The common murre (Uria aalge) is a medium-sized seabird found across the northern Atlantic and Pacific Oceans. Murres are expert divers, using their wings to “fly” underwater in search of fish and other prey. But just how deep can these birds go in their marine hunting expeditions? Here we’ll look at the diving capabilities of the common murre and what allows it to access such deep depths.
Quick Facts on Common Murre Diving
Before diving into the details, here are some quick facts on how far common murres can dive underwater:
- Common murres are pursuit divers, swimming underwater to hunt prey like fish, crustaceans, and cephalopods.
- They can dive to depths of over 200 meters (650 feet), although most dives are much shallower.
- Murres have several adaptations that allow them to dive deeply, including dense bones, collapsible lungs, and high myoglobin levels.
- Dives typically last 2-4 minutes, but murres have been recorded staying underwater for over 5 minutes.
- Most prey is found at depths between 25-50 meters (80-160 feet).
Diving Depth Records
The maximum diving depth recorded for a common murre is 210 meters (nearly 700 feet). However, dives this deep are rare, accounting for only around 2% of all dives. Most common murres do not need to dive nearly this deep to find food.
Still, even “average” dives for murres reach impressive depths. Studies using time-depth recorders attached to murres give us insight into more typical diving behavior:
- One study in Norway found an average dive depth of 42.5 meters (140 feet). Maximum dive depth was 181 meters (594 feet).
- Off the coast of Newfoundland, average and maximum dive depths were 49 meters (160 feet) and 146 meters (479 feet), respectively.
- At the Farallon Islands off California, average murre dive depth was 28 meters (92 feet). Maximum recorded was 167 meters (548 feet).
So while murres are capable of diving over 200 meters, most of their hunting takes place at depths between 25-75 meters (80-250 feet) beneath the surface.
Dive Duration
In addition to diving deep, common murres can also stay underwater for remarkably long periods while foraging for food.
The maximum recorded dive duration for a common murre is 5.5 minutes. However, most dives last between 2-4 minutes. Here are some typical dive times reported by different studies:
Location | Average Dive Duration | Maximum Dive Duration |
---|---|---|
Norway | 2.4 minutes | 4.5 minutes |
Newfoundland | 2.5 minutes | 4.3 minutes |
California | 2.3 minutes | 5.5 minutes |
Murres may spend up to 75% of their foraging trip underwater diving, with the remaining time resting at the surface. Their streamlined bodies and wing-propelled swimming style allow them to stay underwater for extended periods as they pursue fish, squid, and crustaceans.
Adaptations for Deep Diving
Common murres have several key adaptations that enable them to dive so deep and stay underwater for minutes at a time. Understanding the anatomy and physiology that allows murres to successfully hunt at depth helps explain why they are such exceptional divers compared to many other bird species.
Dense Bones
One of the murre’s most important diving adaptations is having dense, solid bones. Most birds have light, hollow bones containing air pockets to minimize body weight for flight. In contrast, a murre’s bones are heavy and lack air spaces.
This greater bone density helps counteract buoyancy, allowing the murre to submerge more easily. The added weight also helps them descend rapidly and maintain depth with less effort flapping underwater. Without heavy bones, murres would have to spend much more energy staying deep or fighting positive buoyancy.
Collapsible Lungs
Another key adaptation of the common murre is having collapsible lungs. As the bird dives, the increased water pressure compresses the air spaces in the lungs and respiratory system. This reduces buoyancy at depth.
The murre’s bronchi (air passages) and lungs are able to collapse down to a smaller volume thanks to having less reinforcing cartilage than the lungs of land birds. This anatomical difference allows the diving murre to minimize air in the body and withstand high pressures underwater.
High Myoglobin Concentration
Common murres have muscles with high levels of myoglobin, an oxygen-storing protein. Myoglobin allows the muscles to function longer during dives without running out of oxygen. It binds and temporarily stores oxygen from the lungs/air sacs before a dive. Then underwater it releases the oxygen to power muscles for swimming and hunting.
Studies have found common murre breast muscles contain myoglobin concentrations similar to those seen in marine mammals like seals and dolphins. This allows murres to cope with oxygen limitation during extended dives.
Slowed Heart Rate
Murres are able to reduce their heart rate while diving, down to just a few beats per minute. This helps conserve oxygen when the bird is submerged and cannot breathe. It also reduces energy consumption that would otherwise be needed to pump blood at the normal heart rate.
Specialized blood vessels in the murre’s wings called rete mirabile act as oxygen reservoirs. They receive oxygenated blood as the heart rate slows during a dive and circulate it preferentially to the heart and brain.
Other Anatomical Adaptations
Several other anatomical features of the common murre assist its diving ability:
- Waterproof feathers help insulate the body and retain body heat in cold water.
- Toothed bill allows them to grip slippery prey like fish.
- Webbed feet provide propulsion and steering when swimming.
- Streamlined body profile and wings reduce drag while swimming underwater.
Hunting Behavior and Strategy
Common murres employ a variety of hunting techniques and strategies to successfully forage at depth.
