Climate Change Impact on the Populations of Goldcrest Regulus regulus and Firecrest Regulus Ingicapilla Migrating Through the Southern Baltic Coast

Since about 2000, we observed the decline in the numbers of Goldcrests and increase in the numbers of Firecrests caught during autumn migration at two bird ringing stations, Bukowo-Kopań and Mierzeja Wiślana, on the Polish coast of the Baltic Sea over 1976–2023 . These two closely related forest bird species share similar biotopes but differ in foraging behavior and migration strategy. The changes in migrating bird abundance in autumn we observed at both ringing stations were correlated with the Pan-European breeding population trends over 1982–2022 from PECBMS project. This suggested that some Europe-wide factors are responsible for these changes. The snow cover in October decreased abundance and delayed autumn migration of Goldcrests on the Baltic coast, probably by exposing them to cold and impeding their feeding and fueling for migration. In Firecrests, the response to October snow was similar but much weaker. Firecrests, in turn, have shifted their migration earlier since late 1980’s, alleviating the effect of early winter. The warmer was October, the larger were the numbers of Firecrests on autumn migration at both stations, which suggest their good breeding success likely due to improved survival of chicks from late broods during warm autumns. Additionally, the increased number of broadleaf trees at the breeding grounds in Fennoscandia can favor the population of Firecrest, which prefers mixed forests and larger prey like spiders, over Goldcrest, which are bound to more coniferous forests, where they feed on smaller insects. If this trend continues, we expect a decline in the number of the currently numerous Goldcrest, but farther expansion of the breeding range to north-east and increase in numbers of the Firecrest, the less abundant of the two species. The change of forest management policy in Scandinavia by planting more broadleaf trees, is aimed at increasing biodiversity and can have a large impact on the forest environment, thus it is imperative to have methods for early detection of such changes. The results we presented show that monitoring the number of migrating birds can serve as a good indicator of environmental changes.