The common murre (Uria aalge) is a seabird found throughout the northern hemisphere. As adults, common murres have few natural predators, thanks to their ability to dive and swim to escape danger. However, common murre eggs and chicks are vulnerable to predation from a variety of animals. In this article, we will discuss the main predators of common murres throughout their range.
Predators of Common Murre Eggs
Common murres nest on ledges of coastal cliffs and islands, laying their eggs directly onto the bare rock. This exposes the eggs to predation from a variety of opportunistic predators. The main egg predators include:
Gulls
Gulls, particularly herring gulls and great black-backed gulls, are major predators of common murre eggs. Gulls scout murre colonies for unattended eggs and swoop in to grab them. Some of the most problematic gull species include:
- Western gull
- Glaucous-winged gull
- Black-legged kittiwake
Research on a common murre colony in California found that gull predation accounted for around 60% of egg loss.
Crows
Crows, especially Northwestern crows and common ravens, also readily predate common murre eggs. Crows will scout murre colonies for opportunities to grab unattended eggs. One study in Oregon found that crows were responsible for up to 49% of murre egg loss at a colony site.
Foxes
Foxes, including red foxes and Arctic foxes, are adept at raiding murre colonies and consuming eggs. One study in Newfoundland found that over a 9 year period, foxes were responsible for between 57-90% of murre egg loss annually.
Other egg predators
Other predators known to occasionally take common murre eggs include:
- Mink
- River otter
- Black bear
- Raccoon
These mesopredators take advantage of any unattended murre eggs they encounter. However, they do not actively hunt murre colonies like gulls and foxes.
Predators of Common Murre Chicks
Common murre chicks face heavy predation pressure in their first few weeks after hatching. Unlike the eggs, murre chicks are mobile and able to move away from predators. However, they cannot fly or swim well in their first 2-3 weeks, which leaves them vulnerable. The main chick predators include:
Gulls
Gulls are again one of the primary predators of murre chicks. Species like herring gulls, great black-backed gulls, and glaucous-winged gulls will actively hunt murre chicks. Gulls grab unattended chicks directly off the colony site cliffs. One study in Alaska found gulls were responsible for up to 60% of murre chick mortality.
Eagles
Bald eagles and golden eagles frequent murre colonies looking for an easy chick meal. The eagles scout from vantage points and swoop down to snatch up chicks when the parents are away. On some colonies, eagles have been documented as causing more than 75% of murre chick mortality.
Crows
Crows like Northwestern crows and common ravens also hunt murre chicks opportunistically. They pluck chicks directly off the colony site cliffs. Some studies have shown crows causing up to 20% of chick losses.
Foxes
Foxes patrol murre colonies and grab any chicks they encounter. Red foxes and Arctic foxes can inflict heavy tolls on murre chick survival some years. For example, at a colony in Wales, fox predation accounted for 95% of monitored chick mortality.
Other chick predators
Additional predators known to occasionally take murre chicks include:
- Mink
- River otter
- Domestic cats
These opportunistic predators account for a small fraction of overall chick losses. But they can still have localized impacts.
Predators of Adult Common Murres
Once common murres fledge and become flighted adults, their risk of predation drops substantially. Their ability to dive and swim makes them difficult to catch. However, some predators do still occasionally take adult murres. These include:
Eagles
Bald eagles and golden eagles are essentially the only significant predators of adult common murres. The eagles ambush murres as they fly to or from the breeding colony. One 15 year study on the Farallon Islands found that bald eagle predation accounted for around 5% of adult murre mortality annually.
Sharks
Great white sharks are known to occasionally prey on common murres, typically ambushing them from below as they float on the water surface. Research using shark tag data found common murres made up around 2% of great white shark diets in some coastal areas.
Killer whales
Some groups of killer whales have learned to hunt common murres cooperatively. The orcas force murres into tight aggregations at the surface, then slap their tails to stun them before consuming them. However, murre predation is generally fairly uncommon among most killer whale ecotypes.
Beyond eagles, shark, and killer whales, most other predators rarely succeed in catching healthy adult common murres. The murres’ diving abilities allow them to escape danger underwater.
Regional Variation in Common Murre Predators
The composition and impact of common murre predators can vary across different breeding colonies due to localized predator abundance. Here are some patterns among different regions:
Pacific Coast of North America
Gulls, bald eagles, crows and foxes are often the most significant murre predators along the Pacific coast. For example, at a colony in California, researchers found gulls and crows accounted for 60-70% of egg and chick loss.
Atlantic Canada
Foxes and gulls tend to be the primary murre predators in Atlantic Canada. At an island off Newfoundland, foxes caused up to 90% of egg loss some years. Gulls accounted for most chick mortality.
Scotland
Great skuas are major predators of murre eggs and chicks in Scotland. One study found great skuas caused around 48% of chick mortality at a colony site in Shetland. Gulls and great black-backed gulls are also significant.
Norway and Russia
White-tailed eagles are serious predators of adult and young murres in parts of Norway and Russia. They can inflict heavier tolls than bald eagles in Alaska and Canada. Foxes and gulls also contribute to egg and chick loss.
So while gulls, eagles and foxes are often the main murre predators, the relative importance of each species varies across regions.
Predation Impacts on Common Murres
Heavy predation on eggs and chicks often leads to poor breeding success for common murres. The impact of predation can be severe enough to limit murre colony growth. For example, at an Atlantic Canadian colony, fox predation kept the breeding population stable despite plenty of food resources to support growth.
Some studies have estimated predation could remove up to 95% of murre eggs laid over a whole season. Chick survival to fledging age can be as low as 10% on colonies with heavy eagle predation. This shows how substantial the numerical impact of predation can be.
Climate change may increase predation rates in some areas. For instance, declining sea ice in the Arctic has allowed foxes to access more murre colonies. Predation pressure from gulls has also intensified with increasing gull populations in Alaska and Canada.
On the positive side, some targeted conservation efforts have reduced murre chick predation. Culling gulls, live-trapping foxes, or relocating eagle nests away from colonies has lowered losses on some sites. Continuing targeted management will be important for enabling murre population growth.
Conclusions
In summary, common murres face predation pressure from a diverse array of predators throughout their life cycle:
- Gulls, foxes, crows, and eagles are the primary predators of eggs and chicks.
- Bald eagles represent the main predation threat to adult murres.
- Great white sharks and killer whales occasionally prey on murres.
- The relative importance of different predators varies across regions.
- Heavy predation limits murre breeding success and colony growth in many areas.
Managing problematic predator populations through techniques like culling, relocation and live-trapping may help reduce predation rates and boost murre productivity. Continuing research and monitoring of murre colony predators will be important for guiding effective conservation.
