When the climate changes, organisms can respond in just a few possible ways:  change their behavior in their current habitats — for example, plants may flower earlier or retain their leaves longer to accommodate a longer growing season, and birds may change their migration times;  change their ranges — for example, the Carolina wren is now commonly seen in New England, where it was essentially unknown 20 years ago;  the species may evolve in response to changed conditions (or  may go extinct_.
But climate change effects (e.g. more volatile weather, changed precipitation patterns, more frequent high temperatures, less frequent low temperatures) interact with many other environmental factors, which makes it challenging to establish the relative contribution of these different factors to an observed change in the biology or ecology of an organism. A recent study by Plummer et al. published in Global Change Biology uses long-term data on the migration of an Old World warbler, the Eurasian blackcap, to understand how changes in climate, and in human bird-feeding behavior, have contributed to the evolution of new migratory behavior in central European blackcaps.
Blackcaps in southern Germany and Austria (see a brief description and photos here on Wikipedia) historically migrate southwest to the Iberian Peninsula for the winter. In the past 60 years, however, many have migrated northwest, to winter in southern and western Great Britain. The authors cite research that has established that there is a genetic component to this — that is, that this population of blackcaps has evolved in response to the emergence of new winter foraging conditions.
Plummer and her colleagues noted that, while average temperatures have increased over times time, so, too, has the intensity of human bird-feeding in Britain. They tested their hypothesis that the two factors together could (largely) account for this striking change in migratory patterns, using long-term data sets of several kinds. They used climatic data, specifically regional changes in average temperature over the study period; historical records of weather in specific locales within the region; information on human bird-feeding from the long-term Garden Bird Watch (GBW) program of the British Trust for Ornithology; and observational data on winter presence of blackcaps, also from GBW. The team analyzed data across a 12-year span starting in 1999.
Their analysis shows that blackcaps did not become more common in winter-time Britain over these 12 years, but they did show an increasing tendency to winter in Britain's south and west regions, regardless of the winter temperatures each year. Moreover, blackcaps were more prevalent where food was more frequently available, and especially in suburban areas (where there are likely to be more feeders per hectare than in rural areas. There is also a possible "heat island" effect, making the gentler climate even more welcoming. Another trend is that the association between blackcaps and artificial food availability got stronger over time. The authors comment, "These novel findings indicate that blackcaps are adapting their feeding habits to exploit human-provisioned foods, and thereby support the hypothesis that the blackcap’s new migratory strategy is likely to have evolved in response to increased human supplementary feeding activities in Britain."
The increased reliance on the seeds and fats supplied by human bird-feeders may not only related to the evolution of migratory habits in this population of blackcaps. The authors suggest that the growing contribution of such foods to the diet of these birds is reflected as well in changes in the beaks of these east-west migrants: "Rolshausen et al. (2009) demonstrated that blackcaps wintering in Britain have relatively narrower and longer beaks than those wintering in Spain, suggesting that British migrants have adapted to a more generalist diet. Our results provide empirical evidence that selection for a beak morphology facilitating the handling of seeds and fats supplied in bird feeders has enabled blackcaps to make greater use of supplementary foods provided in British gardens over time."
Ecological time is more leisurely than the human life-span, though quicker than geological time. Trends at the level of ecosystems, populations, and landscapes are often very complex, and we often do not know enough about the components in any system to make confidence predictions about the future, nor explain past developments. To study such systems for long enough, and with enough detail to bring understanding, we need long-term data sets, and networks of observers that give us data across several scales of space as well as time. Such data sets are precious, because their uses will change as the times change, and as the data set grows. This study is a case in point: Long-term data sets such as the GBW were mostly not created in order to address questions related to climate change. Such questions are of urgent interest right now; who knows what other questions may become urgent, which data sets like this one can help answer?
(This Centerpiece draws from Plummer et al. (2015) Is supplementary feeding in gardens a driver of evolutionary change in a migratory bird species? Global Change Biology (2015) 21, 4353–4363, doi: 10.1111/gcb.13070)