when young were present. Birds of prey will fiercely protect their nests.” Most agreed that crows become much more cautious when hawks have eggs or young. In that context the crows would be viewed as a potential threat to the hawks’ reproductive success. The crows would no longer be merely pests. All the ornithologists I spoke with agreed that if a red-tailed hawk reached out and grabbed an offending crow with its talons, that would be the end of the crow. As one put it in nonscientific but easy-to-understand terms, “The crow would be toast.”

One colleague who worked with bald eagles noted, “None of us have ever seen crows or raccoons be predators on an eagle nest with eggs or young in it.” Another said, “I don’t know for sure, but when there’s a nest involved, the stakes are higher, and the raptor would probably fight back.” The fact that such attacks on raptor nests seldom occur may be because crows already know the answer about the consequences of entering an eagle’s no-fly zone. However, exceptions undoubtedly occur and sometimes the big predator’s eggs or offspring become the smaller predator’s prey.

Observations of the nonchalant manner in which hawks respond when crows engage in mobbing behavior are frequent. The outcome of any particular encounter is problematic, and variations to all the general rules about animal interactions are rampant. One thought, which has no scientific merit but is fun to consider, what if we are looking at the wrong side of the equation? Maybe hawks get bored and amuse themselves by sitting in trees just to annoy the crows.

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