Ecology of the Northeast US Continental Shelf
Historically seabirds have been exploited as an exotic food source and for their plumage. As a result, many populations were decimated or driven to near extinction. In the early 20th century, the government enacted the Migratory Bird Treaty Act of 1918. The act established a Federal prohibition, unless permitted by regulations, to "pursue, hunt, take, capture, kill, attempt to take, capture or kill, possess, offer for sale, sell, offer to purchase, purchase, deliver for shipment, ship, cause to be shipped, deliver for transportation, transport, cause to be transported, carry, or cause to be carried by any means whatever, receive for shipment, transportation or carriage, or export, at any time, or in any manner, any migratory bird, included in the terms of this Convention . . . for the protection of migratory birds . . . or any part, nest, or egg of any such bird.”
As many of these species have become endangered or threatened due to population declines, further protection has been given under the Endangered Species Act of 1973. On top of the initial exploitation of these birds, other anthropogenic factors limit population recovery and strong reproductive success. Habitat infringement, pesticide usage, and bycatch in fisheries are a few of the factors potentially jeopardizing seabird populations.
An important component of the National Marine Fisheries Service's (NMFS) marine stewardship role is the responsibility to protect seabirds and other migratory birds. This responsibility is supported by several directives and dictates that NMFS work both domestically and internationally to gain a better understanding of seabird bycatch and pursue ways to reduce bycatch. It is our goal to develop an effective program, working with avian scientists, regional fishery management councils, other agencies, such as U.S. Fish and Wildlife, and various stakeholders, to foster and sustain seabird and migratory bird populations.
In the scientific literature, a “seabird” generally refers to those birds that obtain at least part of their food from the sea. Most often “seabird” refers to birds traveling some distance over the ocean’s surface to obtain food, spending the majority of their lives on the water, only returning to land to breed. However, many scientists group birds that wade into the sea, “shorebirds”, or birds that are “inshore feeders”, also as “seabirds”. For the purpose of this overview, there are three groups that will be categorized as “seabird”. Group one is the classic definition of a “seabird”, those birds that are marine pelagic species, spending the majority of their time on open ocean, not returning to land at night, and only coming to land to breed. Group two encompasses those marine birds that feed inshore, typically referred to as “shorebirds”. Group three includes those marine birds that seem to migrate behaviorally between group one and group two; that is, these species tend to be found feeding and living in both inshore and offshore habitats. This overview will explore the general ecology, biology, and ecosystem roles of five families of representative seabird species in the Northwestern Atlantic region.
Loons are large waterbirds with wingspans that range from 3-4 feet. Loons are commonly known for their wails and elaborate displays during breeding season. Most encounter loons during migration or winter, when they frequent lakes, coastal marshes, and open ocean coasts.
Loons mainly eat a wide variety of fish up to about 10 inches long. They are pursuit divers, hunting first from the waters surface, submerging their bill and eyes. Once prey is detected, they dive, where dives last an average of 40 seconds and potentially as deep as 250 feet. Loons prefer to nest on secluded lakeshores from boreal to Arctic regions. In general, they have synchronous mating displays, in which the mating pair chase, dive, swim and lower their bills into the water together. Usually, the nest consists of vegetation 1-5 inches high placed on the edge of the shore. Loons are noted for returning to prior nest sites and are territorial. Males establish territories with a loud yodeling call.
Most loons winter in coastal waters as far south as central Mexico, with peak abundance at a latitude of 35°N. Typical wintering ground behavior is a period of no flight while they molt their flight feathers. Specifically, the common, arctic, and red-throated loon, breed in northeastern North America, while only the common and red-throated loons migrate south along the Atlantic seaboard. Red-throated loons remain in neashore waters and travel as far out as the western edge of Georges Bank, whereas the common loon journeys across Georges Bank to the continental shelf edge waters particularly in May and November.
Shearwaters and Petrels
The shearwaters and petrels (Figure 1) are small to relatively large seabirds somewhat similar to gulls. At close range they are easily distinguished by their naricorns (raised, horny tubes at the base of the upper bill that enclose the nostrils). They are highly specialized at harnessing the energy from the wind, allowing them to perform a sailing flight over vast distances on the open ocean. Members of this family spend their lives at sea, only returning to land to nest or when driven there by storm.
