The common loon, scientifically known as Gavia immer, is a captivating aquatic bird celebrated for its haunting calls and striking appearance. Also known as the great northern diver in Eurasia, this species is a member of the loon, or diver, family of birds. As a top predator in its aquatic ecosystems, the common loon's diet plays a crucial role in its survival and the overall health of its environment. This article delves into the dietary habits of the common loon, exploring what it eats, how it hunts, and how its diet varies across different seasons and habitats.
Physical Characteristics and Distribution
Before diving into the specifics of the common loon's diet, let's briefly touch on its physical attributes and distribution. Adult common loons range from 66 to 91 cm (26 to 36 in) in length and have a wingspan of 127 to 147 cm (4 ft 2 in to 4 ft 10 in). Their weight can vary from 2.2 to 7.6 kg (4.9 to 16.8 lb). Males are often significantly heavier than females, with some populations showing males averaging up to 27% more massive than females.
Breeding adults sport a broad black head and neck, often with a greenish, purplish, or bluish sheen. Their upperparts are blackish or blackish-grey, while the underparts are pure white, with some black on the undertail coverts and vent. Non-breeding adults have brownish plumage with a dark neck and head marked with dark grey-brown.
Common loons are primarily Nearctic, breeding from 48° N to the Arctic Circle, and locally south to 40° N and north to 78° N. During the breeding season, they inhabit lakes and waterways in the northern United States, Canada, and southern Greenland. They also breed in Iceland, Svalbard, and sporadically in Arctic Eurasia. In winter, they migrate to coastal areas and inland lakes that remain ice-free.
Dietary Preferences and Composition
The common loon is an expert fisher, with fish accounting for about 80% of its diet. It forages on fish of up to 26 cm (10 in) in length. The freshwater diet primarily consists of pike, perch, sunfish, trout, and bass. The saltwater diet primarily consists of rock fish, flounder, sea trout, herring, Atlantic croaker, haddock, and Gulf silverside.
Read also: The Diet of the Common Warthog
Besides fish, the common loon also consumes a variety of other animal prey, including crustaceans (especially crayfish), insect larvae, molluscs, and occasionally aquatic plant life. During the summer, adults eat many species of freshwater fish, including yellow perch, suckers, shiners, and occasionally small trout. In the winter, loons shift their diet because they live in saltwater along the coast. Fish such as flounder and herring, as well as crustaceans, are regular components of their winter diet.
Specific Prey Items
The common loon's diet includes a wide range of fish species, such as:
- Minnows
- Suckers
- Gizzard shad
- Rock bass
- Alewife
- Northern pike
- Whitefish
- Sauger
- Brown bullhead
- Pumpkinseed
- Burbot
- Walleye
- Bluegill
- White crappie
- Black crappie
- Rainbow smelt
- Killifish
The young typically eat small minnows, and sometimes insects and fragments of green vegetation.
Hunting Techniques
The common loon is an expert fisher, catching its prey underwater by diving as deep as 60 m (200 ft). With its large webbed feet, the common loon is an efficient underwater pursuit predator and adroit diver. The common loon uses its powerful hind legs to propel its body underwater at high speed to catch its prey, which it then swallows head-first. If the fish attempts to evade the common loon, the bird chases it down with excellent underwater manoeuvrability due to its strong legs. Loons are visual hunters, so they need relatively clear water conditions to find prey. Adults prefer to hunt in shallow water and eat whatever is easiest to catch and most abundant. Feeding dives are usually less than a minute, although if prey is hard to find, the dives can be longer.
Once a fish or other potential prey is located, the loon dives, holding its wings close to its body, and uses its powerful feet for propulsion while swimming underwater. The fish, crayfish, or other prey is captured in the loon’s bill, and with the exception of large fish, is consumed while still underwater. If the loon catches a large fish, it may release and catch the fish repeatedly until the fish is subdued and the loon can swallow it whole.
Read also: Omnivorous Starling
Most prey are swallowed underwater, where they are caught, but some larger prey are first brought to the surface. After swallowing a large fish, the loon may stretch its neck up and appear to yawn as it tries to move the fish down its throat. Such a big fish will cause a loon's neck to bulge out until the fish moves down into the stomach and is digested.
