by Cortland Henderson
Shifts in vertebrate and plant migration patterns have become a growing center of study for scientists trying to explain the consequences of climate change. Studies generally focus on birds due to their ability to migrate quickly and the large data sets that are collected by avid birders. Most research has concentrated on changes on spring migration, showing that migration shifts forward as temperatures increase. Jaffré et al. (2013) defined the period of time between spring and autumn migrations of birds as Breeding-Area Residence Time (BART). Although no link between autumn departure dates and changes in temperatures were found, increases in BART length are shown to correlate to increasing temperatures in breeding sites of Western Europe. The authors conclude that migration traits that evolved during prehistoric global cooling stages may fade as partial migrant raptor species increase residency in Northern sites. Consistent patterns of change in the phenology and biogeography of organisms are a result of current climate change, but knowledge of the extent of these patterns is often limited. Since most research has gone into raptor pre-breeding migration in the spring, little has been done to understand possible trends of increased residency in raptors in Northern breeding sites. By studying Mean Passage Date (MPD), or the average date of autumn departure, Jaffré and his colleagues were able to expand past studies of migration patterns. Past record of air surface temperatures from NOAA were compiled and compared against latitudinal abundances of several partial migrant raptor species in Western Europe. These species included Eurasian sparrowhawk Accipiter nisus, common buzzard Buteo buteo (subspecies buteo), marsh harrier Circus aeruginosus (subspecies aeruginosus), hen harrier Circus cyaneus, merlin Falco columbarius, common kestrel Falco tinnunculus and red kite Milvus milvus. The authors chose these birds due to their status as partial migrant raptor species and because they had a sufficient abundance on monitored watch sites that enabled statistical analyses.
Three-mode principal component analyses (PCAs) were the primary statistical tests used due to their ability to compare sites with multi-dimensional variables. To detect anomalies within the time series, the authors applied the Rodinov’s sequential algorithm to the data sets. In order to visually identify statistical distributions of the anomalies, the results were compiled into histograms. These tests found that there have been three distinct periods of temperature and migrations anomalies since 1980: 1980—1994, 1995—2006, and 2007—2010. One PCA found that in all of the time series, no significant linear correlation was detected between spring or autumn temperatures and the timing of the post-breeding migration. The other PCA was able to show a significant relationship between the MPD and spring temperatures, but not with autumn temperatures.
Investigations into long-term changes of raptor migrations revealed that post-breeding migration was delayed for all species but one (marsh harrier). The marsh harrier anomaly supports previous studies that found that long-distance migrants are more likely to have earlier post-breeding migrations as autumn temperatures increase in middle-high latitudes. The other results suggest that there exists a high sensitivity between post-breeding migration of short-distance migrant raptors and the autumn temperatures. These results are probably not a result of micro-evolution (due to the long generation time of raptors and the relatively short time range), but rather a phenotypic response to improved conditions in breeding areas. Although no statistical significance could be found between autumn temperatures on the breeding grounds and autumn MPD of birds, there was a response detected. These results suggest that BART may increase as short-distance migrant raptors delay their departure due to warming autumn temperatures. In this context, residency is a likely result that may develop in some raptor species under moderate IPCC scenarios of global warming.
Jaffré, M., Beaugrand, G., Goberville, É., Jiguet, F., Kjellén, N., et al. 2013. Long-Term Phenological Shifts in Raptor Migration and Climate. PLoS ONE 8(11): e79112. doi:10.1371/journal.pone.0079112 http://dx.plos.org/10.1371/journal.pone.0079112