Age-related changes in migratory behaviour within the first annual cycle of a passerine bird

Abstract

First time migrants (juveniles hereafter) of many species migrate without specific knowledge of non-breeding locations, but experience may aid adults in timing and route decisions because they can migrate more efficiently to their previous non-breeding sites. Consequently, we expect a transition to more efficient migratory behaviour with age, but when and how this happens is little known. We used light-level geolocator data from Cyprus wheatears Oenanthe cypriaca to compare migration timing and route directness between juveniles and adults, and repeatability of their timing and non-breeding locations. We predicted that juveniles would depart and arrive later than adults for both autumn and spring migration; that duration of migration would be greater for juveniles; that routes taken by juveniles would be less direct than those for adults; and that autumn and spring departure timing, and non-breeding locations, would be more repeatable for adults between two years than for juveniles between their first and subsequent migration. We found that juveniles departed significantly later than adults in autumn but there was no difference in arrival timing, and although spring departure timings did not differ, juveniles arrived on the breeding grounds later than adults. Nevertheless, we found no significant age-related difference in the duration of migration in autumn or spring. Yet, juvenile migrations were less direct than those of adults in autumn, but not spring. We found evidence that spring departure timing and non-breeding locations were repeatable for adults but not juveniles. Our findings show that age-related changes in migratory behaviour begin to occur during the first annual cycle demonstrating the potential for early adaptation to environmental variability within an individual's life.

Fig 1. (A-B) Autumn and (C-D) spring migration departure and arrival timing for adult and juvenile Cyprus wheatears.

Timing events are estimated from light-level geolocator data. Geolocator tags were deployed in 2017, 2018 and 2019.

Fig 2. Juvenile Cyprus wheatear migrations are less direct than adults, but only in autumn.

Differences in longitude (degrees) between the median longitude during migration and the longitude at the mid-point between the breeding grounds in Troodos, Cyprus, and non-breeding locations for autumn and spring migrations. Positive values reflect deviations to the east. Data were estimated from light-level geolocation data. Geolocator tags were deployed in 2017, 2018 and 2019.

Fig 3. Non-breeding locations of adult and juvenile Cyprus wheatear.

Locations are median location during December-January. Error bars show inter-quartile range. The black star shows location of the breeding ground field site in Troodos, Cyprus.

Fig 4. Intra-individual variation in (a) migration timing and (b) distance between non-breeding location for adult and juvenile Cyprus wheatears.

Differences are between two consecutive years from light-level geolocation data. Timing differences (a) show the direction of the difference, i.e. whether the difference is earlier or later between year one and two.