About a month ago, NickL (the Birdist) interviewed me on the subject of extralimital birds. We speculated about the number of extralimital species that arrive in North America and how they arrive off-course. Last week an article shed more light on why birds get lost. Scientists in Europe studied the physical dimensions and migratory patterns of 38 species of songbirds. Their goal was to test two competing potential causes for vagrancy. Do extralimital birds have faulty navigational instincts or are they blown off-course by the weather?
Eight species from the leaf-warbler family and six from the thrush family caught the scientists' attention as vagrants. One species that was spotted particularly often was the Yellow-browed Warbler (Phylloscopus inornatus), which was reported by voluntary ornithologists* in Central Europe around a thousand times between 1836 and 1991. This species breeds in the Siberian taiga south of the Arctic Circle and overwinters in the subtropics and tropics of South-East Asia. The other Asian leaf-warbler species were observed much less frequently, if at all, in Central Europe.While vagrancy failed to correlate with weather patterns or body mass, it did correlate with population size. Birds with the largest breeding populations or ranges in Asia were the most likely to turn up in Central Europe. Thus it seems more likely that extralimital birds simply have faulty direction-finding abilities, which would be caused by genetic mutations.
By contrast, five thrush species were reported nearly 100 times. If vagrants were brought by the weather, smaller birds should be blown off course more frequently than larger ones. However, using statistical analyses, the researchers were unable to find any correlation between the frequency of vagrants and their body size. In addition, the Yellow-browed Warbler occurs far too regularly for every sighting in Central Europe to be explained by 'unusual' weather conditions during migration.
One interesting result is that, despite their navigational disabilities, vagrant birds retain their instinct for how far they should fly. Birds that mistakenly wintered in Central Europe were typically about as far from their breeding grounds as they would have been in Southeast Asia.
Unfortunately we do not know what happens to these vagrant birds. No banded birds have been recovered subsequent to vagrancy, so the researchers believe that the birds fail to return to their proper breeding grounds. Given the low rate of band recovery overall, I am not sure we can conclude this with any certainty. However, it does stand to reason that a bird with faulty directional instincts would also have trouble finding its way back.
* I love the term "voluntary ornithologist." Birders should use it more often, especially those of us who contribute to bird databases regularly.