The great black-backed gull (Larus marinus) is a large gull species that breeds on the Atlantic coasts of Europe and North America. As an apex avian predator, the great black-backed gull has few natural predators once it reaches adulthood. However, eggs and chicks are vulnerable to predation by a variety of species. The main predators of great black-backed gulls include other bird species, mammals, and humans.
Bird Predators
The eggs and chicks of great black-backed gulls are preyed upon by a number of avian predators. These include:
Gulls
Other gull species will readily predate great black-backed gull nests given the opportunity. These include herring gulls, lesser black-backed gulls, glaucous gulls, and great skua. As opportunistic predators, they will consume unattended eggs and chicks.
Crows
Crows are intelligent and omnivorous birds that will eat bird eggs and chicks. Predatory species that threaten great black-backed gulls include American crows, northwestern crows, and common ravens. They are aerial predators that can survey nesting colonies from above.
Raptors
Birds of prey are efficient hunters that pose a major threat. Species known to attack great black-backed gulls include bald eagles, golden eagles, peregrine falcons, gyrfalcons, snowy owls, and short-eared owls. Their powerful talons and sharp beaks allow them to kill and consume the gulls.
Mammalian Predators
Mammals are also significant predators of great black-backed gulls, especially in terrestrial environments. The main mammalian predators include:
Foxes
Foxes are cunning predators that feed on eggs and chicks opportunistically. Red foxes and Arctic foxes are the main species that attack gull nests. They often raid colonies at night.
Mink
The American mink is an aggressive predator that feeds on waterbirds and their eggs. Mink have been responsible for large losses in seabird colonies where they have been introduced.
Raccoons
Raccoons are omnivorous mammals that will prey on bird nests. Their intelligence and dexterous front paws allow them to raid nests. Like foxes, they often attack colonies at night.
Feral Cats
Feral and free-ranging domestic cats are harmful predators of ground-nesting seabirds like gulls. They pose a major threat to nesting birds on islands where cats have been introduced.
Rats
Rats are opportunistic nest predators that consume eggs and chicks. Species like the brown rat and black rat have caused devastation when introduced to seabird colonies on islands.
Human Predation
Humans have had a significant impact on great black-backed gull populations through hunting, persecution, and habitat loss:
Hunting
Great black-backed gulls have been hunted by humans for eggs, chicks, and adults. Historical egg harvesting decimated populations. Some hunting continues today in Iceland and the Faeroes Islands.
Persecution
Many consider great black-backed gulls to be pests and kill them intentionally. They are often shot, poisoned, or trapped for preying on livestock, poultry, and pen-reared fish.
Habitat Loss
Coastal development, disturbance of nesting islands, wetland drainage, and pollution have led to habitat loss and lower reproductive success. For example, organochlorine pesticides caused thin-shelled eggs that failed to hatch.
Defense Against Predators
Great black-backed gulls have evolved several defenses to protect themselves against predators:
Nest Defense
Adults aggressively defend nests from predators. They may engage in distraction displays, aerial attacks, or direct physical combat. Their large size aids in deterring many predators.
Colony Nesting
Nesting in dense colonies provides safety in numbers. Predators can be detected faster and mobbing behavior helps drive them away. Synchronized nesting also reduces periods of vulnerability.
Concealed Nest Sites
Great black-backed gulls often nest in concealed locations away from predators, including vegetation, crevices, burrows, and cavities. Such sites help hide the eggs and chicks.
Regurgitation
When disturbed by predators, parent gulls may regurgitate indigestible food like shells, bones, and pebbles. This may deter or distract predators from killing chicks.
Population Impacts of Predation
Predation has had both localized and widespread impacts on great black-backed gull populations:
Colony Failures
In some cases, predators have caused outright breeding failure and colony collapse. For example, a 2013 fox predation event in Maine destroyed most eggs and chicks at four colonies.
Reduced Productivity
More moderate predation reduces overall reproductive productivity. One 1982 study found gull productivity was 34% lower in a Norwegian colony due to egg predation.
Regional Declines
On a larger scale, predation has contributed to regional population declines in conjunction with other factors like toxic chemicals and oiling. The Canadian Atlantic population decreased by 75% from 1970-2020.
Localized Extirpations
Predation has extirpated great black-backed gulls from some nesting islands. Expanding populations of bald eagles caused abandonment of breeding sites in Maine between 1994-2016.
Case Study: Predator Eradication on Islands
Eradicating invasive predators has proven an effective conservation strategy for restoring island seabird colonies. A case study from the Azores archipelago of Portugal demonstrates the dramatic effects:
Área Protegida das Ilhas Selvagens
Selvagens Islands Nature Reserve protects important seabird breeding islands in the South Atlantic. Key actions:
- Feral cats eradicated by 1978 after centuries of predation
- Black rats eliminated in 1974
- Barn owls removed as predator of storm petrel chicks in 2012
Results
- Great black-backed gulls rapidly recolonized and expanded after cat and rat removal
- Population increased from 0-1,695 breeding pairs between 1978-2011
- Now the largest global population of great black-backed gulls
- Indicates high impact of mammalian predation on gulls
Year | Breeding Pairs |
---|---|
1978 | 0 |
1983 | 2 |
1986 | 23 |
2011 | 1,695 |
Conclusion
In summary, great black-backed gulls face predation threats from a diverse array of birds, mammals, and humans. Predation has had detrimental impacts on nesting colonies through reductions in productivity and abandonment of breeding sites. However, management actions like invasive predator eradication on islands can successfully restore populations. Understanding the ecological role of predation is key for properly managing and conserving great black-backed gull populations into the future. Going forward, balancing conservation priorities with issues like livestock depredation will be an ongoing challenge.