The hidden cost of disturbance: Eurasian Oystercatchers (Haematopus ostralegus) avoid a disturbed roost site during the tourist season

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

Klimaatverandering, verhoogde overstromingsrisico’s en kwelderbroedvogels [Climate change, tidal flooding risks and breeding shorebirds]

Het gaat slecht met veel vogels die in de Waddenzee broeden. Naast directe invloeden van menselijke activiteiten wordt vermoed dat klimaatverandering daarbij een belangrijke rol speelt. In dit artikel belichten we één aspect van klimaatverandering waarvan de gevolgen de laatste jaren al merkbaar zijn, namelijk het vaker optreden van overstromingen van nesten in buitendijkse broedgebieden. Lage kwelders vormen‘s zomers belangrijke broedgebieden: in hoeverre beïnvloedt een toenemend overstromingsrisico de levensvatbaarheid van vogelpopulaties, of kunnen vogels hun nestkeuze aanpassen aan deveranderende omstandigheden? Zou uitbreiding van kwelderareaal kunnen leiden tot het creëren van een ‘ecologische fuik’ voor vogels en zijn er maatregelen denkbaar om de overstromingsrisico’s voor kwelderbroedvogels tegen te gaan? [English] Climate change, tidal flooding risks and breeding shorebirds Bird populations are declining in the Wadden Sea. The contribution of climate change has been understudied, but recent evidence suggests that increased flooding frequencies during the breeding season are already affecting bird populations that breed on salt marshes. These effects are expected to get worse in the near future if birds do not adapt their nest-site selection, suggesting that the importance of low-lying breeding areas for shorebirds could decline substantially in the coming decades. The crucial question now is to what extent birds may adapt their habitat selection to the changing environment. Although several studies suggest that the adaptive potential of birds to more frequent flooding events is limited, this is based on anecdotal evidence and systematic long-term studies are now needed. Furthermore, such studies will inform us about the factors that constrain birds from selecting higher nest sites (vegetation, food, predators), such that efficient management strategies may be identified

Spatiotemporal variation in disturbance impacts derived from simultaneous tracking of aircraft and shorebirds

Assessing the impacts of disturbance over large areas and long time periods is crucial for nature management, but also challenging since impacts depend on both wildlife responses to disturbance and on the spatiotemporal distribution of disturbance sources. Combined tracking of animals and disturbance sources enables quantification of wildlife responses as a function of the distance to a disturbance source. We provide a framework to derive such distance–response curves and combine those with disturbance source presence data to quantify energetic costs of disturbance at a landscape scale. We tracked 90 Eurasian Oystercatchers Haematopus ostralegus and all aircraft in a military training area in the Dutch Wadden Sea. We quantified distance–response curves estimating flight probability and additional displacement for five types of aircraft activities, by comparing bird movement prior to aircraft presence with movement during aircraft presence. We then used the distance–response curves to map mean and variation in additional daily energy expenditure due to cumulative aircraft disturbance across the landscape for a 700-day period. Flight probability and displacement responses differed strongly among aircraft activities and decreased from transport aeroplanes, through bombing jets, helicopters, jets to small civil aeroplanes. Since the most disturbing aircraft activities were also the rarest ones, mean additional daily energy expenditure did not exceed 0.25%. However, days with substantial (>1%) additional expenditure occurred between 0.1% and 3.7% of all days across high-tide roosts in the tidal basin. Notably, expenditure particularly spiked on days with transport aeroplane activity (up to 8.5%). Synthesis and applications. We quantified cumulative energetic flight costs due to aircraft disturbance and found that these were low and unlikely to impact survival of oystercatchers in our study area. Our results provide evidence that the legal minimum flight height of 450 m for small civil aeroplanes effectively limits the disturbance of oystercatchers. Mitigation should focus on limiting the number of days when disturbance has a high impact by reducing rare but highly disturbing activities, especially transport aeroplanes. Our approach can be applied to other species and disturbance sources that are automatically tracked, for example boats and walkers, ultimately to quantify the entire anthropogenic disturbance landscape

Balancing food and density-dependence in the spatial distribution of an interference-prone forager

Foraging distributions are thought to be density-dependent, because animals not only select for a high availability and quality of resources, but also avoid conspecific interference. Since these processes are confounded, their relative importance in shaping foraging distributions remains poorly understood. Here we aimed to rank the contribution of density-dependent and density-independent effects on the spatio-temporal foraging patterns of eurasian oystercatchers. In our intertidal study area, tides caused continuous variation in oystercatcher density, providing an opportunity to disentangle conspecific interference and density-independent interactions with the food landscape. Spatial distributions were quantified using high-resolution individual tracking of foraging activity and location. In a model environment that included a realistic reconstruction of both the tides and the benthic food, we tested a family of behaviour-based optimality models against these tracking data. Density-independent interactions affected spatial distributions much more strongly than conspecific interference, even in an interference-prone species like oystercatchers. Spatial distributions were governed by avoidance of bill injury costs, selection for high interference-free intake rates and a decreasing availability of benthic bivalve prey after their exposure. These density-independent interactions outweighed interference competition in terms of effect size. We suggest that the bottleneck in our mechanistic understanding of foraging distributions may be primarily the role of density-independent prey attributes unrelated to intake rates, like damage costs in the case of oystercatchers foraging on perilous prey. At a landscape scale, above the finest inter-individual distances, effects of conspecific interaction on spatial distributions may have been overemphasised