Earlier birds

Natural History, Dec, 2007 by Stephan Reebs

Birds returning from a winter's retreat are showing up in England earlier and earlier each spring as a result of global warming, a new study confirms. Unexpectedly, populations in decline show a less pronounced shift than thriving ones do, sparking fears that ecologists have underestimated the effect of rising temperatures on migratory birds.

Led by Tim H. Sparks, an ecologist at the Natural Environment Research Council in Monks Wood, England, a team of investigators studied Lip to fifty-six years' worth of data, gathered at six English locales, on the arrival and departure dates of thirty-three migrant bird species. On average, they discovered, the birds are arriving in the spring twelve days earlier than they did fifty years ago. That's a big change, and it mirrors advances already noted for migratory birds throughout Europe and in the United States. But the change was even bigger among species, such as the blackcap, whose numbers are on the rise in England: they now arrive as many as twenty days ahead of schedule. By contrast, species with declining populations have been flying in just five days early.

Why the difference between thriving and declining species? Sparks thinks observers may spot the earliest birds more readily when a species is abundant. If so, the true average advance in European and American arrival times may be even greater than actually measured because, sadly, many migrant bird species are on the wane. (Journal of Ornithology)

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