Tidsförskjutningar av flyttfåglars höstmigration och vistelse i häckningsområdet : betydelsen av att kunna variera kullantalet i ett varmare klimat

Sammanfattning: The timing of many biological events, including the onset of spring and bird migration, have changed in the recent years due to climate change. This may affect birds’ length of stay in the breeding area. By using Swedish ringing data from two periods, 1979-1989 and 2006-2016, changes in the length of stay between birds with different migration modes and seasonal number of broods were analysed. The results indicated that changes in the length of stay differed between species depending on their seasonal number of broods. Species with a fixed number of broods shifted their stay by arriving earlier in the spring and migrating earlier in the fall. In contrast, species with a variable number of broods extended their stay by arriving earlier in the spring without changing the timing of autumn migration. An extended stay might enable a higher reproduction output by giving the opportunity to invest more time in offspring. Thus, climate change might favour species with a variable number of broods. Birds with a fixed number of broods might however not experience the same benefits of a warmer climate because their annual reproductive success often relies on one single brood. Species with a fixed number of broods are generally long-distance migrants and therefore their migration is less flexible. If they do not arrive early enough to match their breeding with the advanced timing of peak food abundance a mismatch arises. As a result, they might be more sensitive to climate change