Strategies for the transition to breeding in time-selected migration
Research output: Contribution to journal › Article
In time-selected migration birds adapt their fuel deposition and flight behaviour to maximise sustained migration speed. What are the expectations for the final part of spring migration when the transition to breeding takes place and the criterion of a maximum total migration speed is no longer relevant? Two possible strategies representing different ends of the capital – income spectrum are evaluated. In the first strategy the birds gain an advancement in the breeding cycle by depositing breeding resources while still on migration, as long as the marginal resource deposition rate at the final stopover site, devaluated for the flight transport costs, exceeds that at the breeding destination. In the second strategy an early arrival at the breeding site, before competitors, is of overriding importance, and sprint migration is predicted. In this case migration towards the breeding grounds would to a large degree be a race between competitors, where the birds are expected to change from a maximum sustained speed during much of migration to a final sprint. In such sprint migration the birds exhaust their resources and expose themselves to increased risks in order to obtain the critical priority benefits associated with an arrival before competitors. If and to what degree these strategies exist among migratory birds is unknown. Predictions are given for testing if capital breeding is driven by differential resource gain rates at stopover versus breeding sites. For testing the strategy of sprint migration, investigations of the migrants’ flight behaviour on their final approach to the breeding destinations will be decisive. Inspection of satellite tracking data for two Ospreys Pandion haliaetus revealed an accelerated final approach to the breeding site including nocturnal flights in addition to the regular diurnal thermal soaring migration in one but not the other individual.