Background
The authors write that "recent climate change has caused comparatively rapid shifts in the phenology and geographic distributions of many plants and animals," but they say "there is debate over the degree to which populations can meet the challenges of climate change with evolutionary or phenotypic responses in life history and morphology," which for a warming climate includes a reduction in body size. Hence, they devised an experimental strategy to further explore the issue.

What was done
Van Buskirk et al. studied the body sizes of birds captured in mist-nets and traps between June 1961 and November 2006 at the Powdermill Nature Reserve -- a field station operated by the Carnegie Museum of Natural History in Pennsylvania (USA) at a location that is broadly representative of bird communities in the Appalachian region of eastern North America -- where (1) 35 mist nets were operated five to six days per week during spring and autumn migrations, but where (2) a reduced number of nets was used during summer, and where (3) birds for winter banding were caught in wire traps when the temperature was below freezing.

What was learned
The three researchers report that migrating birds captured at the banding station "have steadily decreasing fat-free mass and wing chord since 1961, consistent with a response to a warmer climate" and confirming that "phenotypic responses to climate change are currently underway in entire avian assemblages," where "size was negatively correlated with temperature in the previous year, and long-term trends were associated with the direction of natural selection acting on size over the winter." In addition, they note that "species undergoing the strongest selection favoring small wing chord showed the most rapid long-term declines in wing [size]," which suggests, as they describe it, that "phenotypic changes are therefore in line with the prevailing selection regime."

What it means
Noting that "in summer, 51 of 65 breeding species had negative slopes of mass against year, 20 of 26 wintering species had negative slopes, 60 of 83 spring migrants had negative slopes, and 66 of 75 autumn migrants had negative slopes," Van Buskirk et al. conclude that their results "offer compelling evidence that climate change has already produced observable adaptive shifts in morphology, behavior, and phenology of a great many species," as, we might add, birds do what they need to do to successfully cope with the climate changes they encounter, which according to the world's climate alarmists have been unprecedented over the past one to two millennia. Yes, birds do it ... and they do it fast, and they do it well.