Shearwaters (Figure 2, Figure 3) and petrels likely detect prey by both sight and smell. The naricorn adaptation is thought to play a role in this sense of smell, as well as likely enabling the birds to detect changes in air pressure and wind direction. Their well-developed olfactory sense allows them to detect prey by day or night. This sense of smell is uncommon in birds, but it seems to function by the birds “scanning” the breeze for scents while flying. Their diets vary greatly, but they mostly eat small fish, squid, and crustaceans. Many species feed mainly at night, waiting for their prey to perform their vertical migration to the ocean surface. Some species are surface-seizers, grabbing prey with their sharp-edged bills. Some species dive for prey, using their wings rather than feet for propulsion, and can dive to depths of 40-60 feet. Some species are known to perform shallow plunges from height while flying.
Shearwaters and related species mate and pair for life, unless the mate is lost. Nest site fidelity is very high with most pairs. Breeding season begins usually in spring or summer, and most species nest only once per year. Generally, they form large breeding aggregations in only a few breeding ground sites. Males and females return to the breeding grounds at the same time and meet at the prior nest site. Some species nest above ground, such as the Northern Fulmar who favors inaccessible ledges on sea cliffs. Most species at the nesting ground are strictly nocturnal. The female lays one large egg that the male begins incubating for a shift, ranging from two to fourteen days. Upon hatching, the thick down covered chick is brooded only until it can regulate its own body temperature. At fledging time, the chick begins exercising its wings, increasing its activity outside the burrow, and is eventually left all alone. From here, the chick has to learn how to fly, find its way to the sea, and learn to forage for food all by itself.
Members of this family cover large distances during the nonbreeding season. Many shearwaters that breed in the northwest Atlantic, winter off the eastern coast of South America. Specifically, five shearwater species occur regularly in the western North Atlantic Ocean. Sooty and Greater Shearwaters are migrants from South America and occur in great numbers on Georges Bank. Up to 200,000 Greater Shearwaters have been observed on a single day on Georges Bank, indicating the importance of Georges Bank as a mid-latitude staging area for nonbreeding migratory birds. Nonbreeding Cory’s shearwaters spread across the temperate North Atlantic during the summer. The Manx Shearwater breeds in the eastern North Atlantic and Mediterranean, but in nonbreeding season is often seen in the Atlantic coast Gulf Stream and on Georges Bank. The Audubon’s Shearwater common to the western North Atlantic, breeds in the Bahamas and the Carribbean. In nonbreeding season they migrate north, but remain in the warm slope waters off the northeastern United States.
Two other species in this family are common to the western North Atlantic, the Northern Fulmar and the Black-capped Petrel. The Northern Fulmar breeds in arctic North America. The distribution of the Fulmar extends across the North Atlantic, with the southern limit of the western North Atlantic populations reaching as far south as the Middle Atlantic Bight. The Black-capped Petrel is distributed in the western Atlantic, traveling from southern Florida and the Bahamas north to southern regions of the Gulf of Maine. They are a pelagic bird and often remain in the Gulf Stream, exploring the warm slope waters for food.
Gannets (Figure 4) are large seabirds found throughout temperate waters of the world ocean. They are closely related to the tropical booby species. Most species have white bodies, with darker feathering on the wings, head, and back. All have long pointed wings and tail, a heavy pointed bill, and webbed feet. The Northern Gannet is the only species to occur in North America and is found over cold water in the North Atlantic.
Gannets eat a variety of schooling fish, including mackerel, anchovy, pilchard, and flying fish, as well as occasionally eating squid. The birds forage at sea for long periods of time and may spend months without returning to land. Their feeding style is plunge-diving, often with many birds over a school of fish. They dive from flight from heights of 300 feet, catching prey at depths of 30 feet below the surface. As the birds approach the surface of the water in their aerial dive, they extend their wings far backward and press them against their body, thus making the shape of a large dart. Their wedge-shaped heads and bills create a streamlined form for piercing the water when diving after fish.
Gannets and the closely related boobies have some of the most elaborate and stereotyped courtship rituals of any of the seabirds. Gannets form some of the strongest pair-bonding among seabirds, and begin nuptials months before nesting. Nest sites are found in a variety of places, from flat ground to cliff faces to small trees and shrubs.
The Northern Gannet breeds only in eastern Newfoundland and the Gulf of St. Lawrence. From there, they travel south to the Gulf of Mexico and the Carribbean from December to February. In the fall they migrate south across the Gulf of Maine and Georges Bank, and travel as far south as the Mid-Atlantic Bight. In April, the spring migration reaches its peak on the Georges Bank and the majority of birds present are adults.
Cormorants are large waterbirds with webbed feet, elongated necks, and hooked bills used to catch aquatic prey while swimming underwater. Most species are predominantly black, some having white patches on their undersides. Cormorants are found on lakes, rivers, and coastal marine habitats.
