To Africa and back

Female (left) and male (right) breeding pair of willow warblers, Flatruet, Harjedalen, Sweden. Photo: Max Lundberg

Over the past several summers we have been working largely in the Swedish mountains chasing willow warblers around the forest with nets and an mp3-speaker system playing the alluring song of the male. There are many questions to be answered and all it takes is a little knowledge of where to look for the birds, trapping them, taking a few measurements, and extracting a feather and some blood.

Here at 62.7° north latitude straddling the border of Sweden and Norway lies a migratory divide for the two migratory phenotypes, “migratypes”, trochilus which winters in West Africa and acredula that winters in East to South Africa. We want to know what happens when these two migratypes meet here during the breeding season. Do they mate assortatively, trochilus with trochilus or acredula with acredula, or do they hybridize and produce offspring?

The data we gather helps separate the individuals in this “contact” zone and ascertain whether they prefer their own kind or interbreed. Over the past two years we have focused specifically on finding breeding pairs and trying to gain insight into these questions.

Although we are still trying to answer these questions we often have interesting surprises from our fieldwork. For example, last summer we found a willow warbler pair with a nest in the bushes when they typically nest on the ground. This year we captured a female with colour-rings that allowed us to identify her as one ringed at her nest in 2011. Typically each year we capture a few males that were ringed in previous years, but female recaptures are rare. A quick check of last year’s territory map shows she nested approximately 500 meters southeast of where we just captured her this year.

Female willow warbler with faded colour-rings, Flatruet, Harjedalen, Sweden. Photo: Max Lundberg.

Looking at her colour-rings we could see that the red ring was faded to pink suggesting that she wintered somewhere with lots of bright sunshine and an open landscape. As she bred here last year that means she is at least three years old and has been back-and-forth from Africa at least twice (or four one-way trips). This little nine gram bird has traveled a minimum of 40,000 km in her lifetime.

//Keith Larson and Max Lundberg