Alcatraz now hosts San Francisco Bay's only colonies of pelagic cormorants, Brandt's cormorants and pigeon guillemots, as well as the bay's largest colony of western gulls and one of its largest colonies of black-crowned night herons. All the success has made scientists practically giddy. Still, no one takes the birds' health for granted. Pollution such as oil spills could be a future hazard. Dredge spoils from the bay containing potentially toxic material have long been dumped in the waters next to Alcatraz.
People are also a threat.
By last summer, so many visitors were being turned away from boats to the island that the National Park Service started allowing night trips as well as day trips. Most of the 1.3 million people a year who visit the island, it is safe to say, come to see the only former federal penitentiary open to the public. But for biologists, the main appeal lies in the habitat and its wildlife. Alcatraz is the sandstone tip of a former mountain that only 200 years ago was a bare hunk of rounded rock in the bay. In the late 1700s, Spanish explorers named it La Isla de los Alcatraces .