The Harlequin duck (Histrionicus histrionicus) is a small sea duck found along rocky coastlines in the northern parts of North America, Europe, and Asia. As their habitat suggests, Harlequin ducks feed primarily on marine invertebrates obtained by diving underwater. Their diverse diet and specialized feeding behaviors provide a fascinating look at the adaptations waterfowl employ to survive in their aquatic environments.
What kind of foods do Harlequin ducks eat?
The Harlequin duck diet consists mainly of invertebrate prey like mollusks, crustaceans, and echinoderms. Their main food sources include:
- Mollusks like mussels, clams, snails, and limpets
- Crustaceans like crabs, shrimp, and amphipods
- Echinoderms like sea urchins and sand dollars
- Aquatic insects like caddisfly larvae
- Small fish like sticklebacks
The relative proportions of these prey items in the Harlequin duck diet vary depending on habitat and seasonal availability. For example, mussels and limpets may dominate the diet in rocky intertidal areas, while crabs and shrimp are more readily consumed in soft-bottomed habitats like estuaries. Harlequins are flexible and opportunistic foragers, adapting their diet to consume the most abundant food sources in their local environment.
How do Harlequin ducks find and catch their prey?
Harlequin ducks employ a fascinating array of specialized hunting strategies and adaptations to locate and capture food in their marine habitat:
Diving and foraging underwater
Like other sea ducks, Harlequins use their strong legs and webbed feet to dive underwater. They can dive to depths over 30 meters to access prey on the seafloor. Their streamlined body shape and robust musculature allow for prolonged swimming and diving activity. Once underwater, they use their keen eyesight to spot prey tucked into rocky crevices.
Flapping wings to overturn rocks
Harlequins sometimes use an unusual technique called “wing-flapping” while diving. They flap their wings underwater to overturn small rocks and disturb the sediment. This exposes hidden prey like amphipods, larvae, and small crabs that they then swiftly eat.
Specialized bill morphology
The Harlequin’s thick, heavy bill is specialized for prying mollusks off rocky surfaces. The rounded tip allows them to crush the shells more easily. Their bill also has fine comb-like projections along the inside edges that help extract snails from their shells.
Rapid surface feeding
Harlequins can spot and snap up prey with lightning speed while swimming along the water surface. This allows them to grab small fish and aquatic insects that dwell near the surface. Their nimble maneuverability makes them well-equipped for surface feeding.
How do they swallow and digest their hard-shelled prey?
Eating prey with tough shells and exoskeletons poses challenges for digestion. Harlequin ducks have evolved some specialized adaptations to help them effectively process and assimilate their diet:
Ingesting pebbles as grit
Harlequins deliberately swallow small stones and pebbles. These stones reside in their muscular gizzard and help physically grind up and crush hard-shelled food items. This makes the shells easier to digest. The abrasive grit also aids in extracting snails from their shells.
Powerful muscular gizzard
The Harlequin’s gizzard has incredibly thick, powerful walls made of dense smooth muscle tissue. The muscular contractions of the gizzard lead to forceful compression that aids in physically crushing prey. This mechanical breakdown makes the shells easier for their digestive enzymes to penetrate and digest.
Secreting higher levels of hydrochloric acid
Harlequins produce particularly high levels of hydrochloric acid in their proventriculus stomach gland. This helps dissolve calcium carbonate shells more efficiently. Their gastric acidity is higher than that of many other duck species without such hard-shelled diets.
Longer intestinal tract
Since their diet is difficult to digest, Harlequins have a relatively long intestinal tract that provides more time and surface area for digestion. Their intestinal length is over twice their body length, compared to only 1.5x body length in Mallards with a similar size but softer plant-based diet.
How much do Harlequin ducks eat per day?
The daily food intake of Harlequins can vary based on factors like their size, activity level, stage of breeding cycle, and food availability. Some general estimates suggest they may consume approximately 15-30% of their body weight in prey daily.
A study observing captive Harlequins fed ad libitum found they ate an average of 277 grams (about 0.6 pounds) of food per day. Their average body mass was around 800 grams (1.8 pounds), equating to roughly 35% of their weight. However, wild ducks are likely more active and may consume even more by percentage.
Do Harlequin ducks feed nocturnally or diurnally?
Harlequin ducks are considered diurnal feeders, meaning they feed during daylight hours as opposed to nocturnal feeding at night. Peak foraging activity often takes place in the early morning and late afternoon/evening. They utilize daylight to hunt visually for aquatic prey while diving underwater.
However, some occasional nocturnal feeding may occur. One study in Iceland found they made foraging dives at night during the arctic summer when daylight hours are exceedingly long. But night diving was much less frequent compared to daytime dives. Their eyes are adapted for visual hunting and they are not well equipped to locate prey in darkness.
Do the ducklings eat the same diet as adults?
Harlequin ducklings have a more varied diet than the adults. They consume small aquatic invertebrates like insect larvae and tiny crustaceans. But a portion of their diet may also include plant material like algae, aquatic vegetation, seeds, and marsh grasses. Their smaller, less specialized bill allows them to feed on a diverse mix of small food items.
