Alcids in marine ecosystems
This article was researched and written by a student at the University of Massachusetts, Amherst participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.
Alcidae is the name for a family of marine birds that includes murres, razorbills, dovekies, puffins, guillemots, auklets, and murrelets. They are commonly referred to as alcids or auks. These birds of the open ocean spend most of their lives at sea, coming to shore only to breed. Most alcids are colonial breeders, nesting in dense colonies on mainland cliffs or offshore islands, where they are inaccessible to terrestrial predators. They are long-lived and socially monogamous, often returning to the same nesting site to mate with the same partner, year after year. Females lay a single egg (or two eggs in a few species), and both parents contribute to egg incubation, chick brooding, and feeding the nestling(s) until they are ready to leave the nest for the sea.
Three distinct chick-rearing strategies exist among the alcids. Chicks of most species remain at the nest site and are fed by both parents until they are fully grown (adult size) and capable of flight, at which point they depart for the sea and begin life on their own. A few species have precocial chicks that depart the nest for the sea within 1-2 days of hatching, and are raised by their parents at sea. Yet another strategy, unique to the Common Murre, Thick-billed Murre, and Razorbill, is intermediate to the previous two. Chicks are raised at the nest site by both parents until they reach approximately one-third the size of adults, at which time the male parent takes the chick to sea while the female remains behind at the colony or departs by herself. The male parent continues to raise the chick at sea until the chick is capable of flight and obtaining its own food.
Auks are sexually monomorphic, meaning that males and females look the same. Their plumages are relatively plain: a palette of black, white, grays, and browns, often with a dark back and lighter belly. During the breeding season, many species develop ornamentation in the form of contrasting head feathers (“whiskers” or “crests”) or horny beak sheaths (e.g. puffins’ brightly colored bills).
Although the black and white coloration, upright posture, and small wings can make some alcids appear similar to penguins, the two families are not closely related. They do, however, fill similar ecological niches. Their physical similarities may relate to similarities in their lifestyles, as each family has evolved to exploit similar resources using similar behavior (convergent evolution), albeit in different hemispheres. Members of both the alcids and the penguins obtain their food on underwater dives, during which their small wings allow them to “fly” through the water with minimal drag in pursuit of prey.
The 22 species that make up Alcidae live exclusively in the Northern Hemisphere. The strongholds of the family are the arctic, sub-arctic, and boreal latitudes, but many species ranges extend south into temperate regions, and a few include subtropical latitudes. The alcids are true marine birds (rather than shorebirds) in the sense that they generally spend their entire lives, with the exception of the breeding season, far from land. They can be found in near-shore waters during the breeding season because they must remain in the vicinity of their breeding colonies. Because of the difficulty in tracking birds in the open ocean, much remains to be discovered about the locations and behaviors of alcids in the non-breeding season.
Foraging and diving behaviors
All alcids use their wings to propel themselves on underwater dives in pursuit of food. The alcid wing shape, which is small and pointed, is a compromise between features that allow aerial flight and those that are ideal for underwater swimming. The narrow, pointed wing shape minimizes drag when stroking underwater, but it also increases “wing-loading,” or the ratio of body mass to wing surface area. Thus, alcids are powerful and agile on underwater dives, while their aerial flight is fast and direct with rapid wing beats and no gliding.
Alcids can be divided into two general categories based on diet: planktivores and piscivores. Planktivores eat zooplankton (especially krill), while piscivores eat small fish (sometimes called “forage fish” or “bait fish”). Some piscivorous species also consume crustaceans, mollusks, and marine worms, while some planktivorous species occasionally eat fish.
When feeding, alcids typically make several dives in a row with short resting periods between dives; this is known as a bout of dives. A single feeding episode or foraging trip may consist of a single bout of dives, or several bouts of dives which are separated by resting on the surface, swimming on the surface, or flight.
Auks are capable of diving to impressive depths, though the vast majority of dives are much shallower than the maximum depths recorded for various species. Dovekies (one of the smallest auks) can dive to 30 [[meter]s, while murres (the largest auks) can dive to over 150 meters. Auks are visual hunters and must have some light to find prey, but sometimes they dive to depths, and at times of day, where the light is dim.
Researchers study the diving behavior of auks using miniature bird-bourn electronic data-loggers called time-depth recorders (TDRs). Using information gathered with these data-loggers, researchers can piece together an intricate understanding of dive depth and length, frequency of dives, and dive profile (two dimensional shape of the dive). The profiles, which vary by species, location and sex, can be shaped like U, V, or W. Profile shape is thought to be related to the bird’s prey type and foraging strategy.
How alcids locate areas for foraging is not well understood. Some seabirds locate plankton by sensing a chemical given off by invertebrates when other organisms are preying on them. Others have been shown to identify the direction of food by the direction of flight of returning birds. Birds may also remember past locations of successful foraging trips and possibly detect changes in ocean temperatures or fronts where prey would likely be concentrated. Alcids often forage in small groups or large mixed-species flocks. They sometimes congregate over underwater banks or pinnacles, and at oceanographic fronts where two water masses or currents meet, most likely because prey items are more numerous, concentrated, or reliably found in these areas.
