When four typhoons blasted across the islands in 1997, both nests and banded birds were lost. Some mortality is probably normal, says the scientist, but troubles arise when bird populations drop so low that they cannot recover after severe natural events. Jeffrey recalls experiencing a thrill while standing in the pitch blackness of a cave as swiftlets rushed past his head. The birds are unusual because, like bats, they rely on echolocation to find their way into and out of caves. Cave disturbance is one likely candidate.
Pesticides are another possibility.
On Guam, snakes are keeping numbers down at the present, but they may not have caused the original decline. Habitat loss and disturbance have taken their toll in unexpected ways, too. Beck recalls that on Guam, some people initially blamed the plummeting numbers of birds on a rush of development in the 1980s. The damage to native birds wrought by the snake, which has no natural predators in Guam, has prompted much of the wildlife management work now under way in the Marianas. The Guam rail, one of the few flightless rail species left in the world, was saved from extinction by a captive-breeding program.