It’s raining once again here in the mountains of middle Sweden. We are strattling the border with Norway at the secondary contact zone for two subspecies of willow warblers which breed here in the birch forests. Each day we head out with our rain gear, poles, and mist nets to target trap willow warblers using an audio playback of their song. When we first arrive at our field site we get out of the car and listen. Once we have located a singing male, we hike over to his territory and start the playback; most of the time their response is immediate and aggressive. We quickly deploy the net and hopefully within minutes have a bird in the net.
Each year millions of willow warblers (Phyllscopus trochilus) migrate north from Africa to Sweden to breed. There are two subspecies, P. trochilus acredula which winters in east to south Africa and breeds in northern Sweden, while P. trochilus trochilus which winters in west Africa and breeds in southern Sweden. Each takes a different migratory route to Scandinavia where they meet in a secondary contact zone in middle Sweden. This contact zone lies between 61 and 63 degrees latitude. Here we have come for the past two summers in an attempt to better define the contact zone and our understanding of the factors that maintain and limit it to a narrow and apparently stable area.
This year we arrived at the beginning of May driving north from Skåne following the migrating willow warblers north and the leafing out of the birch (Betula pubescens and B. pendula). Birch is a key species indicating the presence of willow warbler habitat. On the first of May just north of Mora the birch made an abrupt transition from leafing out to still having dormant buds. In addition, the song of the willow warbler disappeared from the birch forests. By the time we had reached Ljungdalen, it was still winter transitioning to spring. It was ten days before the first willow warbler showed up on the Flatruet (the highest road in Sweden).
It was another three days the before next willow warbler arrived and then they began to fill into this region. The month of May has been extremely exciting. Tracking the phenology of migration and the subsequent breeding of a single species gave us the opportunity to see the transition of winter to spring and then to summer (we hope) as the snow melts, the trees leaf out and the wildflowers bloom. While the landscape comes to life we witnessed the spectacular migration of all the other birds that breed in the mountains of Sweden. The first hearty migrants to arrive were the thrushes, finches, and gulls. The hay meadows in Ljungdalen were filled with thousands of fieldfares for over a week. The next to arrive were the waders. On cold and snowy days the golden plovers would take refuge in the valley and marshlands around the Storsjön. Gradually the insectivorous birds showed up with the warming weather. The first house martins signaled the abundance of insects was finally sufficient and we began to see willow warblers in larger numbers. By the end of May most of the willow warbler territories were occupied. The last migrant we encountered was the long-tailed jaeger which breeds up on the tundra and in montane habitats above the treeline.
Waiting for the first females to arrive, we went out each day to field sites in both low and high elevation birch habitat to identify all the territories and trap the males. We used playback and mist nets to capture the birds. When the males first arrive on their territories they are less aggressive but still easily captured. Gradually as the territories became denser as they shrink to a stable size tolerated by all the neighbors, the birds became more aggressive and responded very quickly to our playback. Sometimes the males are so aggressive they are jumping into the net before we even have it completely setup!
Next the females arrived, the males continued their aggressive behavior, fluttering their wings, singing, and flying directly towards our speakers until they were caught. Then the females joined in the aggressive behavior. For about one week we saw the females respond to our playback by calling, fluttering their wings, and aggressively flying towards our speaker. When captured most of them were developing their brood patch and appeared to have eggs developing in their oviducts. Now the females have disappeared presumably to attend their clutches and the males have become much more quiet.
Although each bird is captured as they arrive, we have no idea which subspecies they represent. In order to elucidate the precise region of the contact zone, the timing of arrival, and the breeding phenology for each subspecies, we collected biometrics, blood for molecular analysis, and feathers for stable isotope analysis. Previous studies by our PhD supervisors, Susanne Åkesson and Staffan Bensch, have identified differences in stable carbon and nitrogen isotopes between the two subspecies reflecting their wintering grounds. Each subspecies winters and moults in a different region of Africa allowing us to separate the majority of individuals based on the stable isotope ratios within their feathers. In addition, there are two AFLP markers thought to be associated with the migratory divide at the contact zone.
