Sunday, January 13, 2002		Nature

Beware, pirates in the sky!
Nutan Shukla

WHY should a hunter expend enormous amounts of energy in catching food when it can save all that time and effort, and reduce the uncertainty of finding a meal by scrounging food or stealing it from another predator? Piracy or kleptoparasitism involves the procurement of food by one animal only to be stolen by another. The pirate may take the food from the grasp of the predator, worry it until it drops the food or force it to regurgitate food that it has already swallowed. Pirates tend to be persistent and violent, harassing successful predators until they give up their prey. Seabirds, particularly gulls, skuas and frigate birds are such type of birds and are known as the pirates of the bird world.

Arctic skuas dive-bomb other seabirds like puffins in an attempt to steal their catch. Usually puffins catch food far from the coast in the open sea and fly back to their chicks, but the skuas often intercept them midway, grabbing their wingtip and tilling them into the sea. The puffin’s only recourse is to jettison its catch so that the pirate leaves it. As the food is dropped, displaying considerable aerobatic skill, the skuas catch the falling prey in mid-air.

Some gulls shadow diving birds, such as ducks and cormorants, and attack them as soon as they reach the surface. Gulls have been seen to perch right on the large beaks of pelicans trying to get them to release the fish they have caught. Gulls also harass terns, chasing them mercilessly until they drop their hard-won catch. Black headed gulls accompany lapwings foraging for earthworms in fields.

The lapwings find and pull the worms from the soil, only to be chased away by the gulls who steal their food. The gulls spread out amongst the lapwing flocks, each gull watching the activities of several lapwings. During the course of a day, a gull can steal more than 160 worms from its lapwing servants, which represent more than twice its daily nutritional requirements — clearly, a far less energetic and more reliable way of obtaining food than following the plough.

The most skilled flying pirates are the frigate birds. They swoop low over the water, dipping the head and beak into the waves and plucking small fish or squid quite literally from the jaws of pursuing tuna. This behaviour requires many years of practice and so younger birds take to intercepting other seabirds, pulling at tail feathers and dangling feet, forcing them to disgorge their catch. The frigate bird has the largest wingspan to body-weight ratio of any bird and it has a smaller quantity of oil in its feathers, reducing the actual body-weight. The lack of oil means that it cannot enter the water to catch its prey and must, therefore, depend, to a certain extent, on ‘professional’ piracy in order to obtain its food.

In India, black drongos sometimes chase smaller insect-eating birds, buzzing them until they drop their food. And in the New World, turkey vultures have discovered an easy way to get food from young, great blue herons. The very young chicks fall prey to the vultures, but the older birds defend themselves by regurgitating food and spitting it at their assailant. What better way for a vulture to obtain a partially digested meal that it can take straight back to its own chicks?

Aerial piracy seems to be commonest among birds that have a fairly catholic diet, and there are some surprises. Even little sparrows can be pirates. They have been seen to steal crickets and grasshoppers from digger wasps.

This feature was published on January 6, 2002