Orientation in willow warblers
The young inexperienced willow warblers migrate alone during the night without guidance from their parents, so they have to rely on their genetic migratory program and the environmental cues en route. Such compass cues for the migratory passerines have been shown to be the Sun, stars, polarized light and the Earth’s magnetic field.
Stensoffa field station
At Stensoffa Ecological Field Station we study the migratory orientation of willow warblers by doing experiments using Emlen funnels. Emlen funnels are cone-shaped cages where birds leave claw marks on a thermo-paper when jumping on the sloping walls of the cage in their preferred direction.
We were mostly interested in the use of the magnetic field cues for orientation and more specifically how the birds orient when passing the magnetic equator. The birds have been shown to use an inclination compass, which means they orient sensing the angle between the geomagnetic field lines and the Earth surface. In the northern hemisphere this angle is getting smaller in the direction to the magnetic equator and then increases again in the southern hemisphere towards the South Pole. Thus if a bird from the northern hemisphere wants to migrate south it should follow the direction of decrease in this angle. Since the willow warblers pass the magnetic equator during their migration and go further south for wintering they should keep their heading despite the opposite pattern of the magnetic lines south of the equator. During this time other orientation cues probably help the birds to orient in the correct direction.
With our study we wanted to see how the willow warblers would react to simulated equator crossing if they had access only to magnetic field information, but not to celestial cues. We were also curious to check if presented also with the celestial cues of southern Sweden the willow warblers would change their orientation to correspond with spring migration. In order to check this we simulated the equator magnetic conditions by keeping the birds in horizontal magnetic field for a few days. Before and after this we tested them with and without availability of celestial cues.
We hope to advance our understanding about the orientation mechanisms of the willow warblers, and the passerines in general, after analyzing the collected data.