Warming sends body weight of birds into sharp decline

[depakid]

Shrinking Jackie Winter
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CLIMATE change is shrinking Australia's birds. During the last century, biologists say, some birds decreased in size by almost 4 per cent. Shrinking was an evolutionary response to rising temperatures, Janet Gardner, from the Australian National University, said yesterday. Dr Gardner said it was known that birds living close to the equator were smaller than those of the same species found at much at higher latitudes.

The reason, she explained, was that smaller birds lost heat more quickly, allowing them to keep cool in hot climates. Dr Gardner, her university colleague Associate Professor Robert Heinsohn, and the CSIRO's Dr Leo Joseph studied more than 500 birds, from eight species, collected by Australian museums between the late 1800s and 2000, recording their wing lengths. On average, birds collected late in the 20th century were about 1.8 per cent to 3.6 per cent smaller than those from 100 years earlier.

''It is certainly a significant change,'' said Dr Gardner. ''Some declined in size more dramatically than others, but all the species were showing the same trend.'' During the century Australia's mean temperature rose by 0.7 degrees. The scientists also found corresponding shifts in the size gradient - the pattern by which birds become smaller the closer they live to the equator.

Sydney birds had shrunk to become as small as Brisbane's birds, 7 degrees further north, once were. ''This means global warming is affecting fundamental biological processes,'' said Dr Gardner. ''The birds are adapting to global warming by getting smaller.'' The scientists ruled out other possible explanations, including nutritional changes.

Dr Gardner said previous studies in Europe and New Zealand had also noted declines in bird sizes. However, until now, ''nobody has been able to pin down the underlying mechanisms''. Although the birds appeared to have found a way to deal with warming habitats, she warned it was impossible to predict how they would cope in the future if climate change continued.

''The shift to reduced size may become greater, but there are costs to becoming smaller. It gets harder, beyond a certain point,'' Dr Gardner said. ''If they can't adapt to change fast enough to keep up they have to move or become extinct.''

The research, published in Proceedings of the Royal Society B, studied brown treecreepers, jacky winters, white-browed scrub wrens, grey-crowned babblers, hooded robins, speckled warblers, yellow-rumped thornbills and variegated fairy wrens.

More: http://www.smh.com.au/environment/warming-sends-body-weight-of-birds-into-sharp-decline-20090812-eie0.html