Withdrawals from the marine ecosystem
Alcids are predators in marine ecosystems. With variation in body and bill sizes, each species takes advantage of a suite of marine organisms as prey. The planktivorous alcids (auklets, dovekies, murrelets) participate in a relatively short food chain, as krill feed directly on phytoplankton. Piscivorous alcids (murres, razorbills, puffins, guillemots, murrelets) feed further up the food chain, as they consume bait fish.
Alcids process large amounts of biomass from marine ecosystems. Individuals consume between 10-90% of their mass in food each day, and some species of alcids are among the most numerous of all seabird species. Predation by alcids probably has significant impacts on populations of prey species, and some populations of alcids may be locally limited by the availability of prey. Some populations specialize on one or a few species of zooplankton or fish, but others are opportunistic and will take prey in proportions relative to abundances in the local environment. In this way, alcids can influence both the total amount, and the relative proportions of prey species in their ecosystems.
Contributions to the marine ecosystem
As mentioned above, many alcid species are flexible as to the exact species of prey that they consume. As a result of this behavioral flexibility they may help regulate prey populations and maintain biodiversity in their ecosystems by reducing the numbers of the most abundant prey, thus freeing up food resources for competing species.
Alcids are also responsible for a large transfer of nutrients from marine to terrestrial or near-shore ecosystems. During the breeding season, individuals spend much time at the breeding colonies, and deposit large amounts of guano at the colonies. Seabird guano is extremely rich in nitrogen and is an important source of nutrients for the cliff and island terrestrial ecosystems where seabirds nest, as well as the surrounding oceanic waters via runoff from the colonies.
Additionally, alcids become prey for other marine animals. While adults are generally considered apex predators, they are occasionally eaten by a variety of predators including terrestrial and marine mammals, gulls, and falcons. Although predation on adults is infrequent, minor to moderate predation on eggs and chicks is common in some species. Records show alcids (adults, chicks, or eggs) being eaten by marine mammals, large fish, gulls, skuas, corvids (ravens, crows, jackdaws), falcons, eagles, foxes, weasels, brown bears, feral dogs, feral cats, and rats. Some smaller alcid species visit breeding colonies only at night, coming and going under the protective cover of twilight or full darkness. This is thought to be an evolutionary adaptation to avoid predation on adults by gulls and other large seabirds.
Anthropogenic threats to alcids
Like many other organisms, alcids face a multitude of anthropogenic (i.e. human-caused) threats in today’s world. Some of the most significant are mentioned below.
Commercial fishing has direct and indirect effects on alcids. Direct effects include drowning deaths when birds become caught in fishing nets (in use or abandoned) while diving for food. Also, fishing boats and other water craft create disturbances on the water surface, causing wary alcids to take flight and possibly leave areas of good foraging. In addition to these direct effects, commercial fishing can affect alcids indirectly in many ways. Perhaps the most significant of these is that commercial fisheries can lead to dramatic reductions in the amount of fish available for alcids to eat, as many of the species consumed by alcids are also important commercial fisheries (e.g. capelin, herring, sand lance). Other examples of indirect effects are reduced habitat for young fish due to fishing gear damaging the sea floor, and fishing harvests leading to altered species assemblages or trophic balances within the alcids’ food webs.
Climate change may affect alcids through changes in food availability, and through loss of breeding areas due to sea-level rise. As the earth’s climate changes at an unprecedented rate, birds may have difficulty adapting to changes in prey distributions, abundance, or timing due to increased sea temperature and altered current and wind patterns. Higher global temperatures are likely to cause average sea-level to rise, which might lead to flooding of low-lying breeding colonies during high tides and severe storms, making them unsuitable for nesting.
Oiling, or the fouling of feathers by oil, leads to death in seabirds because they lose the ability to thermoregulate, and they become too waterlogged to fly. Alcids are particularly vulnerable to oiling because they spend such a large portion of their time resting on the surface of the ocean. Oil on the surface of the water occurs from oil spills and dumping of oily wastewater by ships at sea. On the open ocean, where alcids spend the majority of the year, detection and clean-up of oil spills are difficult, and anti-dumping regulations are hard to enforce.
Introduced species can have detrimental effects on seabird breeding colonies. For example, invasive predators (foxes, cats, rats, dogs) eat numerous eggs and chicks, and sometimes decimate adult populations too. Invasive plants can overgrow breeding colonies, restricting bird’s access to their nest sites. Additionally, introduced livestock and reindeer trample eggs and burrows, and rabbits remove vegetation leading to erosion.
Human exploitation in the form of hunting and collecting eggs has occurred for thousands of years, and still occurs in some areas and cultures. Hunting caused the extinction of the Great Auk in 1844, and has had significant impacts on other species as well. While laws now regulate the take of alcids by hunters, hunting pressure may still negatively affect some populations by removing a significant number of breeding-age birds.
Alcids are marine birds that are integral to their pelagic ecosystems. They face numerous threats in a world dominated by humans. Their well-being is dependent on survival of adults, successful breeding, and access to food, all of which are heavily influenced by human activities.
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