Cormorants are highly skilled divers and all North American species consume small fish. Cormorants take their prey on their underwater pursuit, propelled by their webbed feet rather than their wings. Cormorants, once having caught their prey, bring it to the surface, maneuver it head first toward the gullet, and swallow. Marine foraging cormorants nest on islands, cliffs, or other remote coastlines. Males display for females from a nest site that the male has chosen, employing a wing-raised posture. Once paired, the male brings nesting material to the female at the nest site, where the female constructs the nest. Cormorants have larger clutch sizes than other seabirds: often 4 eggs are laid, but up to seven is not uncommon. On average a successful nest usually yields one to three fledglings.
The Great Cormorant breeds on coastal islands in the Gulf of St. Lawrence and the western edge of NewFoundland. It winters south, as far as South Carolina, occasionally as far south as Florida. The Double-crested Cormorant is very common and widespread. It breeds along coasts, inland lakes, and rivers in the Northern U.S. and southern Canada, as far east as Newfoundland, and as far west as Manitoba. The Double-crested Cormorant winters south from breeding grounds through the Gulf of Maine, as far south as the Bahamas and Cuba. It is very adaptable, living in fresh, salt, and brackish waters. Breeding season for both species is from the end of May through early August.
Gulls, Terns, and Their Relatives
The family Laridae is a large group of small to relatively large web-footed waterbirds with very diverse foraging strategies. The group is divided into four subfamilies: (1) Gulls are predominantly white as adults, while having various stages of gray-brown while immature. (2) Terns are white as well, while tending to be smaller and having sharper, pointier bills than gulls. (3) Skimmers are striking, black and white birds with large red and black bills, where the lower mandible is longer than the upper. And (4) Jaegers and Skuas are mostly brown and have deep, strong bills.
These species have widely varying diets and have numerous different foraging styles. Gulls, skuas, and jaegers are opportunistic omnivores, eating almost anything that will satisfy their nutritional needs. Terns and skimmers are more specialized in food choice and methods of prey capture.
Gulls are remarkable opportunists. They can be seen feeding in multiple habitats in the same day, from a fleet of fishing boats, to the intertidal zone, to agricultural fields, to a restaurant parking lot, to a landfill, to a pond for bathing, to a marsh, and perhaps ending up a river roosting far from sea. In natural environments, most gulls feed on small fish and marine invertebrates. Most species though are opportunists and omnivorous. Their diet may include small mammals, reptiles, amphibians, invertebrates, carrion, dung, waste grain, berries, and much, much more.
Terns, like the gulls, have adapted to human activities and are seen feeding above agricultural fields rich with insects, or following behind fishing boats eating discarded fish. In general though, terns are more specialized than gulls, primarily eating small fish found near the water surface that are present in small bays, marshes, rivers, estuaries, and oceans. Most terns capture their prey with their sharp, pointed bills by dipping and plunge-diving while in flight.
The Black Skimmer forages for small fish at shallow coastal bays, rivers, and marshes. Skimmers fly a few inches above the surface of calm waters, with the lower bill partly submerged. When they make contact with a small fish, the bill snaps shut and the fish is plucked from the water. This strategy, frequently repeated by skimmers over the same area of water, can produce capture rates that equal or even exceed those of terns. Skimmers are by far the most specialized group of feeders within the Laridae.
Skuas and Jaegers use a variety of foraging methods, like those of the gulls and terns, from aerial dipping for tiny prey to predation and stealing prey from other bird species. They eat a variety of things including berries, insects, rodents, chicks and eggs, other birds, carrion, and offal. The Pomarine and Long-tailed Jaegers often try to pirate their food, and often rely heavily on rodents, such as lemmings, for their food source.
Gulls, terns, and skimmers nest mostly in colonies, often very large ones, although some species are known to nest solely. Within the colonies, Laridae are highly territorial, albeit over only a few square feet of beach. Any intruders seen as a threat are attacked. Often, breeding is highly synchronized, all pairs laying their eggs at the same time, thus reducing the likelihood of all the chicks and eggs being eaten by a predator due to the large number. Colonial nesters tend to have elaborate breeding displays, including sky-pointing, bill-drooping, billing, wing-drooping, and throughout the whole breeding season, incessant calling. Most members of the family are monogamous and pair with the same mate annually, until one partner dies.
