Night moves

2019-03-07 01:05:01

By Kate Douglas WHY do bats hunt at night? Most experts thought they were simply avoiding predators, but John Speakman of the University of Aberdeen says his observations of bats in the 24-hour daylight of the Arctic summer cast doubt on that theory. Most probably, he says, bats fly at night to avoid competition with insectivorous birds. Insect-eating bats live on a knife edge. Flying consumes so much energy that each female is only able to produce a single offspring a year. An insectivore on the ground, such as a shrew, however, might produce five young every six weeks. If bats flew during the day, when most insects are active, they could eat enough to overcome this problem—but they would then be vulnerable to raptors such as kestrels and sparrowhawks. In the Arctic summer, raptors are active round the clock. Speakman and his colleagues expected that Norway’s northern bats, Eptesicus nilssonii, would respond by switching their peak activity to the times at which insects were most active. “But they didn’t shift their time of feeding at all,” says Speakman. It’s hard to say what causes this intransigence, but Speakman suspects that competition is the main factor at work. Sand martins were least active between 11 pm and 3 am, during the bats’ period of peak activity. Although at present there are too few sand martins to offer much competition, following a population crash in the 1980s,