© Marcel Burkhardt
Bauer, S., B. J. Ens & M. Klaassen (2010)
Many routes lead to Rome: potential causes for the multi-route migration system of Red Knots, Calidris canutus islandica.
Ecology 91 (6): 1822–1831
Migrants, such as birds or representatives of other taxa, usually make use of several stopover sites to cover the distance between their site of origin and destination. Potentially, multiple routes exist, but often little is known about the causes and consequences of alternative migration routes. Apart from their geographical distribution, the suitability of potential sites might play an important role in the animals’ decisions for a particular itinerary. We used an optimal-migration model to test three non mutually exclusive hypotheses leading to variations in the spring migration routes of a subspecies of Red Knot, Calidris canutus islandica, which migrates from wintering grounds in Western Europe to breeding grounds in Greenland and the Canadian Arctic: the breeding location hypothesis, the energy budget hypothesis, and the predation risk hypothesis. Varying only breeding location, the model predicted that birds breeding in the Canadian Arctic and on West Greenland stopover on Iceland, whereas birds breeding in East and Northeast Greenland migrate via northern Norway, a prediction that is supported by empirical ﬁndings. Energy budgets on stopover sites had a strong inﬂuence on the choice of route and staging times. Varying for aging-intensity and mass-dependent predation risk prompted the birds to useless risky sites, if possible. The effect of simultaneous changes in the energy budget and predation risk strongly depended on the site where these occurred. Our ﬁndings provide potential explanations for the observations that C. canutus islandica uses a diverse array of migration routes. Scrutinizing the three alternative driving forces for the choice of migratory routes awaits further, speciﬁc data collection in rapidly developing ﬁelds of research (e.g., predation risk assessment, GPS tracking). Generally, the type of modelling presented here may not only highlight alternative explanations, but also direct follow-up empirical research.
Keywords: Calidris canutus islandica, food intake rate, Icelandic Red Knot, migratory connectivity, optimal migration, predation risk, shore birds, stochastic dynamic programming model.