Blaccap Males at Feeder by Fizzog2

Over the past 50 years, some of the southwest migrating Blackcaps in Europe are now migrating northwest to the United Kingdom for the winter. New research looks at the physical and genetic variations between these two different migrating groups.

A Blackcap is a type of sylviid warbler found throughout Europe. Blackcaps were first discovered to migrate to the United Kingdom in the early 1960s. Over the years their numbers have increased significantly.

What makes this new northwest migratory path so much more appealing? In 1996, A.J. Helbig writes:

The selection pressures that may favor wintering of continental Blackcaps in Britain rather than in the western Mediterranean region may include the following.

1. Factors acting in the new wintering are such as progressively milder winters in recent decades, reduced competition compared with Mediterranean habitats and improved food supply during cold spells at bird tables provided by man, an important resource exploited by wintering Blackcaps (Hardy, 1978), and at berry bushes planted in coatal areas and gardens.
2. Factors related to the geographic location of the new winter quarters: the migration distance is shorted by up to 1500 km and owing to theearlier increase in day length, British-wintering birds would be expected to return earlier to the breeding grounds (Berthold an Terill, 1988)

In fact, the northwest migrating Blackcaps arrive back from the United Kingdom at the breeding grounds about 10 days earlier than their southwest counterparts. In 1996, A.J. Helbig writes about the importance of this:

An earlier return to the breeding grounds of British versus Mediterranean wintering Blackcaps should enable them to pair preferentially among each other (assortative mating based on arrival times), to occupy the best territoris and to produce the most offspring.

Based on this, scientists believe that it would be possible for differences in the birds to begin to appear over time.

A. J. Helbig, 1996:

Assortative mating based on different arrival times on the breeding grounds is a distinct possibility that would restrict gene flow between populations that use different wintering quarters. Unfortunately, no markers for birds of different winter origin are yet available that would allow us to test this hypothese.

This new study proves his hypothesis. Scientists at the University of Freiberg in Germany caught Blackcaps returning to their breeding grounds in the Spring. They tested parts of their genome and found small but significant variations. The differences are great enough that they can accurately differentiate between the two migrating groups just by looking at their genetic data.

They also compared their physical traits. The northwest migrating Blackcaps have slightly rounder wings which improve maneuverability but are not very efficient for long distance trips. In contrast, the southwest migrating birds had more pointed wings for faster travel and longer journeys. This difference makes sense because the migratory path for the northwest migrating Blackcaps is much shorter than the southwest migrating Blackcaps.

The Blackcaps migrating to the United Kingdom also have narrower beaks similar to other birds that visit backyard feeders. Their southwest migrating relatives have wider beaks making them better suited for eating the large fruit they eat during the winter such as figs and olives.

Normally adaptions such as this are thought to occur over hundreds or thousands of years in isolation in somewhere like the Galapogos Islands. This however has occured over the past 50 years right in the backyard of many European birdwatchers!

It is important to realize that these Blackcaps are not a new species. Scientists believe that this could just be the very beginning of that process.

Over the past week, there have been some sensational headlines floating around the internet such as Birdfeeders Found to Cause Evolution of New Species, Fill Up Your Backyard Feeder, Create A New Speices or my personal favorite Feed a Bird, Split a Species.

Whoa! They seem to imply that the only reason the Blackcaps are changing is due to birdfeeders. I know this makes for a great headline, but I disagree. Now I’m not saying birdfeeders didn’t have a significant effect on the Blackcaps. I’m just saying that this is not the only reason.