Sound hard to believe? According to a recent journal article published in Current Biology, scientists have shown that some birds are changing migration patterns based on food availability, provided by people through feeders and planted trees, and that can result in the evolution of different wing and shape characteristics as well as breeding isolation.
Gregor Rolshausen and his colleagues have shown that changing migration patterns in a European warbler species, the Blackcap (Sylvia atricapilla), has resulted in genetic diversity in a single breeding population. Historically, the Blackcap has bred in southern Germany and Austria and then migrated to the western Mediterranean for winter. Since the mid twentieth century, a segment of the population, which now is estimated at about 10% of the total, migrates northwest to the UK, rather than southwest to the Mediterranean. This second migration pattern has been driven by a warming climate as well as increasing food sources provided by humans throughout the United Kingdom.
Furthermore, Rolshausen goes on to show that these birds not only migrate differently, but the birds migrating to the UK, which is a shorter route, now show more rounded wings and narrower beaks as well as differences in plumage color than those birds that migrate to the Mediterranean. What makes this situation even more interesting is that both populations of birds then return to breed in the same area.
Since Blackcaps select mates on their breeding grounds, not on their wintering grounds, interbreeding between the two groups should occur frequently. However, this doesn’t occur as the birds arrive at different times and form pair bonds within the available birds. Essentially, one population arrives, forms bonds, and then the second group shows up and bonds within the group since the previous group has already partnered up. To make things even more interesting, when the two groups are crossbred to create hybrids, the hybrids migrate in an intermediate direction!
Speciation in birds that breed in the same area but migrate differently appears to be rather rare, or at least evidence is limited. However, in this case, differences in mate selection due to arrival on breeding grounds may some day result in speciation.
Personally, I really don’t find this all that surprising. I have seen changes in distribution due to human impacts numerous times in Florida. My favorite example is the relationship between the West Indian Manatee and coastal power plants. The manatees that live in coastal Florida move inland, up spring fed rivers, in the winter seeking the warmer 72°F water of the springs. However, in recent decades, large groups of manatees have discovered that power plants emit large amounts of warm water and a pattern developed. Instead of moving inland, some manatees remain in the Gulf waters but hang out by the warm power plant outflows.
Changing migration and movement patterns isn’t all that uncommon. What is remarkable about the story of the Blackcap is the rapid rate of evolution that has occurred in these species. Who knew that we could drive the evolution of wing and bill shape in less than a century?
Source: Rolshausen, Gregor, et. al. “Contemporary Evolution of Reproductive Isolation and Phenotypic Divergence in Sympatry along a Migratory Divide.” Current Biology 19.24 (2009): 1-5.