What was done
Working in what they describe as Europe's "last remaining primeval forest," the authors investigated evidence for climate warming in Bialowieza National Park of East Poland over the period 1975-2007, in an effort to determine "whether local populations of four sedentary birds (Sitta europaea, Poecile palustris, Cyanistes caeruleus, Parus major) advanced their breeding phenology during that time, and how breeding phenology was affected by temperature variation during the period preceding egg laying."

What was learned
Wesolowski and Cholewa report that "mean yearly temperatures varied strongly across years, with a significant warming (~1°C) trend," and that "in all species the onset of breeding was vary variable across years (up to 30 days), as birds started breeding earlier in springs with higher temperatures in the pre-laying period." In this regard, they note that the spring of 1990 -- when February and March were extremely warm with "mean temperatures +5°C higher than these months' averages, equivalent to the 'worst' IPCC warming scenario" -- served as a natural experiment, showing "what would happen when the late winter/early spring period got substantially warmer." And that response was such that "all four species were able to react to this challenge by advancing the onset of breeding to unusually early dates that are usually found only in the Mediterranean populations."

What it means
The two Polish researchers write that "the ability to react even in these extreme conditions shows that the range of phenotypic plasticity in these birds is far greater than normally observed, and suggests that there is still unexpressed potential in lay date flexibility." Hence, they conclude that "local birds are already prepared to cope with envisaged warming, suggesting that there is no need for new response mechanisms." In addition, they say that this phenomenon is "clearly noticeable among different groups of organisms in Bialowieza National Park (e.g. timing of bud burst in trees, Wesolowski and Rowinski, 2006, 2008; development of herbs on the forest floor, Falinski 2001, Sparks et al., 2009)," and they thus also conclude that "birds, trees and herbs in this primeval forest apparently 'read' environmental cues in a similar way and react to them in a comparable fashion," likely due to the fact that "they have been exposed to the same vagaries of local climate over ages if not millennia, and there was plenty of time to match phenology of interacting species with one another."

References
Falinski, J.B. 2001. Phytophenological atlas of the forest communities and species of Bialowieza National Park. Phytocoenosis NS 13: 1-176.

Sparks, T., Jaroszewicz, B., Krawczyk, M. and Tryjanowski, P. 2009. Advancing phenology in Europe's last remaining primeval forest: Are we in danger of underestimating the response to increasing temperatures? Climate Research (in press).

Wesolowski, T. and Rowinski, P. 2006. Timing of bud burst and tree-leaf development in a multispecies temperate forest. Forest Ecology and Management 237: 387-393.

Wesolowski, T. and Rowinski, P. 2008. Late leaf development in pedunculate oak Quercus robur: an antiherbivore defense? Scandinavian Journal of Forest Research 23: 386-394.