Cumulative energetic costs of military aircraft, recreational and natural disturbance in roosting shorebirds

Abstract

Knowing the consequences of disturbance for multiple species and all disturbance sources is crucial to mitigate disturbance impacts in densely populated areas. However, studies that observe the complete disturbance landscape to estimate cumulative costs of disturbance are scarce. Therefore, we quantified responses, frequencies and energetic costs of disturbance of four shorebird species on five high tide roosts in the Wadden Sea. Roosts were located either in a military air force training area or were predominantly affected by recreational disturbance. In the military training area, infrequent transport airplanes and bombing jets elicited the strongest responses, whereas regular, predictable activities of jet fighters and small civil airplanes elicited far smaller responses. Disturbance occurred more frequently at roosts near recreational than near military activities, as recreation was prohibited in the military area during operation days. On average, birds took flight due to military, recreational or natural disturbance (e.g. raptors) 0.20-1.27 times per hour. High tide disturbance increased daily energy expenditure by 0.1%-1.4%, of which 51% was due to anthropogenic disturbance in contrast to natural disturbance. Costs were low for curlews Numenius arquata, oystercatchers Haematopus ostralegus and gulls Larus spp, but higherand potentially criticalfor bar-tailed godwits Limosa lapponica as they were most susceptible to aircraft and raptors. Given that bar-tailed godwits have previously been found to be least susceptible to walker disturbances, our results suggest that interspecific differences in susceptibility depend on disturbance source type. In our study area, aircraft disturbance impacts can be reduced by avoiding jet fighter activities during periods when high water levels force birds closer to military targets and by limiting bombing and transport airplane exercises.

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

“…The GPS data are supported by our previous observations in the field that even when birds choose to roost at Westerseveld, they are often disturbed and then fly off to Richel (van der Kolk et al . 2020a). The high probability that birds will be disturbed eventually during high tide on Westerseveld may be the reason why Oystercatchers often choose to fly to Richel immediately at the start of the high‐tide period and avoid roosting on this frequently disturbed site, especially during summer when levels of recreational disturbance are high.…”

“…Recreational disturbance is common, especially by walkers (sometimes with their dogs) who are allowed to walk over the dike and thereby disturb roosting birds, causing Oystercatchers to fly up on average 0.26 times per daylight hour (van der Kolk et al . 2020a). Richel is a protected deserted sandflat for which access is denied to tourists and boats.…”

“…In a previous study, we visually observed how often birds were disturbed at five high‐tide roost sites (van der Kolk et al . 2020a) that were influenced by either military aircraft or recreational disturbance, but could not analyse whether birds were less likely to roost on these sites when disturbances occurred more frequently. On one roost site (‘Westerseveld’) on the Wadden island Vlieland, we noticed that Oystercatchers were scarce in summer and early autumn, even though by the beginning of August most Oystercatchers have returned to the Wadden Sea from their breeding grounds.…”

“…The roost location at Westerseveld is accessible for walkers, who disturb roosting birds on average 0.26 times per hour (van der Kolk et al . 2020a). 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.…”

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

“…The impact of aircraft disturbance on birds varies depending on the type of aircraft (van der Kolk et al, 2019;Smit & Visser, 1993).…”