Thursday, May 20, 2010

Ball 3. Saharan Peril for Migrating Raptors

Update: The article in Tuesday's N.Y. Times ( on long-distance migrants missed this study and other work by Dr. Alerstam on tracking migrating birds. (The cover of my edition also had a teaser with a caption about a bar-tailed godwit under a photo of an arctic tern.)

For birds that migrate from Eurasia south through Africa, crossing the Sahara desert is a particularly treacherous leg of the trip. Although scientists have suspected that the 3.5 million square mile Sahara is a death trap for migrants, the magnitude of the mortality risk hasn’t been clear—until now.

Swedish researchers used satellite tracking technology to follow four raptor species whose migration path took them across the Sahara. They found that 31% of juvenile birds—but only about 2% of adults—died during the migration, accounting for much of the overall annual deaths among these species.

The new study is the first to show how satellite tracking can be used to provide a quantitative picture of bird mortality at specific times and places during the animals’ annual cycles, said Thomas Alerstam, an ecologist at Lund University. It also yielded surprising graphical evidence of the frequent stops and starts, retreats and other detours that delayed the birds’ arrival at their breeding grounds.

“These instances of difficulties were significantly associated with late arrival at breeding sites and poor breeding success, showing that a barrier to passage may not only cause important direct mortality but also have consequences that translate into fitness losses later in the annual cycle,” Alerstam said.

The combination of such a limited margin for safety during migration and the Sahara’s southward expansion—a process of “desertification” resulting from global climate change—may have major consequences for raptors and other species that make the trip from the Palaearctic ecozone across northern Africa, the researchers said.

Alerstam’s group reported its findings in Biology Letters (Dec. 2009).

One editor’s response: “We’re going to pass on this, as it doesn’t seem all that surprising that a big barren desert could be bad for birds. Thanks though.”

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