What are the effects of aircraft on birds in the Dutch Wadden Sea?

The Wadden Sea is of great importance to shorebirds, with millions of birds overwintering or fuelling for migration in the area each year. But they also face disturbances from natural and anthropogenic sources, including raptors, walkers, cyclists, boats and aircraft. A new, three-year study takes a detailed look at the effects of aircraft on the shorebirds in the Wadden Sea.

Vliehors on the Dutch Wadden island of Vlieland is a large sandflat that the Royal Netherlands Air Force has used as training area for its military aircraft ever since the end of the Second World War. There is also an airport for small civil aircraft nearby, located on the neighbouring island of Texel. This makes the area highly suitable for studying the impact of different types of aircraft on shorebirds. 

Researchers from the CHIRP project (Cumulative Human Impact on biRd Populations) studied the effect of four different aircraft types: small civil airplanes, jet fighters, helicopters and transport airplanes. The researchers carried out field observations to find out how often bar-tailed godwits, curlews, oystercatchers and gulls had to take flight due to aircraft disturbance. To follow the movements of oystercatchers after a disturbance, GPS trackers were used.

Major differences

The researchers found that the amount of disturbance depended on the distance between bird and aircraft, and varied widely between aircraft types. Small civil airplanes generally caused little disturbance, as long as they flew at a minimum of 450m (1500ft), which is the minimum legal flight height in the Wadden Sea area. The effect of jet fighters was also small - possibly because the birds have become accustomed to the jet fighters' frequent exercises along standardised flight paths.

What did cause major disruption, was the dropping of explosives. This is only allowed a few weeks per year. Helicopter exercises also turned out to be a source of considerable disturbance. But the largest amount of disruption by far was caused by transport airplanes: these would cause birds to fly for many kilometres, often even to other islands. Low flying transport airplanes are not a frequent occurence in the Wadden Sea: military exercises involving Hercules C-130 aircraft are held three days per year on average. Other occasions, such as air shows, are even rarer.

Energy loss

Thanks to the use of GPS trackers, it was possible to quantify the additional distances oystercatchers had to fly, and to estimate the cost in terms of energy. On average, aircraft disturbance increases daily energy expenditure by 0.25%, even in the middle of the training area where aircraft are most frequent. The additional daily energy expenditure is relatively low because the oystercatchers hardly react to small civil airplanes and jets, which are the most common aircraft types.

However, on days with highly disturbing aircraft types - transport airplanes in particular, but also helicopters and fighter planes dropping explosives - the additional expenditure spiked to 10%. And while the impact of aircraft on oystercatchers may appear relatively small when compared to other human activities in the Wadden Sea, it is still recommended to further reduce the amount of disturbance

Harsh conditions

The current policy to maintain a minimum flight height of 450m is an effective measure to reduce disturbance by small civil airplanes. The impact of jet fighters can be further reduced by avoiding exercises at extremely high water levels at high tide. In these conditions, the water forces the birds to roost closer to the jet fighter targets, which increases the probability that they are disturbed.

Low flying transport airplanes have a large impact, and to minimize disturbance it is important that their flights should be limited. This is particularly true when conditions for the birds are harsh, e.g. in periods with cold weather or prolonged high water levels.

These recommendations apply especially to oystercatchers, since this bird species was studied in the most detail. A comparison with field observations of other species revealed that gulls and oystercatchers are relatively insusceptible to aircraft disturbance, whereas bar-tailed godwits are more sensitive. It is important, therefore, to study the impact of aircraft disturbance on such more sensitive species in the future.

(Photograph: Ingrid D. van der Spoel, Text: NIOO, Radboud, Sovon, CAPS)

• Article: Van der Kolk, H., K.L. Krijgsveld, H. Linssen, R. Diertens, D. Dolman, M. Jans, M. Frauendorf, B.J. Ens & M. van de Pol (2020). Cumulative energetic costs of military aircraft, recreational and natural disturbance in roosting shorebirds. Animal Conservation. https://doi.org/10.1111/acv.12546
• Article: Van der Kolk, H., A.M. Allen, B.J. Ens, K. Oosterbeek, E. Jongejans & M. van de Pol (2020). Spatiotemporal variation in disturbance impacts derived from simultaneous tracking of aircraft and shorebirds. Journal of Applied Ecology. https://doi.org/10.1111/1365-2664.13742