Pursuit Diving
Murres do not just swim straight down and back up again. Instead, they “fly” underwater in pursuit of prey, propelling themselves with their wings and steering with their feet. This allows them to chase mobile fish and squid.
Murres normally dive at an angle, descending rapidly while also moving horizontally. This pursuit strategy helps them intercept prey instead of diving straight down to the depths where fish reside.
Vision for Hunting
Murres have excellent underwater vision compared to other birds. Their eyes are specially adapted to focus well while submerged. This helps them spot and pursue fish swimming in the ocean depths.
Studies show common murres are capable of detecting prey-sized objects at up to 80 meters depth. Their specialized visual capabilities allow murres to hunt effectively even in dark, deep waters.
Coordination of Murres
Groups of murres often dive synchronously, swimming and surfacing together. This coordinated diving behavior is thought to help concentrate prey for more efficient hunting. It may also reduce competition from other murres in the area.
Additionally, murres tend to dive sequentially, with a steady stream continually submerging. Researchers propose this steady pattern allows the maximum number of murres to have access to the prey patch below while minimizing interference with others diving and surfacing.
Prey at Depth
Common murres are generalist feeders and eat a varied diet of prey. However, diet often differs depending on the colony location and available food sources in the surrounding marine habitat. Here are some of the main prey items common murres hunt at depth:
- Fish – capelin, sand lance, Arctic cod, herring, blennies, eelpouts, sculpins
- Crustaceans – krill, amphipods, copepods
- Cephalopods – squid, octopus
Diet studies based on murre stomach contents and regurgitations show the majority of prey is caught between 25-50 meters depth (80-160 feet). However, murres are capable of diving much deeper when prey descend farther or disperse.
In one study off California, around 90% of prey were found in the upper 50 meters. Yet almost a third of dives reached deeper than 50 meters, demonstrating the birds’ ability to hunt at greater depths when necessary.
Dive Depth Differences Between Locations
The typical diving depth of common murres can vary significantly between breeding colonies. This reflects differences in ocean floor depth, water temperature, and the vertical distribution of prey.
For example, at colonies off Norway, murres dive mostly between 30-70 meters depth. But at the Farallon Islands near San Francisco where waters are much deeper close to shore, 50% of dives are to 80 meters or deeper.
Ultimately, murres dive as deep as needed to access prey. Average and maximum depths tend to be greater near colonies bordering deep, open oceans compared to those next to continental shelves or shallower seas.
Changes Throughout Breeding Season
Common murre diving behavior changes over the course of the breeding season due to altering energy demands.
During incubation shifts, when one parent guards the egg while the mate is off feeding, murres make relatively short, shallow dives. This likely conserves time and energy when the mate needs to return soon to resume incubation duties.
Later in chick rearing, with hungry chicks to feed, average dive depths increase as parents work harder to provide for offspring needs. Maximum depths reach yearly peaks at this time.
So murres adjust their diving activity based on seasonal reproductive constraints and offspring provisioning requirements.
Impact of Hunting Success on Diving
Common murre diving behavior is also influenced by hunting success. Murres frequently adjust the depth and duration of dives based on conditions and how much prey they have captured so far.
In laboratory experiments on diving murres, researchers found:
- Unsuccessful murres increased depths of subsequent dives.
- Successful hunts resulted in a shallower next dive.
- Multiple unsuccessful dives led to longer dive durations.
This suggests murres strategically alter their diving activity depending on foraging outcomes. When prey patches are scarce or difficult to catch, murres respond adaptively by diving longer and deeper in an attempt to find more profitable hunting grounds.
Threats to Deep Diving Seabirds
The common murre’s exceptional diving capabilities allow it to exploit food resources unavailable to most other birds. However, human activities are increasingly threatening murre populations and their marine habitats.
Some key threats facing deep diving murres include:
- Oil spills – Murres are vulnerable to spills, especially in winter when large numbers concentrate offshore.
- Overfishing – Reduced fish stocks may lower prey availability.
- Climate change – Ocean warming and acidification can impact prey abundance and distributions.
- Disturbance – Boating, infrastructure, and other activity near breeding colonies may displace murres.
Protecting breeding sites and maintaining sustainable fisheries will be important for ensuring healthy murre populations that can continue exploiting the ocean’s depths.
Conclusion
Common murres are superbly adapted for deep diving, with an anatomy allowing them to reach depths over 200 meters and withstand high underwater pressures. While they are capable of very deep dives, most hunting takes place between 25-75 meters beneath the surface.
Dense bones, collapsible lungs, high myoglobin levels, and other specializations permit murres to dive deep and stay underwater for up to 5 minutes at a time. Murres use a variety of hunting strategies, including pursuit diving and synchronous diving in groups.
Their excellent underwater capabilities allow murres to catch prey like fish, squid, and crustaceans that live far below the ocean’s surface. Murres adjust their diving activity based on factors like colony location, breeding stage, and hunting success. These adaptive deep diving specialists remain vulnerable to human impacts, highlighting the need for continued conservation efforts.