Birds risk life and limb every time they mob other birds; the question is why
Published: November 1, 2005
|About a dozen crows were going crazy in the woodlot behind our condo. They were giving an interminable volley of loud, raucous calls and taking turns flying and diving toward the upper part of a tree partially hidden from my view. |
I couldn't see the object of their attention, but I was certain it was an owl. After moving some distance, I saw a Great Horned Owl perched next to the trunk, hidden from most of the world, but not from the crows.
The crows' ganging up on the owl was an example of mobbing - a group behavior in which one or more birds harass a potential predator with noisy alarm calls and dive-bombing flight. Crows commonly mob hawks and owls. Songbirds often mob crows and shrikes, as well as hawks and owls.
When a bird discovers a potential predator and gives alarm calls, neighboring birds converge on the predator. Remarkably, alarm calls frequently elicit responses from birds of other species, thus adding even more birds to the fray. Because mobbing can be initiated by sound as well as by sight, some birders whistle the call of a screech-owl to bring songbirds out into the open.
The function of mobbing is varied. It can alert potential prey species that a predator is in the area, thus removing the element of surprise. It might confuse the predator, minimizing its effectiveness, and it can reduce the risk to each individual because of the numbers of mobbers. Mobbing also teaches young birds how to identify predators, and alarm calls are known to suppress vocalizations of nestlings and fledglings. Mobbing can occur at any time of the year, but it is most common during the nesting season, when eggs or young are vulnerable.
I once observed Red-winged Blackbirds mob a Red-tailed Hawk and actually drive it away. At times, the blackbirds seemed close enough for the hawk to grab a bird with its bill or talons, but it didn't. Why?
How close is too close?
Apparently, the Red-tail would rather get away from its harassers than fight them. It probably knows that it can't match the more maneuverable flight patterns of most songbirds. But sometimes mobbing birds do end up in danger. An observer reported watching a crow in Virginia come too close to a Great Horned Owl, which quickly extended a foot, grabbed the crow, and pinned it against its body and the branch on which it was perched. Later it flew away with the crow, presumably intent on a meal.
Whether mobbing offers advantages to mobbers or is largely a deadly risk is still being debated. It's a matter of cost versus benefit. While the anecdotal observation of a predator killing a mobbing bird is hard evidence, it is not thought to be very common.
The mobbing behavior of chickadees has been studied extensively. Chickadees have two different alarm calls: a high-pitched seet, usually given in response to a flying predator, and a loud, broad-band chick-a-dee that is given when the predator is perched or stationary. The chick-a-dee call is also used to maintain flock structure during winter and to help coordinate territorial defense. The calls of flock members are similar and recognizably different from those of other flocks.
Different functions for the chick-a-dee call are possible because the call consists of four syllables (A, B, C, and D) that can be repeated in different combinations. Chick is composed of syllables A, B, and C, while dee is made by syllable D. Which meaning is communicated depends on the number of As, Bs, or Cs in the chick portion and the number of Ds in the dee portion. The call can be much more complex than the simple chick-a-dee-dee-dee. A higher rate of calling, for example, is thought to indicate a predator that is very close or a threat of great urgency.
When different calls communicate different meanings, they are said to be referential. A study published this summer by Christopher Templeton, Erick Greene, and Kate Davis demonstrated that chickadee calls were referential beyond anyone's expectations. The researchers presented 15 live predators (13 birds and 2 mammals) to chickadees in semi-natural conditions in large outdoor aviaries. The elicited alarm calls were quite variable. They were tape-recorded and played back to other chickadees so their mobbing response could be observed.
Predators that posed the greatest threat to chickadees elicited the most complex alarm calls (more syllables), and more of them, than predators of less importance. And the calls elicited greater mobbing behavior from other chickadees. Smaller, more maneuverable predators known to prey on small birds, such as pygmy-owls, elicited a greater response, for example, than did the larger Great Horned Owl, which preys primarily on mammals.
Birds' ability to recognize predators and share that information is an important behavior for survival. That prey species can communicate whether a predator is flying or perched and whether it poses a mild or serious threat is astonishing, and adds to the amazing behavior of birds.