With the current risks caused by sea level rise and increased extreme weather events, the study of natural coastal systems has never been more important. Erosion and anthropogenic forcing led to disappeared of the majority of coastal bogs in Europe. Here, we report on case study of a unique bog remnant still under influence by seawater which floats during storm floods. We investigated biogeochemical characteristics and discuss mechanisms that influence buoyancy, which is of vital importance for the conservation of the bog and can provide insights into the functioning of coastal bogs and potential consequences of future sea level rise. The studied area is characterized by a steep salinity gradient and marine clay deposits provide the 'hinge' that allows the upper peat layers to float. Our results show out that buoyancy is driven by a combination of factors: the density differences, desiccation along the edges and methane production. If the ability to float is reduced in coastal bogs, the impact of erosion and the sum of several other processes (i.e., peat decomposition, salt stress, clay sedimentation, internal eutrophication and reduced methanogenesis) can cause a shift in environmental conditions and lead to loss of this unique habitat and its characteristic species.
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.
Capsule: Migration studies of tagged Eurasian Bitterns Botaurus stellaris provide information on the behaviour of this secretive species in relation to weather conditions. Aims: To study if cold spells trigger southward migration in resident Eurasian Bitterns. To describe migratory behaviour including flight routes, altitudes, flight direction in relation to wind speed and direction, and general characteristics of stopping sites. Methods: Six adult Eurasian Bitterns from Dutch breeding areas were followed for two to five years using ARGOS-PTT/GPS transmitters or GPS trackers. Results: Four individuals remained at the breeding sites in winter and did not migrate in response to cold spells. Two individuals covered distances of 300 and 1600 km between stopping sites. They migrated predominantly at night, but over the Sahara they extended flights into daylight hours. Depending on wind assistance, flight speeds (groundspeed) varied between 3.6 and 26 ms −1 . Flight altitude varied from just above sea level when facing headwinds, to almost 2000 m above sea level under tail wind conditions. Repeatedly tracked individuals showed substantial variation in routes and stopping sites between successive migrations. These flight patterns appeared influenced by the speed and direction of the winds encountered en route. Conclusions: Eurasian Bitterns were either migratory or resident and the latter individuals did not perform facultative movements at the onset of cold spells. The flight speed, flight altitudes, and routes of migrating birds were influenced by wind conditions. This likely explains the variable use of stopping sites between years.
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