Some birds flock. Some birds don’t. Several hummingbirds may appear at a feeder, but they are definitely not a flock. Robins flock in winter, but in breeding season form territorial pairs. Ditto swans. Starlings, however, are rarely seen singly, and the same applies to flamingos. Other species gather in large groups to roost at night (crows) or only when migrating (sandhill cranes).
|Geese in "V" formation|
When feeding in groups, one or more birds are always pausing to watch for predators, either the fox on the ground or the diving hawk. Warning sounds and sudden movement by these observers triggers mass evasion by the rest of the flock. In the air or on the ground, being in a group can also hinder a predator from targeting and pursuing one individual bird out of the flock.
Group behavior is seen in species other than birds. Fish school. Mammals form herds. There is a saying about being on a hike in bear country: “You don’t have to run faster than the bear to get away. You just have to run faster than the guy next to you.” Generalized to herd behavior, the best strategy against a predator is to put another individual between you and the predator. A grouping may be somewhat scattered to feed, but cluster inward when predators approach, as each individual tries to move to the center. If the grouping has a familial relationship, vulnerable young may be moved to the center while adults form a protective barrier. Think musk ox or elephants.
A different reason for forming large groups is referred to as “predator satiation.” When prey appear suddenly and in large numbers, the probability of any one individual being killed by predators is reduced, the reason being that the predator population is low before the prey arrive and cannot increase fast enough to take advantage of the sudden influx of food. The sudden appearance of prey in great numbers can be from migration by massive numbers, or by near-synchronization of birth of young, or even skipping years. An extreme example of the last is the 17-year cicada
|Painting of passenger pigeons, male on left (internet download)|
Back to birds. We are some 120 years from one of the greatest species extinctions caused by Man – the end of passenger pigeons. This native North American species – larger and with a much longer tail than the non-native pigeons we now know disparagingly as ‘rats with wings’, numbered in the billions before the arrival of Europeans, and actually well into post-colonial times. Their food of choice was acorns and other tree nuts, but seeds of any plant were also consumed, as was fruit, berries and insects. Individual flocks of migrating birds numbered in the millions, and were described as darkening the sky when passing overhead. From an 1855 description, Columbus Ohio: “As the watchers stared, the hum increased to a mighty throbbing…Children screamed and ran for home. Women gathered their long skirts and hurried for the shelter of stores. Horses bolted. A few people mumbled frightened words…and several dropped on their knees and prayed.”
Roosting at night, the birds were so densely gathered as to break the limbs off trees, and in areas where they nested, droppings piled up a foot thick, poisoning ground plants, and in time, the nesting trees. The birds moved on.
Passenger pigeons, more than other flocking bird species, depended on predator satiation, and so were not able to adapt to human harvesting. The end came quickly. Circa mid-1800s, market hunters used shotguns, nets, whiskey-soaked corn and so on to bring birds to ground. Birds were plucked, gutted, cooked, salted and packed into barrels, to be shipped by railroad to the fast-growing cities of the Industrial Age. Their meat ended up in pigeon pie (think today’s meat pies). At the nesting sites, ground fires would be lit when the nestling squabs were nearing adult weight but not yet able to fly. The squabs would fall to the ground, stunned, to be picked up by hand. As flocks vanished from the usual places, telegraph messages would go out about newly found locations. Market hunters followed.
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