Halupka, L., Dyrcz, A. and Borowiec, M. 2008. Climate change affects breeding of reed warblers Acrocephalus scirpaceus. Journal of Avian Biology 39: 95-100.
What was done
The authors documented various breeding parameters of reed warblers -- long-lived passerine birds that spend their winters in Africa but breed in reed beds of marshlands in the Palaearctic, with some of them nesting in fishponds of the Stawy Milickie Reserve in southwest Poland -- during twelve breeding seasons (1970-73, 1980-83, 1994, 2003 and 2005-06) that encompassed the period 1970-2006, after which they compared trends in what they measured with concomitant trends in mean monthly temperatures.
What was learned
Halupka et al. report that mean breeding season (April-August) temperatures increased significantly between 1970 and 2006, as did the mean temperatures of each individual month of the breeding season, with average temperatures for the May-July period rising by 2°C. In response to these changes, they found that in 2005 and 2006, egg-laying (measured by the first egg date of the earliest pair of breeding birds) started three weeks earlier than in 1970, and that the median first egg date shifted forward in time by eighteen days. The end of egg-laying, however, did not change significantly in either direction, so there was a corresponding increase in the total length of the egg-laying period.
With the longer laying period available to them, more birds were able to rear second broods. In the 1970s and 1980s, for example, the Polish researchers report that "only about 0-15% of individuals laid second clutches," but that "between 1994 and 2006 up to 35% of birds reared second broods." In addition, they report that "during seasons with warm springs, early nests were better protected by being hidden in newly emerged reeds," and that "as a result, these nests suffered fewer losses from predation."
What it means
As stated by the researchers in their concluding paragraph, "it would appear that the studied population of reed warblers benefits from climate warming," which is something we probably won't read about any time soon in the popular literature ... which is a matter of some significance in and of itself!!!