In 1769 Spanish explorers mapped a small island at the entrance to San Francisco Bay, and called it Alcatraces after the abundant seabirds sighted there. By 1850, the island sported a defensive fortress, and the Bay’s first lighthouse. As the US military arrived and settled, the birds left, sensing they were no longer welcome. It wasn’t until 1972 when the island was again uninhabited, that the birds began to return, first the seagulls and then other species. They began nesting in the trees, thickets, natural rock crevices and the man-made concrete structures.
Recently I visited Alcatraz for a tour of bird-nesting areas with park ranger Tori Seher. She identified 9 bird species that nest on Alcatraz annually: gulls (California and Western), herons (Great Blue and Black-Crowned), pigeon guillemots, cormorants (Brandt’s and Pelagic), snowy egrets, and black oystercatchers. These populations are significant in that they are large (over 1000 pairs make up the 2nd largest Western Gull colony on the West Coast), and unique (pigeon guillemot have only one other small nesting area near Berkeley). The rangers keep a census of bird sites and populations from March to September, the primary nesting season.
Alcatraz has no natural source of fresh water, and no natural predators. Ravens, however, have taken up residence on Alcatraz and have been known to eat black- crowned night heron eggs. To control the raven population, the park service regularly oils the raven eggs to prevent them from hatching.
Sensitive nesting sites are off-limits to visitors to ensure a refuge for the birds. We viewed some of the species through binoculars and Tori’s telescope, and saw the cormorants through frosted windows in the New Industries Building. Fortunately, we were able to get very close to the egrets. Tori also delighted us with a story about a lone northern gannet, a bird from the Atlantic Ocean, sighted recently and repeatedly at Alcatraz. This wandering bird may have flown over the Arctic Circle but has arrived in California only to look for a mate fruitlessly.
Today Alcatraz struggles to balance the needs of their various seabird populations with that of the historical structures on the island. Some of the challenges include the long term effects of a large Western gull population on the island, the interruption of regular maintenance on the structures, the corrosive effects of bird excrement, and the ongoing relationships among the bird species within the ecosystem.
