Alcatraz Island in San Francisco Bay is famous for its abandoned prison, but there is another Alcatraz Island, located off the coast of a sleepy beach town in Mexico, that is equally eccentric, given its unusual mix of desert cacti, surf, sand and seemingly more birds than a Hitchcock thriller.
Alcatraz means “pelican” in Spanish, so both islands have been noted for their bird colonies, but Mexico’s Alcatraz Island remains literally for the birds, and especially wading ones. A recent study, published in the journal Waterbirds, determined that 13 species of nesting wading birds live in the region. Eleven of those are year-round residents.
“Most wading bird species are colonial nesting birds, so they need extensive, undisturbed areas of dense vegetation to nest and raise young, as well as close proximity to rich feeding grounds -- all of which are provided in the area,” senior author of the study, Mark Riegner, told Discovery News. “The climate is subtropical, with very hot summer days (100-plus degrees F.) and cool, breezy winter days (60–80 degrees F.). The region is a subdivision of the Sonoran Desert.”
Riegner, a professor of environmental studies at Prescott College, is this island’s “birdman of Alcatraz,” given his longstanding research in the area. The recent survey alone occurred over a five-year period.
Alcatraz Island, located off of Bahía de Kino Viejo in Sonora, Mexico, is a desert island in the Gulf of California. On any given day, multiple wading birds can be found there, often simply hanging out in the desert scrub. Like workers, though, they must regularly commute.
In this case, “the wading birds on Alcatraz Island must make a 3- to just over 6-mile commute to arrive at the estuary where they spend a few hours each low tide cycle,” study co-author Abram Fleishman told Discovery News. Along with lead author Emily Clark, Fleishman worked with Riegner at the site.
The Alcatraz Island colony of reddish egrets is one of the largest colonies of the species in the world. Their range extends from Baja California to the Bahamas. For this and other reasons, the nearby estuary was designated as a “Ramsar site,” meaning a wetland of international conservation importance.
The researchers noted the unique hunting style of this gregarious bird.
Riegner said that reddish egrets will “jump, twist and flash open their wings to presumably startle or confuse prey, which can then be snatched up.”
The scientists documented the foraging habits and desired prey of various wading birds. They observed that the yellow-crowned night-heron mostly eats crustaceans, and especially crabs.
“As ‘crab crunchers,’ yellow-crowned night-herons regurgitate indigestible parts of their prey, such as crab claws, and spit them out as pellets, much as do owls with rodent bones and skulls,” Riegner said.
He added, “The pellets can then be collected, washed and teased apart to identify what species of crabs the herons are eating.”
Meanwhile, the white ibis, also found at Alcatraz, has an altogether different method of feeding.
“It probes into the mud with its remarkably long, down-curved bill, feeling for small prey animals,” Riegner said.
The feeding activity of the white ibis and the other wading birds is strongly regulated by the tidal cycle. In the wetlands of the region, there are two low and two high tides per day.
“As the water level drops due to an outgoing tide, the birds leave their roosts and descend upon the shallow pools, channels, and mudflats,” Riegner said. “They will spend a few hours feeding and then return to their roosts as the water level rises with the incoming tide. At the roosts, the birds will preen, stretch and doze.”
While the prisoners at San Francisco’s Alcatraz got into some famous fights, the wading birds at Mexico’s Alcatraz often get along with each other surprisingly well. Riegner explained that they each have different behaviors and feeding norms, so that they experience minimal competition.
Smaller species tend to feed in the shallow water, mostly on small fish and other tiny prey. Some small wading birds even just perch themselves on a mangrove root and stab at the minnow-like fish that swim by.
Taller wading birds, on the other hand, can move to greater depths to catch bigger fish.
In this photo, a white ibis was captured peacefully feeding one of its chicks, as multiple other birds looked on with curiosity.
Life on Mexico's Alcatraz is a veritable bird soap opera, with mini dramas happening there daily. Here, a snowy egret engages in a visual display at a nest for an attentive audience.
Such mating shows must be working, as the researchers determined that the most abundant wading bird species in the area is the snowy egret, which had a peak of 234 nests in 2012.
Alcatraz Island provides good refuge spots for the wading birds, such as this reddish egret mating pair. That's why they bother to make the lengthy commute to and from the estuary. During the winter, however, many of the birds get a break from their tiring travels, since the mangroves act to hinder predators like coyotes, Fleishman said.
Another resident mammal is the Mexican fishing bat, which roosts on Alcatraz Island and other islands in the Gulf.
In terms of marine mammals, bottlenose dolphins are common in the estuary, as are California sea lions.
“Farther offshore, the Gulf of California is home to almost one third of the world’s marine mammal species,” Riegner said. “Fin whales, humpbacks, and common dolphins are regularly seen, and occasionally blue whales, gray whales and orcas.”
Besides wading birds, other birds in the region include sandpipers, plovers, curlews, godwits, ospreys, double-crested cormorants, yellow-footed gulls, Craveri’s murrelet and, as the Spanish meaning of Alcatraz indicates, lots of pelicans.
This great blue heron has cleverly built its nest on a giant cardon cactus, keeping it safe from most predators. Herons are perhaps the bird bullies of the diverse wading bird population, since they have broadened their diet to include eggs and young chicks of other nesting birds at Alcatraz.
“Great blues will snatch, for example, a cormorant chick from its nest, work the chick in its bill, then throw its head back and swallow the chick whole,” Fleishman said.
While nestlings such as these provide tremendous hope for the future of the region’s wading birds, the researchers note two major human-related threats.
The first is shrimp aquaculture, which often involves clearing mangrove forests that the birds need for nesting and roosting. Many species of fish and crustaceans use the mangrove estuary as a sheltered nursery while they grow and develop, so they too can be harmed by mangrove forest removals.
Various antibiotics and other chemicals used in shrimp aquaculture can also flush into the estuarine waters, leading to harmful pollution.
“Another big threat, specifically to nesting wading birds, is human disturbance,” Riegner said. “If people carelessly enter a breeding colony, it may cause adult birds to abandon their nests and young, which can be disastrous.”
He and his colleagues are concerned about growing numbers of tourists and weekend residents from the city of Hermosillo, which is only 90 minutes away by car. Already, boaters and jet skiers have started coming to Alcatraz Island.
The scientists hope that raising awareness of the rich biodiversity in the region will help facilitate care and stewardship. They further hope that development of shrimp aquaculture will be curtailed, allowing for a sustainable ecosystem in the estuary that would ultimately benefit not only the wading birds and other wildlife, but also the local fishermen.
The researchers also call for increased signage in the area, alerting the public to the sensitivity of nesting birds. “Even better,” Riegner said, “it would be very effective if there were funds to support a seasonal park ranger who could patrol the area, educate the public, and engage in sustainable management of the wading bird habitats.”
M. Clay Green is an associate professor and director of the Aquatic Resources Ph.D. program at Texas State University. He supports the idea of hiring one or more park rangers to help ensure protection of the birds at and around Alcatraz.
“This is an important issue that the authors state, and I agree that waterbird colonies that seem to do best are ones that have signage to inform people and deter human access, and ideally rangers/wardens that can enforce the protection,” Green told Discovery News.
He added that long-term monitoring of waterbird colonies is not very common.
Green said, “We need more studies like this that track changes in colony size and species over time.”