Scottish seabird conservation action plan: vulnerability report

Threats To Safe Breeding & Foraging Seabird Habitats

Invasive Non-Native Species (INNS)

All seabird species, to varying degrees, are vulnerable to predation of chicks and eggs whilst at breeding colonies. As such, seabirds have strong tendencies to breed on islands free from land-based predators. While native predators like otters and great skuas are part of the natural food web, invasive non-native species (INNS) can devastate seabird populations. The presence of INNS at seabird colonies is the most significant cause of global bird extinctions over recent centuries (BirdLife International, 2017).

In Scotland, non-native predatory species include brown and black rats, American mink, domestic cats and house mice (Burnell et. al., 2023). On Scottish seabird islands, non-native predators can also include species that are native elsewhere in Scotland such as wood mouse, stoats, hedgehogs and red fox. The presence of mammalian predators on seabird islands can serve to eliminate and exclude breeding species, to alter behaviour and breeding habitat occupancy, and/or to suppress breeding numbers and distribution. Impact severity differs between species, with small burrow-nesting petrels and auks at particular risk.

INNS are introduced to islands through accidental transport on vessels, swimming from invaded areas, or even deliberate human actions (Russell et al., 2017).

Grazing animals such as deer, sheep, goats and rabbits can also potentially impact seabirds by altering habitats through over- or under-grazing and erosion. Similarly, non-native plant species such as tree mallow, can encroach breeding sites rendering them no longer suitable for breeding.

Disturbance

Breeding seabirds are vulnerable to disturbance from a wide range of activities at their colonies. Most breeding colonies are in remote or rural locations and so disturbance can be a marked change in conditions they are used to at the site. Seabird responses to disturbance varies greatly between species, with some species or species groups showing a high sensitivity to disturbance (e.g. terns) where others are much more tolerant (e.g. gulls) (Goodship and Furness, 2019 & 2022).

Noise or visual disturbance from the presence of people and/or anthropogenic activities at or close to a colony can cause adult birds to move off their nests, leaving chicks and eggs vulnerable to chilling or predation. Continued or severe disturbance can cause reduced breeding success, desertion of individual nests or colony abandonment. Even where there is no obvious escape response, physiological responses to stress from disturbance have been shown to reduce fitness in some species, and can affect immunity (Ellenberg et al., 2006).

Visual disturbance is also associated with artificial lighting on vessels and structures, as well as lighting from buildings on land close to the shore. Artificial light can cause disorientation to sensitive species, which can result in injury or death (Watson et al., 2014 and Deakin et al., 2022). The young of species that fledge at night, such as Atlantic puffin, Manx shearwater and storm-petrels are particularly susceptible to being disorientated by artificial light on land and at sea.

Breeding and non-breeding seabirds are vulnerable to vessel disturbance at sea, eliciting escape responses (birds taking flight or diving to avoid vessel) and potentially displacement of birds from foraging areas or other important areas. Escape responses and the potential requirement to have to travel further for food as a result of displacement increases energy expenditure and possible loss of fitness which can affect breeding success and survival.

Habitat Loss

Anthropogenic activities on land and at sea can result in the loss of breeding, wintering or foraging habitats for seabirds which in turn can adversely impact seabird populations, breeding success and behaviour. On the coast, infrastructure such as landfall for offshore cables and pipelines; ports and harbour development; coastal protection and flood defence works and laying of outfall and intake pipes, can result in the loss of intertidal foraging areas and nesting habitats for species such as terns and gulls.

At sea, offshore energy infrastructure and development can result in the displacement of birds from important foraging areas. When a bird is displaced to a location further from its source colony, it can cause increased flight time. This extended travel can deplete the bird’s energy reserves and reduce the time it has available for essential foraging activities, potentially affecting its energy for attending to nests (Searle et al. 2018).