Merganser Feeding Frenzies: Unusually Active Foraging by Red-breasted Mergansers

Narrow, ice free bodies of water are great places to observe birds in the winter as they sometimes contain hundreds to thousands of birds. Red-breasted Mergansers (Mergus serrator) are a species of duck that frequents these habitats when ice closes fresh water lakes and streams in the northeast. At times, these birds charge through inlets in frenetic feeding frenzies that can readily be described as blitzkriegs or “lightning wars” – a term coined by the Germans to describe the extremely rapid ground-covering tactics of the German army during World War II.

Different Behaviors Within a Feeding Flock

Groups of up to seventy-five mergansers will often assemble, but only twenty to forty-five may be above water at any given time. Birds in the middle of the flock will feed in the fashion most often observed for this species: groups of six to twelve birds swim rapidly and dis¬appear almost in synch¬ronization while other groups pop out of the water, move ahead a few yards and disappear again just as the first groups pop up to replace them.

At the leading and trailing edges of the force, the birds exhibit the behaviors that characterize the blitzkrieg: birds just behind the front runners churn for up to 100 feet to move ahead of the leaders, causing the water to seemingly boil by thrashing the surface with their pumping feet as they run full tilt. While still moving at top speed, they cannonball below the water’s surface, to be replaced only seconds later by new birds from behind.

When birds at the tail end of the flock explode out of the water and see how far ahead the main group has moved, they spatter across the top of the water as they scamp¬er to become airborne, fly low over the assemblage, scatter spray both right and left as they hit the water with both feet extended, and plummet below the surface almost as soon as they settle.

What Happens When the Fish are Gone

The total effect is of a frenetic, chaotic churning of ducks rapidly moving across the water as they feed on swarms of fish that rapidly disperse when they are attacked from all sides. The mergansers may roil the water along one shore, rapidly cross to the opposite shore, and return to the original side as they charge after prey scattering through the shallow waters. The overall speed of flock movement is often much faster than that which these active ducks usually exhibit, and an entire sortie might last only a few minutes. While fishing is good, head tosses indicate successful captures. When fishing is finished, the birds disperse and the flocks disperse.

Several Species of Birds are Affected by Feeding Frenzies

The active foraging of these mergansers both attracts and repels many bird species. Hooded Mergansers (Lophodytes cucullatus) and Buffleheads (Bucephala albeola) are drawn to feeding Red-breasted Mergan-sers (personal observation). The churn¬ing of the water’s surface appears to be the stimulus to these other diving ducks that there is a plentiful supply of fish available. In Florida, Snowy (Egretta thula), Great (Csmerodius alba), Reddish Egrets (Egretta rufescens), and Tricolored (Louisiana) Herons (Egretta tricolor) fly from some distances to feed near merganser frenzies.

Little Blue Herons (Egretta caerulea) will fly away from a merganser boil, preferring to sedate¬ly stalk the shallows by themselves. In contrast, Great Blue Herons just ignore these disturbers of small fish – neither moving toward nor away from them (personal observation).

The species of herons and egrets attracted to merganser flurries often stand in wait for the mergansers to arrive then abandon their usual stalking behaviors and begin to chase and pursue fish disturbed by the ducks. Thus, the egrets use much more active behaviors when near mergan¬sers than these larger birds do when hunting in the absence of mergan¬sers. The egrets often gather near shallow lagoons just before low tide and wait for the mergansers to show up. They are rarely disappointed. Almost on cue, the mergansers fly into the lagoons and ponds, begin chasing the fish – sometimes in water only three to six inches deep, and the egrets join the fray.

Buffleheads also sometimes churn the water when feeding. Their feeding frenzies often attract Red-breasted Mergansers to the Buffleheads where the two species will feed side by side until the fish scatter or are depleted by the two predators. Although mergansers will be attracted to Buffle¬heads by the churning of the water, the mergansers once there are no available fish, the association disintegrates and the two species swim away from each other.