Abstract
Disturbance may impact individual birds and ultimately bird populations. If animals avoid disturbed sites this may prevent them from being disturbed directly but may also negatively impact their movement patterns and energy budgets. Avoidance is, however, challenging to study, because it requires following individuals over large spatial scales in order to compare their movement rates between sites in relation to spatiotemporal variation in disturbance intensity. We studied how 48 GPS‐tracked non‐breeding Eurasian Oystercatchers Haematopus ostralegus used two neighbouring roost sites in the Wadden Sea. One roost site is highly influenced by seasonal recreational disturbance whereas the other is an undisturbed sandbar. We analysed roost choice and the probability of moving away from the disturbed roost site with regard to a seasonal recreation activity index, weekends and night‐time. Oystercatchers often chose to roost on the undisturbed site, even if they were foraging closer to the disturbed roost. The probability that Oystercatchers chose to roost on the disturbed site was negatively correlated with the recreation activity index and was lowest in the tourist season (summer and early autumn), indicating that birds used the site less often when recreation levels were high. Furthermore, the probability that birds moved away from the disturbed site during high tide was positively correlated with the recreation activity index. The choice to roost on the undisturbed site implies that birds must fly an additional 8 km during one high‐tide period, which equates to 3.4% of daily energy expenditure of an average Oystercatcher. Our study tentatively suggests that the costs of avoidance may outweigh the energetic cost of direct flight responses and hence that avoidance of disturbed sites requires more attention in future disturbance impact studies. Nature managers should evaluate whether high‐quality undisturbed roosting sites are available near foraging sites, and in our case closing of a section of the disturbed site during high tides in the tourist season may mitigate much disturbance impact.
Highlights
In additional energy expenditure and loss of foraging time (Platteeuw & Henkens 1997, Frid & Dill 2002)
Shorebirds are an interesting case study to quantify the impacts of disturbance on space use, as they commute between foraging areas at low tide and roost sites at high tide
No study has yet focused on how roost choice is related to recreational disturbance that varies throughout the year, and in general studies focusing on roost choice of shorebirds are lacking for the Wadden Sea area
Summary
In additional energy expenditure and loss of foraging time (Platteeuw & Henkens 1997, Frid & Dill 2002). Shorebirds are an interesting case study to quantify the impacts of disturbance on space use, as they commute between foraging areas at low tide and roost sites at high tide. In other seasons, when birds were present at Westerseveld at the start of the high tide, we observed that upon disturbance birds often flew to Richel, a deserted sandbar that is located 4 km eastwards and is protected by bird wardens Based on these observations, we hypothesized that Oystercatchers avoid roosting at Westerseveld during periods when most tourists are present on the island (i.e. in summer and early autumn), and instead directly fly to Richel at the start of the high-tide period, even though this roost is farther away from their feeding grounds. There are no other frequently used high-tide roost sites in the vicinity, meaning that these two sites provide a simple and powerful case study to compare roost choice between a disturbed and undisturbed roost location
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