Within their first few weeks, ducklings transition to the typical invertebrate diet of adults. They practice diving and foraging techniques and gradually acquire the physical diving capabilities and specialized bill morphology needed to feed underwater like adult Harlequins.
What adaptations allow Harlequins to feed underwater?
Several key adaptations allow Harlequins to dive and swim effectively in search of submerged prey:
- Streamlined body shape – Their compact, rounded shape reduces drag and allows for swift diving and swimming motions.
- Dense plumage – Their feathers are coated in oils that help repel and shed water, keeping their underlayers dry and well insulated.
- Robust musculature – Powerful breast and leg muscles provide thrust and strength for diving.
- Reduced buoyancy – Solid bones and proportionally small air sacs increase their body density to submerge more easily.
- Keen underwater vision – Their eyes are adapted to see clearly while diving, aided by nictitating membranes that protect and act like goggles.
In combination, these adaptations provide Harlequins with great agility, stamina, and sensory abilities for finding food beneath the water’s surface.
How do Harlequin ducks access their hard-to-reach prey?
Harlequin ducks use ingenious techniques and behaviors to access difficult-to-reach marine invertebrates:
Prying prey off rocks
They use their strong bills to pinch and pry small mollusks like limpets and periwinkles off rock surfaces. The force exerted by their bills can reach over 9 kilograms to lever shells free.
Flipping rocks over
Diving Harlequins overturn rocks using their bills and feet, revealing and consuming the exposed prey living on the rock underside.
Digging in sediment
They excavate holes in sandy or muddy seabeds using their bill and feet in search of buried bivalves. This allows them to reach deeper prey like clams.
Hammering through shells
The Harlequin’s heavy bill can deliver pounding blows to break open sturdy shells of prey like blue mussels and thick-shelled snails.
Crushing hard shells
Their powerful gizzard allows them to crush and grind up the fractured shells of prey. Small ingested pebbles act as grit to aid in physically pulverizing shells.
Extracting snails from shells
Specialized fine combs along their bill enable them to firmly grip snails and pull the soft bodies out of tightly coiled shells.
These methods provide Harlequins unique access to well-protected marine invertebrates that provide a protein and energy-rich food source.
How do Harlequin ducks find adequate calcium for eggshell formation?
Female Harlequins require high calcium intake prior to the breeding season to produce strong eggshells. They obtain ample calcium from their mollusk-rich diet in several ways:
- Consuming calcium-rich shells of prey like mussels and limpets
- Eating soft mollusk bodies which filter and accumulate calcium from seawater
- Ingesting whole snails as a package of shell and body
- Retaining calcium from crushed shells digested with aid of grit in their gizzard
Their specialized adaptations for shell digestion and assimilation allow Harlequins to gain enough dietary calcium for proper eggshell formation, even while breeding at nutrient-poor northern latitudes.
Do Harlequins exhibit differences in foraging behavior between breeding and non-breeding seasons?
Some differences in Harlequin foraging ecology occur between breeding and non-breeding life stages:
- Non-breeders tend to forage more in intertidal zones where prey is very abundant.
- Breeding birds dive in deeper subtidal areas to find adequate food for rearing ducklings.
- Breeding males continue foraging further from shore while females incubate eggs.
- Parents accompany ducklings and may feed closer to shore in warm shallows.
- Molting birds often aggregate and feed together in late summer.
While Harlequins feed on similar prey year-round, factors like changing energy demands and seasonal prey availability lead to shifts in fine-scale foraging behaviors between breeding stages.
What effect do changing environmental conditions have on Harlequin foraging?
Shifting environmental conditions can impact Harlequin duck foraging in several ways:
- Prey availability – Food supply can fluctuate greatly between years, with effects on growth and reproduction.
- Seawater temperature – Warmer water increases metabolic demands and the energy needed for foraging dives.
- Weather – Storm swells and high winds make diving more difficult.
- Tides – Low tides expand foraging areas and access to more intertidal prey.
- Light level – Less daylight in winter constrains their visual foraging time.
- Sea ice – Ice cover reduces suitable foraging habitat and food sources.
Harlequins exhibit flexibility in their feeding ecology to cope with variable and challenging marine environments across their northern range.
How do Harlequins survive harsh winters when prey is scarce?
Harlequins use several key strategies to meet their energy needs during harsh northern winters when prey is limited:
- Switch to more intertidal foraging in calmer waters
- Expand home ranges and foraging movements
- Spend more time feeding and diving each day
- Reduce energy expenditures by settling in sheltered areas
- Rely more on lipid reserves accumulated during summer/fall
- Prioritize self-maintenance over breeding during the harshest months
Their behavioral adaptability provides resilience, enabling Harlequins to endure seasonal bottlenecks in food availability.
Conclusion
In summary, Harlequin ducks are strikingly adapted for extracting hard-to-reach marine invertebrates across turbulent northern seas. Flexible foraging strategies, diving capabilities, bill morphology, and digestive systems allow them to persist in rocky intertidal habitats rich in their preferred mollusk prey. Yet they also exhibit resiliency in their feeding ecology in response to variable environmental conditions. The Harlequin’s specialized diet and efficient foraging abilities are key to surviving in the extreme habitats they occupy.
