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THEBACHA/FORT SMITH - Whooping cranes from Wood Buffalo National Park have had a tough season on their wintering grounds in Texas. A record number of the endangered birds have died at the Aransas National Wildlife Refuge.

Last fall, 270 whoopers arrived at the refuge after a 3,800-km migration from the NWT.

A whooping crane dines on a blue crab in Aransas National Wildlife Refuge in Texas. - photo courtesy of Steve Sykes, Aransas National Wildlife Refuge

"Now we're down to 249," said Dan Alonso, manager of the Aransas National Wildlife Refuge.

The death of 21 of the birds works out to a 7.8 per cent mortality rate - the highest ever recorded at the refuge.

"We're speculating they're lost, because we don't have them in hand," he said.

It is estimated that 15 chicks and six adults have been lost this winter in the flock.

Alonso said the primary reason for the population decline is a drought and the subsequent decline in the population of blue crabs, which make up 85 per cent of the whooping cranes' diet.

Brian Johns, a wildlife biologist and whooping crane expert with the Canadian Wildlife Service, said it hasn't been a good winter for whooping cranes in Texas.

Last year, there were 66 nests in Wood Buffalo, but Johns said the number will probably fall this summer. He predicts the number of nests this summer may be down to approximately 50.

The drought in Texas means less fresh water is flowing into the marshlands at the mouths of two rivers where whooping cranes feed on blue crabs, he explained.

That means the marshes become saltier with water from the Gulf of Mexico and blue crabs stay out of the marshlands, and the birds have to fly inland for fresh water.

Plus, Johns said there have been fewer acorns and wolfberries, which are other food sources for whooping cranes.

"There's sort of a triple whammy there on the birds," he said.

Johns said whooping cranes normally build up fat reserves in Texas for migrating north to nest in Wood Buffalo, but they may be in a weaker than normal state this year.

The birds will start to head north in the latter part of March and the first two weeks of April.

If the drought isn't bad enough, a virus - infectious bursal disease (IBD) - has been detected in the flock for the first time.

Johns said the virus usually weakens young birds.

Alonso said the virus was detected in the carcass of a young whooping crane.

Previously, the virus was detected in a whooping crane flock in Florida.

Alonso said the presence of the virus is a concern, but it's hard to say how serious the situation may be.

He said this winter's decline in the whooping crane population can't be blamed on the virus.

Johns said it is unknown if the virus is just an isolated incident. "It could be there for a long time or could be something new."