The sharp-shinned Hawk in the Mojave Desert needs a lot of water to stay alive. (Courtesy of Sean Peterson)

We found out last year that hotter, drier weather due to climate change is likely causing bird populations in the Mojave Desert to collapse at an alarming rate. A new study published today suggests one big reason why: Birds are having a hard time staying hydrated, which means they're having a hard time staying cool.

Over the past century, temperatures in the Mojave Desert have risen about 3.5 degrees Fahrenheit, while precipitation has declined in some parts. That's coincided with a roughly 40 percent decrease in the number of bird species documented there.

Adapting has been harder for some birds than others.

"Birds that required more water over the last century to cool off experienced more decline in the desert," said Eric Riddell, postdoc in museum of vertebrate zoology at the University of California, Berkeley and co-author of the paper. "If birds had an unlimited amount of water they could probably deal with a lot more heat."

The water requirements for desert birds are increasing as heat increases. (Courtesy of Eric Riddell at UC Berkeley )

SOME BIRDS DO BETTER

Different species of birds get water in different ways.

Birds with primarily plant-based diets hydrate by eating seeds, some insects and by drinking from pools of water.

Primarily carnivorous birds, on the other hand, hydrate mostly by eating other animals, and don't tend to drink from oases. The problem is that they have to hunt in order to eat, which means expending lots of energy in increasingly hot environments.

"Compared to 100 years ago, some birds needed to collect up to 60 more bugs per day just to replenish their water reserves," Riddell said. "So that extra cost per day of having to go out and find a little more bugs and a little more bugs, we suggest has contributed to the collapse of the desert bird community."

Larger birds with high energy demands have an even harder time. The daily grind can lead to a decline in reproductivity and premature death.

Birds like the American kestrel, prairie falcon and turkey vulture have all suffered.

The authors estimate that there's been a roughly 10% to 30% increase in water requirements for desert birds over the past century. That need could increase by up to 80% by 2100.

To estimate how bad things could get, the researchers created computer models of 50 different types of desert birds, all of which they subjected to increased heat due to climate change. Thirty-nine of the species declined significantly.

"For people that want to go out and see these birds, not only will they see fewer of them, but they will have much smaller windows of time when they can see them," Riddell said. "Not only that but the conditions that make the desert hard to live and be active in for birds will also be true for humans, as well."

A separate, larger study published earlier this month estimated that 3 billion birds have disappeared from North America in the last 50 years.