Why Do Small Birds Chase a Hawk? Numbers Change the Odds
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Prey animals frequently approach a predator rather than fleeing it, calling loudly and diving at it until it leaves. Doing so is dangerous, which makes the reasons worth examining.
What the behaviour looks like
On spotting a predator, an animal that might be expected to hide instead moves towards it while giving a loud distinctive call, which attracts others of its own and of other species until a group has gathered. The group follows the predator, calls continuously, and individuals make repeated diving passes close to it, occasionally striking. The predator is rarely injured and usually leaves. The behaviour is commonest in birds and is well documented in mammals, in fish and in some insects, and the calls used are acoustically different from the calls used for a predator that is actually attacking.
What it achieves
Several benefits have been demonstrated and they are not mutually exclusive:
- •Driving the predator out of the area, which it frequently does
- •Removing surprise, since an ambush hunter that has been seen usually gives up
- •Alerting nearby relatives and neighbours to the danger
- •Teaching young in the group what a predator looks like
- •Attracting a larger predator that may attack the first one
- •Advertising fitness to potential mates, which is disputed
The risk being taken
Approaching something that eats you is obviously dangerous and participants are sometimes killed, which is what makes the behaviour theoretically interesting. Participation is not random. Animals join more readily when the predator is a species that hunts adults rarely, when their own nest is close by, when the group is large, and when they are in good condition. They hold back in the reverse conditions. Distance is calibrated finely, with birds approaching an owl much more closely than a hawk, because an owl in daylight is comparatively helpless while a hawk can turn. The calls themselves encode information, and several species vary the number of notes according to how dangerous the predator is.
What the calls carry
The calls used are not a single undifferentiated alarm and analysis has shown they carry more information than expected. Black-capped chickadees add more notes to the end of a call for a more dangerous predator, with small agile hawks that hunt them effectively producing the longest calls, and other birds respond in proportion. Several species use acoustically different calls for aerial and ground predators, which prompt different responses, one sending the group into cover and the other bringing it out to gather. The calls are also deliberately easy to locate, being broadband and repeated, which is the opposite of the pure tones used when a predator is actually hunting and the caller does not wish to be found.
How other species exploit it
Because the calls attract animals of many species, they have become a general signal that other animals listen to and sometimes abuse. Birds of several species respond to each other's calls and join gatherings, and mammals including squirrels and primates respond to bird calls. Some species give false calls to clear a feeding site, driving competitors away from food by announcing a predator that is not there, which has been documented repeatedly and works until it is used too often. Researchers exploit the response deliberately, playing recorded calls to draw secretive birds into view for counting, which is a standard survey technique and raises its own questions about disturbance.
The takeaway
Approaching a predator while calling and diving at it drives it away, removes its surprise, warns relatives and teaches young what to avoid. Participants are sometimes killed, so the decision is calibrated by how dangerous the predator is, how close the nest is and how large the group is, with birds approaching an owl far closer than a hawk. Some species give false alarms to clear a feeding site.