Red-throated divers (Gavia stellata) use marine areas in the North Sea, Irish Sea and Baltic Sea during the non-breeding season. They are known to be displaced by various marine industry activities, including construction and operation of offshore wind farms. However, the consequences of displacement for individuals and consequently on the population as a whole are unknown. On 2nd May 2017, seven scientists with particular expertise in redthroated diver ecology and/or the energetic and demographic consequences of displacement for marine birds were invited to a workshop. Using their expertise and knowledge of divers and other marine birds, plus fundamental ecological principles, some generalisations about how red-throated divers might be affected by displacement were sought. This report comprises the main points of discussion and agreement during the workshop.
For displacement to cause a population-level change the following needs to occur. Divers need to show a behavioural response to anthropogenic marine structures, leading them to forage in a different area. This needs to cause a change in energy budgets either through increased energy costs of displacement and barrier effects or decreased energy intake through reduced prey acquisition and assimilation. This, in turn, would lead to poorer body condition with associated impacts on demographic rates, e.g. reduced breeding success or survival. Finally, a sufficient number of individuals need to have altered demographic rates for it to affect population growth rate and size.
It is difficult to draw concrete conclusions about how displacement will affect individuals and populations of red-throated divers, in the absence of empirical evidence. Based on our understanding of the ecology of red-throated divers and similar species, the following would seem to be more likely.
Red-throated divers appear capable of utilising a range of marine habitats and prey species. They also tend to occur at relatively low densities and not in large aggregations. Consequently, reduced prey intake caused by increased density-dependent competition or interference would seem unlikely. Red-throated divers are highly mobile in winter which may mean they are better able to find alternative foraging sites following displacement. However, individuals tend to be relatively site faithful in winter and in their choice of staging/moulting areas (even though there is large variation among individuals in choice of site). Redthroated divers show a strong stress response and this could temporarily limit their ability to exploit new locations following displacement, even if prey is abundant. Without understanding why divers are so mobile and how their behaviour changes following displacement, it is not possible to infer the energetic consequences of displacement with any certainty.
Year-round energetic budgets of red-throated divers are unknown but this information is key to understanding the possible consequences of displacement. An understanding of redthroated diver time budgets during winter, e.g. proportion of time spent foraging, would indicate when individuals are struggling to find sufficient food and might be in poorer condition. If red-throated divers tend to be in poorer condition in the non-breeding season when displacement is occurring, displacement could have an impact on survival and productivity. However, if individuals are in relatively good condition during the non-breeding season and spend only a small proportion of their daily activity budget foraging, they may have the capacity to buffer against any additional energetic expense of displacement and barrier effects.