Sunday, September 30, 2007

Intelligence and Instinct - Part 5

Helping others to find food has advantages

Sparrows often tell one another about a good food supply by making chirruping calls from a safe perch. Once a group has gathered, they all begin to feed.

There is a good reason for this unselfish behaviour, however. In Britain, the house sparrow population is thought to number about 12 million. But they have found that life around humans can be dangerous. Domestic cats pose the main threat, and the recovery of the suburban sparrowhawk population in recent years has renewed another.

By breeding and feeding in a large group, each bird cuts down its risk of being caught by a hungry predator, so it can spend less time on the lookout for trouble and more time searching for something to eat. But when the food, a piece of bread perhaps, cannot be shared easily, the sparrow reverts to its natural selfishness and secretly gorges itself.

Juvenile North American ravens also appear to help each other out. In winter they range widely for food, and often find it on an adult bird's territory. The youngsters use a special call to summon others from more than 1 mile (1.6 km) away to help them hold off the stronger adults. A young bird that is alone when it finds food will not call, however, for fear of reprisals from the older birds. It waits until there are sufficient youngsters in the vicinity to put up a defence.

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