Tracking of adult males and females across a migratory divide: migration strategies of a western baltic common tern (Sterna hirundo) population

BACKGROUND: European Common Terns' migration routes along the eastern Atlantic flyway and wintering areas in western and southern Africa were identified in ringing studies. Recent tracking studies revealed a migratory divide and the use of an eastern African migration route and wintering areas in the Mozambique Channel. METHODS: 71 Common Terns breeding in a colony at the German Baltic coast were tracked with light-level geolocators. Linear Models were used to test influence of sex, year, migration route, and wintering area on timing of migration, and duration of migration and stopovers. RESULTS: A total of 58 individuals used the western African migration route, while 13 individuals used the eastern route. 45 individuals wintered at the southern African coast, while 12 individuals wintered in the Gulf of Guinea, and 13 individuals wintered in the Mozambique channel. Migration route and wintering area were identified as the most important factors influencing the duration of migration and stopovers, with birds using the eastern route showing significantly longer stopovers and duration of migration. Sex was identified to especially impact the start of migration, with females starting both autumn and spring migration significantly earlier than males. Arrival in the breeding area was only affected by the year, but this might be an effect of very low sample size in some years. Birds wintering in western Africa showed a tendency to a longer spring migration, while those wintering in eastern Africa tended to longer autumn migration. CONCLUSION: The eastern African migration route and wintering area seem to be much more common than ringing had indicated. Route and sex explain a substantial part of the observed variation in timing and duration of migration, but many factors such as annual variation in weather, wind, food availability as well as individual variation in fitness and breeding success are likely to affect timing of migration and duration of migration and stopovers. Comparison of duration of autumn and spring migration indicates that contradicting results reported in tracking studies from different European breeding areas might be explained by wintering area rather than breeding area, but are pronounced in birds with shorter migration distance.