Conserving bird populations in the Anthropocene: the significance of non-breeding movements

Abstract

Advances in tracking technologies have revealed the diverse migration patterns of birds, which are critical for range mapping and population estimation. Population trends are usually estimated in breeding ranges where birds remain stationary, but for species that breed in remote areas like the Arctic, these trends are often assessed in over-wintering ranges. Evaluating population trends during the wintering season is challenging due to the extensive movements of birds in these ranges, which require a deep understanding of the movement dynamics. However, these movements remain understudied, particularly in the mid-latitudes, where many Arctic breeders overwinter, increasing uncertainty in their ranges and numbers. Here, we show that the Arctic breeding raptor Rough-legged buzzard, which overwinters in the mid-latitudes, has a specific wintering strategy. After migrating ca 1,500 km from the Arctic to mid-latitudes, the birds continue to move throughout the entire over-wintering period, covering an additional 1,000 km southwestward and then back northeastward as the snowline advances. This ongoing movement makes their wintering range dynamic over the course of the season. In essence, this movement represents an extension of the quick migration process, albeit at a slower pace, and we have termed this migration pattern foxtrot migration, drawing an analogy to the alternating fast and slow movements of the foxtrot dance. These results highlight the potential errors in range mapping from single mid-winter surveys and emphasize the importance of this migration pattern in assessing the conservation status of bird species. Understanding this migration pattern could help to correctly estimate bird populations in over-wintering ranges, which is especially important for species that nest in hard-to-reach regions such as the Arctic.