Despite all the rain and cold this year we have had great success trapping willow warblers. The most exciting aspect of the field work is the large number of recaptures of individuals we trapped last year. This gives us a unique opportunity to compare feather stable isotope ratios of these birds from multiple years. Of course the short-eared owl we saw yesterday and the stunning mountains also makes for a very enjoyable field season…
Each year millions of willow warblers (Phyllscopus trochilus) migrate north from Africa to Sweden to breed. There are two subspecies, P. trochilus acredula which winters in east to south Africa and breeds in northern Sweden, while P. trochilus trochilus which winters in west Africa and breeds in southern Sweden. Each takes a different migratory route to Scandinavia where they meet in a secondary contact zone in middle Sweden. This contact zone lies between 61 and 63 degrees latitude. Here we have come for the past two summers in an attempt to better define the contact zone and our understanding of the factors that maintain and limit it to a narrow and apparently stable area.
This year we arrived at the beginning of May driving north from Skåne following the migrating willow warblers north and the leafing out of the birch (Betula pubescens and B. pendula). Birch is a key species indicating the presence of willow warbler habitat. On the first of May just north of Mora the birch made an abrupt transition from leafing out to still having dormant buds. In addition, the song of the willow warbler disappeared from the birch forests. By the time we had reached Ljungdalen, it was still winter transitioning to spring. It was ten days before the first willow warbler showed up on the Flatruet (the highest road in Sweden).
It was another three days the before next willow warbler arrived and then they began to fill into this region. The month of May has been extremely exciting. Tracking the phenology of migration and the subsequent breeding of a single species gave us the opportunity to see the transition of winter to spring and then to summer (we hope) as the snow melts, the trees leaf out and the wildflowers bloom. While the landscape comes to life we witnessed the spectacular migration of all the other birds that breed in the mountains of Sweden. The first hearty migrants to arrive were the thrushes, finches, and gulls. The hay meadows in Ljungdalen were filled with thousands of fieldfares for over a week. The next to arrive were the waders. On cold and snowy days the golden plovers would take refuge in the valley and marshlands around the Storsjön. Gradually the insectivorous birds showed up with the warming weather. The first house martins signaled the abundance of insects was finally sufficient and we began to see willow warblers in larger numbers. By the end of May most of the willow warbler territories were occupied. The last migrant we encountered was the long-tailed jaeger which breeds up on the tundra and in montane habitats above the treeline.
Waiting for the first females to arrive, we went out each day to field sites in both low and high elevation birch habitat to identify all the territories and trap the males. We used playback and mist nets to capture the birds. When the males first arrive on their territories they are less aggressive but still easily captured. Gradually as the territories became denser as they shrink to a stable size tolerated by all the neighbors, the birds became more aggressive and responded very quickly to our playback. Sometimes the males are so aggressive they are jumping into the net before we even have it completely setup!
Next the females arrived, the males continued their aggressive behavior, fluttering their wings, singing, and flying directly towards our speakers until they were caught. Then the females joined in the aggressive behavior. For about one week we saw the females respond to our playback by calling, fluttering their wings, and aggressively flying towards our speaker. When captured most of them were developing their brood patch and appeared to have eggs developing in their oviducts. Now the females have disappeared presumably to attend their clutches and the males have become much more quiet.
Although each bird is captured as they arrive, we have no idea which subspecies they represent. In order to elucidate the precise region of the contact zone, the timing of arrival, and the breeding phenology for each subspecies, we collected biometrics, blood for molecular analysis, and feathers for stable isotope analysis. Previous studies by our PhD supervisors, Susanne Åkesson and Staffan Bensch, have identified differences in stable carbon and nitrogen isotopes between the two subspecies reflecting their wintering grounds. Each subspecies winters and moults in a different region of Africa allowing us to separate the majority of individuals based on the stable isotope ratios within their feathers. In addition, there are two AFLP markers thought to be associated with the migratory divide at the contact zone.
Despite all the rain and cold this year we have had great success trapping willow warblers. The most exciting aspect of the field work is the large number of recaptures of individuals we trapped last year. This gives us a unique opportunity to compare feather stable isotope ratios of these birds from multiple years. Of course the short-eared owl we saw yesterday and the stunning mountains also makes for a very enjoyable field season…
Keith Larson, Nils Müller, and Sieglinde Kundisch
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