How the amount of water in two rivers may determine the success of the Whooping Crane recovery
Published: April 20, 2012
|The largest population of Whooping Cranes is also the most important. It breeds in Wood Buffalo National Park in Canada and spends the winter largely in Aransas National Wildlife Refuge, on the edge of San Antonio Bay, Texas. |
Three smaller populations — the one migrating between Wisconsin and Florida and two non-migratory populations in Florida and Louisiana — play important roles in the long-term effort to bring the species back from near extinction, but I want to focus here on the Wood Buffalo-Aransas birds.
From a low of 15 birds in 1941, the population is now approaching 300. The cranes were listed as endangered in the United States in 1970 and designated as endangered in Canada in 1978. Between 1967 and 1996, U.S. and Canadian officials removed eggs from nests of the Wood Buffalo cranes, incubated them in captivity, and reintroduced the youngsters into the wild. (The protocol took advantage of the crane’s usual habit of laying and hatching two eggs but fledging only one chick.) Captive-breeding programs were also established to provide chicks for reintroduction to the wild.
Whooping Cranes don’t start to breed until they’re about five years old, they typically raise a single offspring, and they require very large territories both on their breeding and wintering grounds. Consequently, the species can be slow to build up numbers.
Whooping Cranes arrive at Aransas from October through early December and set up elongate territories that are 200-500 acres in size, placing a premium on good habitat. Juveniles stay in their parents’ territory, and offspring from previous years are often found immediately adjacent to the parents’ territory.
Crane territories tend to include shallow near-shore marshes of smooth cordgrass but extend outward to embrace different depths of coastal marsh interspersed with ponds.
The cranes arrive during high tides and feed largely on blue crabs. They have huge appetites; one bird ate 80 crabs in a single day. Later, the cranes feed almost exclusively on wolfberry, a perennial shrub that tolerates saltwater and has fruit available in December. In January, the tides go out, exposing flats, shorelines, and deeper ponds that allow cranes to switch to clams.
In March, high tides return, flooding marshes and bringing in blue crabs, which again become the cranes’ main food source. By early April, the cranes are ready to migrate, although they wait for southerly winds before winging their way north. They typically make the trip to Canada in 10-14 days.
Problems with fresh water
A major problem for the Wood Buffalo-Aransas cranes today is reduced water flow in the two main rivers that feed the refuge: the Guadalupe and the San Antonio. The rivers supply both fresh water, which the cranes must have to drink, and nutrients for the marsh.
Although the region and the cranes have experienced droughts in the past, the high rate at which water is being withdrawn from the rivers is new, as the water-removal rights of businesses and industries are being added to the historical riparian rights of farmers.
Recently, the Aransas Project, a coalition of local governments, tourism-dependent businesses, and non-profit organizations including the International Crane Foundation, filed a lawsuit against the state of Texas, claiming that it had mismanaged the water in the rivers, leaving the cranes vulnerable. The trial, conducted in federal court in Corpus Christi, concluded in December, but a decision is not expected until the summer. If the judge rules for the cranes, the state would be required to assure that a sufficient and agreed-upon amount of water reaches the gulf.
The seriousness of reduced water availability was highlighted by the drought that peaked in 2008-09, when about 23 Whooping Cranes were believed to have died at Aransas. Even in the fall of 2011, as the drought continued, many birds had to fly off their territories to find fresh water for drinking. In fact, last fall several cranes did not fly to Texas at all; they wintered in Kansas and Nebraska. Since climate change is expected to make drought in the Southwest more likely, this problem could recur with greater frequency in the years ahead.
Climate change could also add to the cranes’ troubles by making cold snaps less likely. An occasional freeze is necessary to keep black mangroves from taking over Aransas marsh sites. Mangroves and Whooping Cranes do not coexist. A freeze every 10-15 years, the historical average, is sufficient to prevent establishment, but temperatures are warming. The last freeze, in January 2011, was not severe; the evergreen mangroves lost their leaves but survived and later regenerated new leaves.
The persistent efforts of biologists working on the recovery of the Whooping Crane have been successful, but the problems are many. Fresh-water availability and occasional cold snaps weren’t originally among them. Whether the river flow can be controlled will surely help determine whether the birds continue on the road to recovery. Follow the lawsuit, and support the cranes.
Eldon Greij is professor emeritus of biology at Hope College, located in Holland, Michigan, and the founding editor of Birder's World (now BirdWatching) magazine.
Read more by Eldon Greij.