Causes and Consequences of Partial Migration in Skylarks
Should I stay or should I go? This is not just the title of the famous 1981 rock song by The Clash, but also a question many birds face every autumn. In so-called partial migrants, some individuals of a breeding population remain on the breeding grounds all year, while others leave in the fall to spend the winter in milder regions. A new study by Arne Hegemann and colleagues investigated the causes and consequences of partial migration in skylarks, a partial migrating bird. This study, recently published in The American Naturalist, suggests that a combination of size and immune function can explain why birds that are neighbours in summer spend the winter in very different regions.
“That the size of an individual influences its decision to migrate or to remain resident has been shown before, especially in fish. However, that individuals can switch forth and back between strategies and that immune function is also involved is an exciting new finding—one that gives a new perspective on the ecology and evolution of partial migration,” says Dr. Arne Hegemann, lead author of the paper.
That resident skylarks, when compared with migratory conspecifics, are less likely to return to breed in future years has also important implications for conservation planning. The costs in survival of residency may contribute to the ongoing sharp decline of this species.
You can find a press release here http://www.amnat.org/an/newpapers/OctHegemann.html and the paper here http://www.jstor.org/stable/info/10.1086/682667