Increase in Whooping crane deaths leads to population decline – Starts alarm bells

27/08/2009 12:11:58

Whooping crane mortality increases
August 2009. The world's only naturally migrating whooping cranes, and the species' best chance for survival, suffered an increased mortality rate, dying at about twice their normal rate last year, which will cause a drop in the population. This worrying spike in their death rate has started alarm bells ringing amongst those who are responsible bringing the whooping cranes back from the brink of extinction.

Three distinct flocks
There are three distinct flocks in North America; one that migrates without human help, flying each autumn from Canada to the Gulf Coast in Texas, one that is guided by micro-light planes on migration, and one in Florida that does not migrate.

Average 10% mortality - 21% in the last year
On average, 10% of the flock dies each year, but last year around 21% died. Including new births, this year's flock is expected to drop by about 20 birds from last year's 270 when counted after returning to the Aransas National Wildlife Refuge in the autumn, according to Tom Stehn, who oversees efforts to help the whooping crane for the U.S. Fish and Wildlife Service.

First decline since 2002
That would mark the first population decline for the flock since 2002. The Aransas flock typically grows by about six birds each year, but it was reduced by19 birds between April 2008 to April 2009, as 57 of the flock's 266 birds died and were replaced by just 38 surviving hatchlings. Hatchlings aren't counted in the total population until they have made it to Aransas. This year only 52 birds hatched to the flock - a six-year low - and only 22 of those survived, Stehn said.

"We're trying to figure out what's killing all these whooping cranes," Stehn said. "It's disappointing," he said. "It'll be interesting to see how it turns out this year. The flock's population tends to dip about once each decade, but last year's spring decline was so sharp and unexpected it was "alarming," Stehn continued.

Because the flock that migrates 2,400 miles from Wood Buffalo National Park in northern Canada's boreal forest to Aransas is the only self-sustaining flock, it is the species' best chance for survival, he said.

"The species remains highly endangered, and the threats are rising," Stehn said.
It's difficult to know exactly how the birds die in part because they're not individually tracked and their 200-mile wide migration corridor is so large.

Causes - Drought?
One likely cause for the population decline could be changes in habitat, Stehn said. A drought in Texas severely affected the whooping crane's foods of blue crabs and berries. Corn feeders were set up to supplement the cranes' diets, but only about half of the birds used the feeders. And wetlands and prairie have been making way for cornfields along parts of the flock's flyway, which runs from northern Canada through Montana and the Dakotas, New Mexico, Nebraska, Oklahoma and Kansas.

Satellite tracking
Among efforts under way to understand the increase in whooping crane deaths is a plan to track about 20 of the Aransas birds using radio transmitters when they leave Texas next spring. Funding for a habitat conservation plan for the crane's migration corridor has also been approved, but work on the actual plan has yet to begin.