Tuesday, November 06, 2007

Migratory Displacement

Two interesting experiments were conducted using displaced White-crowned Sparrows, specifically the gambelii race and yet yielded slightly different results with regard to juvenile birds. In September 2005, I reported about a Lund University study where:

"Scientists found that both adult and juvenile birds abruptly shifted their orientation from the migratory direction to a direction leading back to the breeding area or the normal migratory route, suggesting that the birds began compensating for the west-to-east displacement by using geomagnetic cues alone or in combination with solar cues."

Today there is a new study by Princeton University, which found:

"Though they had likely never before been east of the Rocky Mountains, once released, the adult birds quickly realized they were significantly east of where they wanted to be and began to fly in a southwesterly direction to their normal wintering grounds. The juvenile birds, on the other hand, went with their innate migratory instinct. 'The juveniles, they just kept on flying south as if nothing had happened,' said study leader Kasper Thorup of Princeton."

The Princeton birds were displaced further directly east from a normal stopover site in Washington, while the Lund birds northeasterly, past the article circle, including the north magnetic pole, where they wouldn’t have dark skies for celestial visual cues. The results seem to indicate there must be some east-west point where juvenile White-throated Sparrows are able to correct for longitudinal displacement, but too far to the east and they "decide" to migrate directly south. Adults figure it out either way. Any theories? Do juveniles need the Rocky Mountains as a visual cue? Why could they correct longitudinally further north from Ellef Ringnes Island but not when released in New Jersey?

White-throated Sparrow © 2007 Mike McDowell

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