Nutritional Needs and Consumption Rates
Loons typically eat fish weighing 10-70 grams. They usually eat the fish and other prey they catch underwater, although they will bring food to the surface to feed their chicks, and also large fish to subdue them before swallowing them. An adult loon eats approximately two pounds of food daily. Adult birds, especially large adult males, occasionally catch a large fish (up to 250-300 grams).
Loons also ingest small pea-sized stones to help them digest fish bones and the shells of crustaceans. Like other animals, they swallow it to make digestion easier. Since they eat prey with exoskeletons and bones, their gizzard needs help crushing these hard parts, and pebbles are great at doing this.
Feeding the Chicks
Juvenile loons are fed fish appropriate to their size. Adults carry small minnows and sunfish, crayfish, and aquatic insects sideways in their bill to bring food to their chicks. The young birds often watch underwater as their parents are hunting and begin trying to catch small prey when they are ~4-6 weeks old. By the time they are in the immature feathered plumage, juveniles are capable of catching and eating larger items- up to several inches in length. However, they will continue to beg for food incessantly from the parents until the late fall, when the adults leave for the coast.
Both parents feed the chicks live prey from hatching to fledging. As they grow, chicks are able to catch an increasingly large proportion of their diet by themselves; they can feed and fend for themselves after about two months, although many juveniles continue to beg from adults well beyond this age.
Read also: Feeding Your Degu
Adaptations for Aquatic Feeding
Adult loons are highly specialized for swimming and diving, and thus well-suited for aquatic environments. Their bodies are streamlined and their legs are laterally flattened - both of these characteristics greatly help reduce resistance when swimming underwater. Loon bones are very dense, which helps them dive and stay underwater. To compensate for the added weight of extra dense bones, loons must flap their wings continuously when flying to remain airborne. Loons’ large webbed feet are used for propulsion and making quick turns while underwater. Their legs are positioned so far back on their body that they are virtually incapable of walking on land. Loons only come to land to breed, nest, defecate (occasionally), or when they are gravely ill or injured.
Loons have a salt gland to excrete excess salt that they ingest while feeding on the ocean.
Impact of Diet on Loon Behavior and Ecology
The common loon's diet influences its behavior and ecological role in several ways:
- Territoriality: Common Loons are very territorial and have a set of unique vocalizations to communicate with their mate, chicks, and other loons.
- Habitat Selection: Common Loons are a classic bird of the North Woods lakes. They are excellent indicators of water quality as they require crystal-clear lakes (which makes it easier for them to see prey underwater) with abundant populations of small fish.
- Social Behavior: Loons sometimes form social gatherings of several adults in late summer. Loons are actually very social animals when they are not defending a territory or chicks. They gather together to feed, often doing a “circle dance,” in which the birds swim in a circle, taking turns peering and diving into the water. Vocalizations, primarily hoots, and ritualized displays such as bill dipping, wing rowing, and wing flapping, are regularly observed during these events.
Threats to Loon Populations Related to Diet
Several threats to common loon populations are linked to their diet:
- Mercury Pollution: Mercury, from the burning of coal, can build up in lakes through rainfall, and this has led to poor reproductive success for Common Loons in Canada, New England, and Wisconsin.
- Lead Poisoning: Lead fishing sinkers, which loons ingest when they scoop up pebbles off the lake bottom to store in their gizzards, have been a significant cause of loon deaths from lead poisoning.
- Acid Rain: Acid rain can acidify lakes, reducing fish populations that loons depend on.
- Fishing Line Entanglement: Loons face numerous threats, including fishing line entanglement.
Conservation Efforts
Several conservation efforts are in place to protect common loon populations:
- Protection Under the Federal Migratory Bird Treaty Act: Common Loons are a Species of Special Concern in New York State and are protected under the Federal Migratory Bird Treaty Act.
- Restoration Programs: Restoration programs have increased productivity of loons in Vermont.
- Artificial Nesting Platforms: Artificial nesting platforms are occasionally placed in suitable water bodies.
- Maintaining Distance: Avoid getting within 300 feet of loon nesting and nursery areas during the breeding season (May through August). Maintain a respectful distance from wild animals.