A fundamental study by Ens et al. (1992, Journal of Animal Ecology, 61, 703) developed the concept of two different nest‐territory qualities in Eurasian oystercatchers (Haematopus ostralegus, L.), resulting in different reproductive successes. “Resident” oystercatchers use breeding territories close to the high‐tide line and occupy adjacent foraging territories on mudflats. “Leapfrog” oystercatchers breed further away from their foraging territories. In accordance with this concept, we hypothesized that both foraging trip duration and trip distance from the high‐tide line to the foraging territory would be linearly related to distance between the nest site and the high tide line. We also expected tidal stage and time of day to affect this relationship. The former study used visual observations of marked oystercatchers, which could not be permanently tracked. This concept model can now be tested using miniaturized GPS devices able to record data at high temporal and spatial resolutions. Twenty‐nine oystercatchers from two study sites were equipped with GPS devices during the incubation periods (however, not during chick rearing) over 3 years, providing data for 548 foraging trips. Trip distances from the high‐tide line were related to distance between the nest and high‐tide line. Tidal stage and time of day were included in a mixing model. Foraging trip distance, but not duration (which was likely more impacted by intake rate), increased with increasing distance between the nest and high‐tide line. There was a site‐specific effect of tidal stage on both trip parameters. Foraging trip duration, but not distance, was significantly longer during the hours of darkness. Our findings support and additionally quantify the previously developed concept. Furthermore, rather than separating breeding territory quality into two discrete classes, this classification should be extended by the linear relationship between nest‐site and foraging location. Finally, oystercatcher′s foraging territories overlapped strongly in areas of high food abundance.
Wetlands such as the World Heritage Site in the Wadden Sea include important habitats for breeding waterbirds. Its saltmarshes and adjacent conservation polders are used by thousands of breeding birds. However, some alarming population declines have been recorded during recent decades and previous studies found indications that predation pressure from red foxes (Vulpes vulpes) and more recently from invasive racoon dogs (Nyctereutes procyonoides) played an important role. The current study aimed to assess habitat utilisation by foxes and racoon dogs along the coast of the Wadden Sea. We equipped 21 foxes and seven racoon dogs with GPS collars and recorded a total of 37,586 (mean: 2,088) GPS fixes during a total of 2,617 (mean: 145) equipment days for red foxes and 3,440 (mean: 573) GPS fixes during a total of 272 (mean: 45) equipment days for racoon dogs. Foxes showed high individual variability in Kernel 95% home range sizes, with a mean of 172.2 ha (range: 3 to 824 ha) and little overlap among territories. Males had significantly larger home ranges than females, and there were no differences in home range sizes between adults (n = 14) and young (n = 4). Racoon dogs had smaller home ranges than foxes (mean: 52.8 ha). The preferred habitat type of both predators during daytime was the conservation polders along the Wadden Sea, while foxes also selected saltmarshes during the night. In contrast, both species avoided farmland areas. Foxes showed 20% of their activity during daylight hours and spent this time largely in areas with dense vegetation cover. None of the tagged individuals entered areas with particularly high bird densities (i.e. Wadden Sea islands or Halligen). However, our data suggest that foxes and racoon dogs frequently make use of linear structures such as dykes and dams and patrol along the tide line for carcasses. This suggests that at least single individuals of both species are prone to enter islands that are connected by dams to the mainland.
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