Inheritance and learning in raptor migration
This particular project aims to obtain tracking records of the entire migration history of some individuals - from their first migration journey as juveniles and further on in their life as adults. Analyses of repeated journeys and of individual migration histories are fundamental for revealing the relative importance of inheritance and learning in bird migration. How is inherent spatiotemporal program complemented by imprinting and learning during the first and successive journeys? How do wind drift and other deviations during one journey affect succeeding journeys?
Larger sample of juveniles
Due to a much higher mortality rate among juveniles, it is a much more difficult task to obtain multi-year migration data for individual juveniles than for adults. Hence, we wish to make a special effort during the coming years to obtain information about the entire migration history of individuals by carrying out GPS satellite tracking of a larger sample of juvenile ospreys to make sure that at least 5 are expected to survive several years into adulthood. Here a juvenile osprey is released that just has been fitted with a GPS-satellite transmitter.
We have already taken the first step of such analyses in a study of repeated migratory journeys by individual adult ospreys demonstrating the existence of intermediary goal areas as well as important differences between autumn and spring migration. Routes followed by the same individual osprey during three successive autumn (blue) and spring (red) migration journeys as recorded by satellite tracking. The bird showed fidelity not only to its breeding and wintering site but also to an intermediary stopover site in Europe that was visited on all journeys (yellow). Additional possible goal areas, where the routes converge either on autumn or spring migration, are indicated in blue (potential autumn goal area) or red (spring goal area).
Individuality in routes and timing
In a second study we examined the individual consistency in migratory routes and timing of adult marsh harriers. Unexpectedly, there was a much stronger individual repeatability in time (b, c) than in space (a). While individuals (in different colours) had consistent annual migratory schedules, their routes varied considerably between years across the whole migration corridor of the population. Endogenous control of migratory timing in combination with navigation flexibility in response to short-term environmental variation (winds and habitats) may explain the contrasting degrees of individual.
We will use this approach to further analyse the much more detailed multi-year GPS satellite tracking data that we have obtained from ospreys and marsh harriers since 2006 - positions every hour in 3 dimensions with 10-20 m accuracy during the birds’ repeated journeys between Sweden and Africa! Course changes during days will be analysed in relation to different possible compass mechanisms as well as to wind, landscape and possible goal areas and landmarks from preceding journeys. Here a GPS-tag is fitted to an adult male marsh harrier.