Bird Study Linked to Water Debate
The marshes along San Antonio Bay produce enough food — even when little water is flowing from the Guadalupe River — to sustain the flock of endangered whooping cranes that winter there, according to a new study that could play a key role the state’s ongoing debate over water rights.
The seven-year, $2 million study suggests that the iconic birds aren’t as dependent as previously believed on the availability of blue crabs, which are their favorite food and need a certain amount of freshwater to survive.
When a saltier bay reduced the blue crab population in the marshes, the whoopers turned to clams, among other foods, according to the study released Wednesday by the two regional water managers that funded it.
“The whooping crane is a predator, and predators tend to eat many things,” said Douglas Slack, a Texas A&M University wildlife biologist who led the study. “They can’t afford to be specialists. They eat what’s available.”
23 birds perished
The findings come at the end of the deadliest year on record for the cranes, with 23 birds having perished along the coast, roughly 10 percent of the world’s last migratory flock.
Federal and state wildlife biologists have blamed the die-off on a dry spell that reduced the Guadalupe’s flow so severely that the supply of freshwater and food for the cranes dwindled in the bay.
But Slack said the study showed that while blue crabs are the most important source of protein for the whooping cranes, the birds can survive when blue crabs are as little as 20 percent of their diet. The whoopers also eat wolfberry fruit, insects, snails and smaller fiddler crabs.
Slack said he is at a loss to explain why so many cranes died this year.
His computer models showed that the delta’s ecosystem works even with 10 percent of the river’s average flow.
In 1956, the height of the worst drought on record for Texas, the Guadalupe’s flow was at 13 percent, without a noticeable effect on the cranes’ population, he said.
It’s not yet known how much water flowed from the river into the bay during the whooping cranes’ most recent stay, said Todd Votteler, executive manager of intergovernmental relations and policy for the Guadalupe-Blanco River Authority, one of the agencies that funded the study.
Ecological relationships
They commissioned the study amid plans to pump water near the Guadalupe’s mouth to quench the city of San Antonio’s growing thirst. Although that proposal was shelved in 2006, the study continued because of its potential to influence future projects.
Mary Kelly, an Austin-based expert on rivers and deltas for the Environmental Defense Fund, said the study shows that “these ecological relationships are so complex, and we’re just beginning to understand them.”
But Jim Blackburn, a Houston attorney who wrote a book about Texas bays, found fault with the study in the wake of the die-off.
“Everything that happened this winter belies their study,” Blackburn said. “The cranes died, and they died of malnutrition because there were no crabs in the marsh.”
