Earlier springs with warmer temperatures over the past 30 years have prompted a ubiquitous North American bird species, tree swallows, to begin laying eggs, on average, a week or more earlier. But whether these harbingers of global warming are being adversely affected by changing weather patterns isn't clear, biologists in New York, Wisconsin and California report in Proceedings of the National Academy of Science (PNAS ).

When tree swallows start earlier, they often lay more eggs, say the biologists, referring to data collected by thousands of volunteer citizen-scientists who have watched the birds' nest boxes for 40 years.

"We don't know whether earlier lay dates and larger clutch sizes will be good in the long term for populations of tree swallows," says David W. Winkler, a Cornell University professor of ecology and evolutionary biology. "And tree swallows are just one of the many organisms that potentially can be affected by climate change."

After an exhaustive, three-year statistical analysis of bird and weather data, Winkler, Peter O. Dunn of the University of Wisconsin-Milwaukee and Charles E. McCulloch, a biostatistician at the University of California-San Francisco, report the effects of climate change on swallows in the PNAS Online Early Edition, week of Sept. 23, 2002. Their article is titled "Predicting the effects of climate change on avian life history traits."

Tree swallows (Tachycineta bicolor ) are astute weather monitors, Winkler explains, because of three characteristics:

• They are aerial insectivores, hunting the insects they crave "on the wing." (An adult tree swallow can capture as many as 50 insects before returning to the nest and feeding its young.)
• Tree swallows are "income breeders" that rely, more than many other species, on their daily foraging intake – both before and during the spring breeding season. (Tree swallows begin breeding once their source of insect income looks large enough, but the future of their growing family is at the mercy of sometimes-fickle weather.)o Insects the swallows need do not fly during cool weather, and swallows will not forage on the ground. (A sudden cold snap and a local shortage of insects can kill 5- to 8-day-old nestlings before their developing bodies learn to thermoregulate and grow insulating feathers. When adult tree swallows are forced by cool weather to travel greater distances in search of insects, they may be forced to abandon their chicks.)

Professional ornithologists rely on trained amateurs in volunteer programs, such as the Cornell Lab of Ornithology's Nest Record Card Program, to report on birds throughout a wide geographic area. In 1999, after studying 21,000 nest records from Cornell's database and similar programs in Canada, Dunn and Winkler reported that the lay date of tree swallows shifted an average of nine days earlier between 1959 and 1991.

Since that report, which was among the first to link animal-behavior changes to global warming, Winkler and Dunn have worked with McCulloch and extended the analysis to another key life-history trait – the number of eggs birds lay each year.

"One of the strongest patterns in this data set showed birds that begin earlier in a given season tend to lay larger clutches of eggs," Winkler recalls. "We wanted to see if earlier average lay dates over the past 30 years have led to larger clutches. However, it is interesting to find that, despite the change in lay dates, there has been no significant increase in clutch size across the years."

To say more with any certainty will require a much better understanding of how birds respond to climate change – and more detailed, hands-on research than even the most dedicated legions of volunteers can conduct. Nevertheless, the PNAS authors believe that their statistical analysis of tree swallows' response can be a template for studies by other researchers of how climate change might affect various plant and animal species.

"Tree swallows are doing a fine job of observing seasonal climate conditions and responding in a way that's easy for us to measure," Winkler notes. "Clearly, they're laying eggs earlier on average. Our job as biologists is to learn more about the birds and their food organisms in order to understand the effects of this and other responses to climate change."

The study was sponsored, in part, by the National Science Foundation and Cornell University.