Diving seabirds, also known as plunge-diving seabirds, are seabirds that specialize in hunting underwater. They capture prey by diving into the water from flight or while swimming on the surface. There are about 50 species of diving seabird in the world, filling a variety of ecological niches based on their anatomy, foraging behaviors and habitat preferences.
Characteristics of Diving Seabirds
Diving seabirds share some common anatomical and morphological adaptations that enable them to pursue prey underwater. These include:
- Streamlined bodies to reduce drag while swimming
- Wings designed for propulsion underwater – short, narrow, stiff and positioned far back on the body
- Feet set far back on the body to act as rudders while swimming
- Legs placed so close together that they act as a single paddle
- Webbed feet for more efficient swimming
- Salt glands to remove excess salt from the bloodstream
- Nostrils that can close to prevent water entry
- Dense plumage to provide insulation and waterproofing
- Excellent eyesight to spot prey from the air or underwater
They also exhibit behavioral and physiological adaptations to the marine environment such as the ability to dive to substantial depths and stay underwater for minutes at a time. Their wings provide the main source of propulsion for swimming underwater. Many species also use their feet for added thrust when pursuing prey.
Major Groups of Diving Seabirds
There are several taxonomic families of diving seabirds. The main groups include:
Penguins
Penguins (Spheniscidae) are flightless diving seabirds found exclusively in the southern hemisphere. Their wings have evolved into specialized flippers that enable ‘flying’ through the water. They use their feet for steering while swimming. There are 17-20 species of penguins, varying in distribution from Antarctica and subantarctic islands to the coasts of Australia, New Zealand, southern Africa and South America. Well-known species include the Emperor, King, Gentoo, Chinstrap and Little (Fairy) penguins.
Auks
Auks (Alcidae) include murres, guillemots, auklets, puffins and other extant members of this northern hemisphere family. There are around 22 species. They have compact bodies, short tails, small wings and pointed bills. Their legs are set far back on their bodies and their feet are webbed for swimming. Most species nest in large colonies on cliffsides and rocky coasts.
Cormorants and Shags
Cormorants and shags (Phalacrocoracidae) number around 40 species worldwide. They have long necks, slender bills with hooked tips, short legs set far back on the body, and all four toes joined by webbing. Most species are black or dark colored. They dive from the surface to catch small fish and are often seen perched on rocks, poles or trees with wings spread to dry their feathers.
Pelicans
Pelicans (Pelecanidae) are large waterbirds with a distinctive throat pouch used for catching fish. Of the 8 living species, 7 dive from the surface to hunt for prey. The exception is the pink-backed pelican which feeds by scooping up fish while swimming. Pelicans swim with their webbed feet and wings. Species such as the brown pelican dive from heights of 20-40 feet to plunge into the ocean after fish.
Gannets and Boobies
Gannets and boobies (Sulidae) are medium to large seabirds with long pointed wings and long bills. There are around 10 species worldwide. They plunge dive from heights of 30-100 feet into the ocean to hunt fish and squid. The impact stuns their prey and the streamlined body shape allows them to dive to depths of 30-50 feet in search of food.
Tropicbirds
Tropicbirds (Phaethontidae) are named for their predominantly tropical range. There are 3 species worldwide distinguished by their elongated tail streamers. Their legs are set far back on their bodies and their plumage is mainly white, with black markings on the face and wings. They catch fish by hovering then plunge diving into the ocean.
Loons/Divers
Loons or divers (Gaviidae) are freshwater diving birds found primarily in the northern hemisphere. There are 5 living species. Their legs are positioned far back on the body and their toes are connected by webbing for swimming. They use their feet and wings in unison to ‘fly’ through the water when hunting fish and invertebrates.
Grebes
Grebes (Podicipedidae) are small to medium-sized diving birds with lobed toes and pointed bills. There are 22 existing species residing on freshwater lakes and rivers around the world. They use their feet for propulsion underwater while their wings provide steering and stability. Several species have elaborate breeding plumages.
Plunge Diving vs Pursuit Diving
Diving seabirds employ two main foraging strategies – plunge diving and pursuit diving:
Plunge Diving
Plunge diving birds spot prey from the air or surface before diving vertically down into the water to catch their prey. Species such as gannets, boobies, pelicans, tropicbirds and some terns are classic plunge divers. They rely on the momentum gained from their aerodynamic body shape and high diving speeds to overtake fast moving fish.
Pursuit Diving
Pursuit divers swim underwater to catch prey, using their wings for propulsion and feet for steering. Penguins, auks, cormorants, grebes and loons are excellent pursuit divers. Their streamlined bodies and specialized wings enable them to swim rapidly after fish, squid and other prey. Some species such as auks also use their wings to ‘fly’ through the water.
Some diving seabird groups like shags contain species that exhibit both plunge and pursuit diving behaviors depending on the circumstances and type of prey targeted.
