Timing is Everything

While there is a huge diversity of behaviors birds use during mating, it is important to remember that in many cases there are outside factors that can affect them that may not be obvious at first. For instance, Bearhop et al (2006) have fairly recently done some interesting work on migration. Specifically, how the migration of the European Blackcaps may end up undergoing speciation in the long run.

The European Blackcaps are a species of European passerines, a fairly common little bird found across Eurasia and northern Africa, have historically bred in south central Europe. However, instead of migrating to Iberia and Northern Africa for their overwintering grounds a rapidly increases number of the birds are showing up in the British Isles. And as time goes on, it seems that they are staying.

Bearhop and his colleges used a stable isotope technique for tracking these birds across their migration distances, something that had previously been quite difficult as Iberia and the British Isles are a fair distance away from one another and these birds are rather small. Lots of room for error and lost birds. However the stable isotope technique allowed them for the first time to get accurate, traced results and what they found could have some interesting long term implications.

They found that there is a genetic basis for the new migratory pattern displayed by the birds overwintering in the British Isles yet still returning to the south central Europe breeding grounds. And their numbers are growing, hinting that there may be some benefit towards fitness to change up the overwinter areas.

European Blackcap at a feeding table.

There are several reasons why the birds heading to the UK and Ireland may be experiencing higher fitness. The level of urbanization is quite high, presenting easy access to food from feeders, rubbish bins, as well as exotic plant species which fruit and flower more frequently throughout the year. The shorter migration distance is less energetically exhausting, and some have even theorized that the colder climate better prepares the birds for the early breeding ground climate upon their return. However, what may be the most important observation is that critical photoperiods during juvenile development that trigger gonad development occur in northern climates about 10 days earlier than the ancestral breeding ground.

That change shifts the development and breeding cycle of the birds up by several days, triggering migration back south sooner than those in Iberia return north. What we’re seeing is assortive mating being driven by a genetically driven behavioral change. This could, over time, isolate the species by migration pattern, and eventually even lead to speciation as the change is genetically driven.

So there you have it, a gene change for migration behavior may lead the European Blackcap to speciate sometime in the future. A time that may not even bee that far away really, as the migration change of birds towards the British Isles has rapidly occurred just over the past 60 years or so!

References

Bearhop, S, Fiedler, W, Furness, RW, Votier, SC, Waldron, S, Newton, J, Bowen, GJ, Berthold, P & Farnsworth, K 2005, ‘Assortative Mating as a Mechanism for Rapid Evolution of a Migratory Divide’, Science, vol. 310, no. 5747, pp. 502–504.

Picture: http://www.burdr.com/2009/12/blackcaps-adapting-to-a-new-migratory-path/