Sex-related migration distances in the dimorphic Eurasian Bittern Botaurus stellaris breeding in the Netherlands

In many bird species intraspecific variation in migration strategies is related to sex or size. The Eurasian Bittern Botaurus stellaris is a sexually size-dimorphic heron species with a vast breeding and wintering range spanning a range of climates. Ringing data show that Bitterns from northern populations migrate westwards or southwards over thousands of kilometres, while Bitterns from southern or temperate breeding areas migrate much shorter distances or remain resident. So far, any differences in the migrations of males and females have remained unstudied. In temperate climates, relatively benign winters alternate with the occasional harsh winter, and under these conditions, males as the larger sex might take the risk to stay rather than show seasonal migration. In the years 2010-2012 we equipped three females and three males from breeding areas in The Netherlands with a tracking device, recording their movements over periods of 2-5 years. All three males and one female stayed within or near the breeding area during the non-breeding season, but two females moved to distant wintering sites. One female migrated 4900 km to The Gambia providing the first direct evidence for trans-Saharan migration in this species. Another female migrated 700 km to winter in Devon, UK, for five consecutive years. The Bitterns were site-faithful to their breeding area and, mostly, to their wintering areas as well, although one male and one female progressively wintered closer to the breeding area in three successive years. Our results suggest that larger males from The Netherlands, a breeding area with fluctuating winter conditions, are predominantly resident, while the females are partial migrants with individually different and flexible strategies.