Migration strategy and tactics of Eastern Marsh-harriers from the western part of the species range

Abstract

From 2021 to 2024, three sibling Eastern Marsh-harriers from a nest near Irkutsk were tracked using ICARUS transmitters. One male provided data for three full years, and a second individual for two full years. The male exhibited fidelity to its wintering site, returned to its birthplace at two years of age, and successfully bred at the same location at three years old. The second individual changed its wintering site from China to Vietnam between years. As one-year-olds, both immature birds spent the summer within a 30–50 km radius of their natal area, ranging more widely than during the following summer at two years of age. Wintering areas were located in Southeast Asia, from the Yangtze River basin to northern Thailand. Autumn and spring migrations were rapid (up to 660 km per travel day), involved few short stopovers, and likely consisted predominantly of flapping flight. The migration routes of the two individuals were more similar to each other than the autumn and spring routes of a single bird, indicating that the harriers strategically utilized the geographical and climatic conditions of the regions they traversed. In their first year, the young harriers arrived at their wintering grounds and subsequently at their natal area significantly later than in subsequent years, likely due to inexperience. Overall migration speeds were lower during the first migration compared to later ones, although the differences were not statistically significant; the speeds on travel days also showed minor, non-significant differences.