Consistency in long-distance bird migration: Contrasting patterns in time and space for two raptors

As the evolutionary responses to environmental change depend on selection acting on individual differences,
disentangling within- and between-individual variation becomes imperative. In animal
migration research, multiyear tracks are thus needed to estimate the individual consistency of phenotypic
traits. Avian telemetry studies have recently provided the first evidence of individuality across
space and time in animal migration. Here, we compare repeatability patterns of routes and timing between
two migratory birds, the marsh harrier, Circus aeruginosus, and the osprey, Pandion haliaetus, as
recorded by satellite tracking. We found interspecific contrasts with low repeatability in timing and
duration and a high repeatability in routes for ospreys, but the reverse pattern for marsh harriers. This
was mainly caused by (1) larger between-individual variation in routes for ospreys (broad-front
migration) than for marsh harriers (corridor migration) and a higher degree of repeated use of the same
stopover sites among ospreys, and (2) higher within-individual consistency of timing and duration
among marsh harriers, while individual ospreys were more flexible. Our findings suggest that individuality
in space and time is not a shared trait complex among migrants, but may show adaptive
variation depending on the species' life history and ecology.