Willow warbler is a very common migratory leaf warbler. This small bird has shown a ray of hope to those evolutionary biologists who have always been supporting what is called a ring species. There were always more evidence of the mode speciation in biology where one species diverges into two species due to some kind of geographical barrier, may that be a mountain or a sea. The concept that two locally situated and reproductively isolated forms are connected by a long chain of intermediate populations that encircle a geographic barrier was always difficult to swallow for many biologists. On the other hand, there are only two well known cases recognised in the scientific world of ring species. One of them is a species of salamanders in California and the greenish warblers in Asia. The more classical examples, for example, the herring gull or the great tit have mostly been rejected by later workers. A recent study has tried to explain the biogeographic history of the warblers surrounding the Baltic Sea. Its results claim that the modern two forms of the Warbler arose from a common ancestor pretty recently and we can actually see a reproductive isolation happening. It is indeed a nice example of a ring species that once again highlights the gap between the text book definition and the real world phenomenon of speciation. People who want to know more about this issue may read the following articles.
Bensch S, Bengtsson G and Åkesson S (2006) Patterns of stable isotope signatures in willow warbler Phylloscopus trochilus feathers collected in Africa. Journal of Avian Biology, 37, 323–330.