Common loons (Gavia immer), known as Great Northern divers in the UK, spend much of their lives foraging for food underwater, in lakes and in coastal waters, so it won’t come as much of a surprise that fish form the largest part of their diet.
But what kind of fish do loons eat? And do loons eat anything else? Keep reading as we look into the dietary preferences of these streamlined underwater hunters.
Loons hunt by sight, scouring the waters of lakes and oceans for suitable fish to chase, catch and swallow whole. Around 80 percent of their diet consists of fish, while crustaceans, small amphibians and some aquatic insects are also eaten.
Able to remain underwater for up to 5 minutes without needing to resurface, loons generally eat their prey underwater too, only bringing food to surface level during the breeding season when they need to feed their young.
Common Loon with a fish in its beak
If a large fish proves too tricky to handle at first, it may be brought to the surface, then speared again with the loon’s dagger-like beak before being dropped into the water once more and pursued for a second time.
The varied habitats of the common loon around the globe brings them into contact with a wide range of fish, both freshwater and marine species. In the UK, where they are known as Great Northern divers, overwintering birds in coastal areas commonly hunt for herring, sea trout and rock.
Keep reading as we’ll be exploring the hunting techniques used by these skilled divers, and investigating just how many fish a common loon needs to catch each day to maintain its energy levels.
Fish make up the majority of a Common Loon, or Great Northern Diver, diet
Freshwater and saltwater fish form the bulk of a loon’s diet, with fish species between 10g and 70g being the most frequently targeted. Sunfish, perch, smelt, bass, pike, minnows and catfish are among the most caught fish by common loons.
Trout and salmon are also eaten occasionally, although the straight swimming patterns of these species make them less of an easy target for diving loons, who are skilled at pursuing fish with more erratic swimming habits.
Loons spend winter months offshore on coastal waters, where their main diet consists of flounder, rock, Gulf menhaden, bay anchovies, and silversides, as well as crabs, lobster, and shrimp.
Common Loon with a Pumpkinseed Sunfish in its beak
Daily food intake for an adult loon is estimated at around 960g (2.1 lb) of fish. Their most commonly caught fish vary in size between 10 and 70 g (0.4 to 2.5 oz), so it is not unthinkable for an adult bird to catch and eat upwards of 20 fish in a single day.
Fish makes up around 80 percent of a common loon's diet. In addition, they also eat crayfish, frogs, leeches, and snails. They spend winter months at sea, where as well as marine fish, they survive on crabs, snails, shrimp, lobster, and marine invertebrates.
Close up of a Loon with a freshly caught Crayfish
When there is a lack of fish available, it is not unheard of for loons to catch young ducklings to eat. Newly hatched young waterfowl are easy prey for loons.
One unusual case, documented in 2019, was the killing of a bald eagle by a loon, possibly in defense of a nest site or young chicks, as a dead baby loon was found nearby. The bald eagle was found floating in a lake, having been stabbed through the heart by the blade-like beak of a common loon.
Loons are rarely found on land, and their legs are not capable of walking even the shortest of distances without extreme difficulty. Therefore, they have adapted and perfected their hunting methods to work underwater, or occasionally on the surface of a lake, reservoir, or the ocean.
Loons hunt by sight, so rely on clear and clean water to watch and track fish swimming below the surface. They are what is known as “pursuit predators,” chasing their prey through the water and catching it with their dagger-like beaks.
Common loons’ tongues are barbed with sharp spikes called “papillae”, which prevent the fish they catch from slipping out of their beaks.
Loon diving under the water to catch some prey
Common loons tend to eat their prey underwater, swallowing fish headfirst and whole. Their throats stretch to accommodate even the largest fish, which they then swallow without the need for chewing or dissecting.
Fish are typically only brought to the surface by parent birds feeding their young, although occasionally a loon can be observed resurfacing after several minutes underwater when it has successfully caught a larger fish, and will then stab at it, release it into the water and then dive down again to catch it a second time, when it is more subdued and easier to handle.
To aid digestion, it is fairly common for a loon to swallow small pebbles, which remain in its gut and help to pulverize the food in its stomach. These stones, known as gastroliths, are particularly useful as they consume a lot of fish or crustaceans with bones and shells that are hard to digest.
Common Loon stretching out its feathers to dry them, White Lake, Ont, Canada
Loons are carnivores, commonly classified as piscivores due to the importance of fish in their diet. They do also supplement fish with other aquatic animals, such as frogs and salamanders, and may also eat crabs and crayfish. In their early life, before they have fully mastered the art of diving and catching underwater prey, young loons also eat algae and vegetation found in their watery habitats.
Loons spend a large part of each day searching for food. Underwater dives typically last less than a minute, although the species are highly proficient divers and can remain underwater for around 5 minutes.
Close up of a Great Northern Diver (Common Loon), swimming on the lake
In the early stages of life newly hatched young loons are fed by their parents up to 73 times a day. Parental feeding continues for the first 8 weeks, with parents bringing small or partly digested fish and lizards for their young. Baby loons often eat algae and other vegetation as an extra source of nutrients until they learn to dive for themselves.
Common Loon feeding a Sunfish to its young chick
Loons are active hunters, who scan underwater areas for potential kills, and then torpedo themselves towards their target. Due to the nature of their preferred diet, it’s not really feasible for people to ‘feed’ them, as they seek live prey and are adapted for breathing and hunting under the surface of lakes and coastal waters to satisfy their nutritional requirements.
Loon feeding chick fish
Spending several months at sea means fresh (non-salty) water is not always available for loons to drink. Although generally considered freshwater birds, loons spend extensive periods at sea, in the Atlantic Ocean and the Gulf of Mexico, during the winter months, and have a particular physical adaptation that allows them to remain hydrated.
A special ‘salt gland’ located above a loon’s eyes, filters out excess salt, which is then excreted from the body through an opening at the top of its beak.
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