Larids occur throughout the world, many species having wide cosmopolitan distributions and long seasonal movements. Some species of gull, tern, skua and jaeger have circumpolar distributions. Some species are entirely pelagic for long periods of time, not coming to shore until they are ready to breed, perhaps at 10 years of age. Most skuas and jaegers are long distance migrants, such as the Long-tailed Jaeger that breeds in the Arctic and wintering in subantarctic waters. Gulls typically migrate shorter distances, nesting in the interior of North America, migrating to the coast in nonbreeding season, and not traveling further south than Central America. Some terns may move from their breeding grounds of marshes and lakes of the interior to the littoral zone of the Atlantic, and out to the pelagic zone of the Gulf Stream. From the pelagic zone, terns often migrate south, feeding within the pelagic drift community of sargassum, on their way to Western South America where they winter.
Eight gull species regularly occur on Georges Bank. Great Black-backed gulls and Herring gulls are common from Nova Scotia south to the Mid Atlantic Bight from October to April. In winter along the Atlantic coast Herring gulls are abundant, substantial numbers collecting in the greater New York area. Offshore at this time, large aggregations of both Great Black-backed gulls and Herring gulls attend fishing fleets throughout the gulf of Maine. Kittiwakes are found all across temperate North Atlantic and farther north. Other gulls are seasonally present on Georges Bank in low numbers. The Common and Arctic Tern are regularly found on Georges Bank. The common tern breeds coastally in eastern North America from South Carolina to Labrador, and the Arctic Tern breeds from Massachussetts to arctic Canada and Greenland. Both species arrive on Georges Bank in May from their wintering grounds in the southern hemisphere, remaining there till October. Jaegers are migrants on Georges Bank in the Spring and Fall, the Pomarine Jaeger being the most common, although relatively rare. The Parasitic Jaeger is seen more commonly in New England coastal waters, and the Long-tailed Jaeger is rarely seen on Georges Bank. The three species of jaeger are circumarctic breeders. The Great Skua breeds in Iceland, Scotland, and the Faroes, and is seen on Georges Bank from October to March.
In general, seabirds are affected and threatened ecologically by many of the same factors that are conservation concerns of land birds and mammals. Habitat encroachment and disturbance, acid rain, pesticide pollution, domestic predation, and hunting are some of the potential threats to seabirds. Seabirds are particularly sensitive to environmental changes due to their special adaptations allowing them to survive and live in a marine environment. Nesting colonies tend to be a site of the highest sensitivity, due to the fragility of these areas and the gathering of large numbers of seabirds at these areas. Multiple examples of seabird species in peril exist. The extinct Great Auk met its demise in the 19th century through hunting, habitat infringement, and being killed at sea by fisherman competing for fish with the seabird. The Brown Pelican was in major decline in the ‘60’s and ‘70’s due to widespread usage of the pesticide DDT, that found its way up the food chain through fish to the Pelicans, limiting their reproductive success. One of the largest threats to seabirds is oil spills. Oil spills both directly and indirectly affect the survival of seabirds. Oil laden birds have difficulty flying and foraging for their food, as well as the oil interrupting the insulative properties of their down feathers, making them unable to stay warm and often leading to death by hypothermia. Oil that makes its way to the coastal islands that are breeding grounds of these birds wreaks havoc on the environment, making the islands uninhabitable and thus reducing the reproductive success of those bird populations. An estimated 350,000 birds died as a result of the Exxon Valdez oil spill in 1989.
In the late 19th century fishermen on the Georges and Grand Banks often used Shearwaters caught in nets as bait in handline fisheries. This practice is probably not used today, but seabirds continue to have interactions with the commercial fishing industry, both directly and indirectly.
The information available to evaluate the distribution and abundance of many seabirds and the risk posed by human activities is scanty. A relatively large body of data that has accumulated, and is accumulating, is the number of bycatches of marine birds in observed fishing gear. From 1989 – 2005, 2828 seabirds were recorded as bycatch in the Northwest Atlantic, falling into 11 different families of bird species, although the primary observer effort has only been established over the last few years. Of the 11 families of marine birds reported as bycatch, the Procellaridae, or Shearwaters and Petrels, were the most commonly observed taken. They were followed by the Gaviidae, or Loons, the Laridae, or Gulls, the Phalacrocoracidae, or Cormorants, and finally the Sulidae, or the Northern Gannet (Figure 3). Interestingly, the fisheries that were most responsible for these bycatches were not the longline fisheries, as on the west coast, but rather were the more common fisheries in the Northeast. The largest numbers of observed seabirds taken during 1989-2005 were in the bottom otter trawl fishery followed by the sea scallop dredge, drift gillnet, and midwater paired otter trawl fisheries. These observed seabird takes could be used to estimate the numbers taken annually from the entire fisheries and could also be used to determine which types of gear characteristics are associated with bycatch which could then possible be used to develop bycatch reduction plans.