Behavioural responses of non-breeding waterbirds to marine traffic in the near-shore environment

Two important wintering grounds for waterbirds have recently been granted Special Protection Area (SPA) status in the Orkney near-shore zone: Scapa Flow and North Orkney. This means that any potential developments in the marine environment must be subject to robust environmental impact assessments, which rely on a clear understanding of wintering waterbirds’ responses to disturbance. 

This study focussed on 11 species that qualify the two proposed areas as SPAs, such as Eider, Long-tailed Duck, Black-throated Diver, Great Northern Diver, Red-throated Diver and Slavonian Grebe. Observations were recorded from 116 ferry journeys between November 2016 and March 2017, resulting in data for over 6,000 flocks. The researchers categorised the birds’ behaviour into four responses to the ferry: ‘flight’, ‘swim away’, ‘evasive dive’ or ‘no response’. This enabled them to compare the sensitivities of different species to the ferry traffic, and also how the different response types varied between species. 

The study found that the likelihood of a response was greater for all the investigated species when the ferry passed birds at closer distances. Red-throated Diver, Black-throated Diver and Slavonian Grebe were the most likely species to respond, but the species’ response behaviour differed; while Red-throated Diver and Slavonian Grebe were more likely to take flight, swimming away or diving accounted for almost all the evasive responses of Black-throated and Great Northern Diver. These species have higher wing-loading (the total mass of a bird divided by the area of its wings) so require more energy to take flight, which may explain why they almost exclusively displayed swimming or diving behaviour when the ferry passed. 

The researchers also found that rougher sea conditions increased the proportion of flight responses. This effect was strongest for Red-throated Diver, which were around three times more likely to take flight in the roughest sea states. The increase in flight responses coincided with a decrease in other responses, suggesting that birds were substituting diving and swimming away with flight responses in rougher conditions. The reduced energetic cost of take-off in strong winds may make flight the most energetically effective response in these conditions.  

In addition to providing comparative information for species’ sensitivity to marine traffic, the study also demonstrated the importance of considering different behavioural responses in sensitivity assessments. For Great Northern and Black-throated Diver, for example, a ‘flight response’ analysis would show these species to be very low sensitivity; however, including all behavioural responses show these species to have relatively high sensitivity to marine traffic. Further, long-term research is required to assess how this sensitivity affects foraging behaviour, energetic budgets and body condition, particularly in cases where repeat disturbance occurs. 

The findings of this study will be used by regulatory bodies such as NatureScot and Marine Scotland to inform the licencing process in the marine environment in Orkney and other similar marine areas. This should allow evidence-based decisions to be made which facilitates increased protection for sensitive species, but also doesn't unnecessarily restrict marine activity in other areas.