Thursday, July 18, 2024

A female hooded merganser takes off from Horn Pond in Woburn on Nov. 16. (Photo: Manish Shukla)

Found only in North America, a hooded merganser is a carnivorous diving sea duck with a serrated bill, which helps it catch and hold food. Primarily a fish eater, it will also eat crabs and insects depending on what food is available. Mergansers are the only ducks that specialize in eating fish. A merganser can forage underwater for up to two minutes, resurfacing to swallow its prey. To avoid being impaled by spiny fins, a merganser turns a fish around in its bill and swallows it headfirst.

Hooded mergansers need good vision when they fly rapidly through the air (as fast as 50 mph), when they paddle on the surface and when they dive into clear water to search for fish. Water refracts light differently than the air, so this difference presents a visual challenge. Eyes that are in focus in the air tend to be farsighted underwater. Eyes that are in focus in the water tend to be nearsighted in air. For an image to remain sharp in the air and underwater, an eye must be able to adapt to different rates of refraction. To accomplish this feat, hooded mergansers when underwater push the lens of their eye forward so it bulges through the pupil. Now the underwater world is in clear focus.

A male hooded merganser swallows a fish on Horn Pond in Woburn. (Photo: Tom Murray)

True to its name, the hooded merganser has a crest on its head that resembles a hood. It compresses the crest before it dives, and keeps it smoothed down when relaxed. During mating season the crest is important, though: When a pair of love-struck hooded mergansers are interested in each other, they show their attraction with their heads. They raise their crests and shake their heads, throw back the head until it touches the back, pump the head while flapping the wings and much more.

Males have yellow feet and a white fanlike crest surrounded by black on the head. (Photo: Tom Murray)

Hooded mergansers nest in tree cavities near a body of water. To get into the tree cavity, these agile swimmers must fly up to the cavity, stop abruptly, grab the edge of the cavity with webbed feet and plunge inside the hole. It sounds difficult, but these ducks are good at accomplishing this feat. (Some other types of ducks occasionally crash and miss the cavity).

A female hooded merganser selects a nest cavity, often abandoned by another bird. She prefers a cavity that is 4 to 15 feet off the ground, but hooded mergansers have been known to nest in holes 50 feet off the ground. Once a nest site is selected, the female lays about 10 or 12 thick-shelled eggs. The male leaves the female at this point to migrate to a safe area where he molts. The female incubates the eggs for about a month.

A male hooded merganser prepares to swallow a large fish on March 6, 2022. (Photo: Richard George)

When the chicks are born, they can swim and forage for themselves almost immediately. Within 24 hours they leave the nest. The female calls to them from below, and the chicks jump to the ground – as far as five stories below! Then they waddle after their mother to the nearest body of water, which might be a half-mile away. The female leads the chicks around on the water to good hunting sites for several weeks, but she abandons them before they can fly – also flying off to molt. Young hooded mergansers have longer claws on their feet than dabbling ducklings. This helps them climb a tree to the nest cavity at night, for they cannot fly until they are about 70 days old.

Hooded mergansers may lay eggs in the nests of other ducks (called egg dumping). Although each female lays about a dozen eggs, some nests contain as many as 40 eggs! You might see a female merganser leading a troop of 30 or more chicks through the water. These large troops can be the result of egg dumping, but they can also result if a parent is killed or if the chicks swim after the wrong parent. This is most likely to occur when a predator scatters groups of ducks; the groups re-forms as best they can.

A hooded merganser pair in Cambridge’s Alewife Brook Reservation on Nov. 25, 2020. Females have a cinnamon head and green feet. (Photo: Ann Schlesinger)

Many hooded mergansers migrate here in the late fall from farther north, where they breed in the summer. These migrants are looking for unfrozen lakes or saltwater bays where they can fish and be protected throughout the winter. Some hooded mergansers actually migrate north to the Great Lakes, which are large enough not to freeze over completely in winter. In the spring, hooded mergansers migrate again, arriving back in their northern breeding grounds within a few days of the ice melting!

Hooded mergansers do not breed until they are 2 years old; therefore, they have low productivity rates compared with many other birds. In addition, their numbers are relatively small and they are slow to colonize breeding areas. Hunters kill about 95,000 of these birds each year. Because they rely so heavily on aquatic prey, water pollution can harm these ducks. Noxious chemicals such as mercury or dioxins can accumulate in fish they prey upon. Acid rain can kill fish and other prey. Forest destruction has reduced the size of their breeding grounds. For all of these reasons, if we want future generation to be entertained by this secretive pint-sized creature, it is important to preserve and care for the waterways and forests where this marvelous little duck breeds and overwinters.

A male paddles in Cambridge’s Alewife Brook Reservation on Nov. 22, 2020. (Photo: Richard George)

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Seen nearby

Marj Watson photographed this ash-throated flycatcher in Rockport on Nov. 2.

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Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.