Issued by: Terry Higgins
414-229-5560
thig@uwm.edu
Date: Oct. 23, 2002
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Issued by: Terry Higgins Date: Oct. 23, 2002 |
Dunn and his collaborators studied the egg laying date and number of eggs laid (clutch size) of tree swallows, a common bird that breeds in man-made nest boxes throughout most of North America.
In response to warmer temperatures, tree swallows have started laying eggs an average of nine days earlier during the last 30 years. Birds that lay earlier tend to lay more eggs, so the researchers were interested in whether clutch size had increased in response to warmer temperatures.
Dunn and co-authors David Winkler and Charles McCulloch found that, surprisingly enough, there was no trend toward larger clutches with warmer spring conditions and rising temperatures.
They pointed out that clutch size may respond to warmer temperatures only up to some critical level, and beyond that point no major changes will occur. If there is such a threshold, then changes in bird populations may not be as extreme as predicted previously.
Clutch size is just one of many aspects of reproduction that could be affected by climate change. Thus, the scientists noted that we should be careful to distinguish between adaptive and demographic effects of climate change.
Although bird populations may adapt to climate change by adjusting laying date or clutch size, we also need to know about the demographic effects. Birds that "breed earlier and have a reproductive advantage in terms of greater production of offspring might have that advantage eliminated by increases in the mortality of offspring," Dunn noted.
In other words, according to Dunn, "increases in clutch sizes, if they occur, could be offset by more starvation or predation of young.
"We do not yet know enough about the complex interactions among the environment, organisms, their food supply and predators to make good predictions. We need more in-depth studies to predict what will happen in the future."
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