Diving Depths and Times
Different species of diving seabirds vary greatly in their diving capabilities in terms of depth and duration underwater. Some examples:
| Species | Maximum depth | Maximum duration |
|---|---|---|
| Gentoo penguin | 170m / 558ft | 5.5 minutes |
| Macaroni penguin | 100m / 328ft | 8.5 minutes |
| Little penguin | 60m / 197ft | 2.5 minutes |
| Adelie penguin | 175m / 574ft | 5.6 minutes |
| Northern gannet | 24m / 79ft | 3.6 seconds |
| Brown pelican | 20m / 66ft | 1 minute |
| Great cormorant | 40m / 131ft | 1.5 minutes |
| Common murre | 180m / 591ft | 3.5 minutes |
Penguins, auks and cormorants are clearly among the most accomplished divers. Gannets and pelicans plunge dive to shallower depths for shorter periods as their hunting strategy relies more on momentum than underwater swimming.
Sensory Adaptations
Diving seabirds also possess specialized sensory adaptations to help them catch prey underwater:
Vision
Many diving species have excellent underwater vision. They can adjust the lens shape and retina properties of their eyes to optimize visual acuity when swimming. Penguins, alcids, cormorants and grebes all have adaptations to enhance prey detection underwater.
Tactile
Whiskers around the beak provide tactile sensation to help some species such as shags locate prey in murky waters. Penguin tongues also have small spines for detecting captured fish in the mouth while swimming.
Auditory
Penguins can hear well both in air and water. Studies show their ears have adaptations that enable good underwater hearing for navigation, communication and prey detection. Other diving seabirds may also have similar auditory capabilities in the marine environment.
Feeding Behaviors
The main prey items taken by diving seabirds include:
- Small fish such as herring, sandlance, anchovies, sardines
- Squid
- Crustaceans such as krill
Feeding behaviors and strategies include:
- Plunge diving from flight by gannets, pelicans, boobies, tropicbirds
- Pursuit diving from the surface by penguins, auks, cormorants, grebes
- Pursuit diving from underwater by penguins, auks
- Seizing prey with the bill – most species
- Filter feeding krill from water using specialised bill – some auks
- Stealing food from other seabirds – skuas, some gulls
- Nocturnal feeding – some penguins, cormorants, shearwaters, petrels
Many species congregate in large feeding flocks and colonies, particularly during breeding season when food demands peak. This allows for more efficient exploitation of localized food sources.
Breeding and Nesting
Most diving seabird species nest in dense colonies on offshore islands, remote coastlines and cliffs. Colony sites are chosen for protection from terrestrial predators. Breeding habitats include:
- Rocky coasts and cliffs
- Inland penguin colonies in Antarctica
- Burrows dug into soil or sand – petrels, shearwaters, some penguins
- Crevices or caves on cliffs
- Tree branches near water – cormorants, some pelicans
- Ice shelves – emperors
- Vegetation mats – terns
Courtship behaviors vary by species but commonly involve vocalizations, preening, sky-pointing and greeting ceremonies. Most perform elaborate visual or vocal dueting displays.
Most species lay 1-3 eggs per clutch. Incubation duties are shared by both parents in most species, with alternating shifts ranging from a few days to over a month depending on the bird. Chicks hatch after 3-10 weeks depending on the species. Both parents typically help feed and protect the young until fledging.
Breeding success is highly dependent on ample food supply, which in turn relies on oceanic conditions and fish/prey abundance in surrounding waters. El Nino Southern Oscillation events can significantly impact breeding outputs in many populations.
Migration
Many diving seabirds disperse or migrate away from breeding colonies after the nesting season. Migration includes movements such as:
- Partial migration – some individuals migrate while others remain resident
- Post-breeding dispersal to offshore feeding grounds
- Trans-equatorial migrations in search of food
Some long-distance migrant species include:
- Arctic tern – round trip from Arctic to Antarctic each year
- Sooty shearwater – from islands off New Zealand to North Pacific and back
- Cassin’s auklet – from Pacific coast of North America to Sea of Cortez and back
- Brown pelican – northern populations winter in south after breeding
Migratory movements allow birds to exploit seasonally abundant food resources in different regions. They show remarkable navigation abilities to locate destination sites across ocean expanses.
Threats and Conservation
Many populations of diving seabirds currently face a range of threats:
- Oil spills – direct fouling and ingestion when feeding
- Plastic pollution – entanglement and ingestion
- Overfishing of prey species
- Disturbance of breeding colonies
- Climate change impacts on prey abundance
- Habitat loss
- Introduced predators at island colonies e.g. rats, cats, dogs
Conservation efforts focus on protecting breeding habitats, managing fisheries sustainably, reducing marine pollution, and mitigating climate change impacts. Some successes so far include:
- Establishing marine protected areas around key breeding sites
- Eradication of invasive species from island breeding colonies
- Mandatory use of bird scaring lines on fishing vessels to avoid bycatch
- Rehabilitation of oiled birds after spills
- Public education to reduce disturbance
Continued conservation action is needed to ensure healthy populations of these unique marine birds into the future.
Conclusion
Diving seabirds comprise a diverse group of species sharing common adaptations for an aquatic life hunting prey underwater. Major groups include penguins, auks, cormorants, gannets, boobies, grebes and loons. They exhibit a range of hunting strategies, some plunging from the air and others swimming in pursuit of fish, squid and other prey. Many species are colonial nesters, breeding in large aggregations on cliffs and remote islands. They face a variety of threats on land and at sea. Conservation of these remarkable diving specialists presents many challenges but is vital to maintain healthy seabird populations into the future.
