Common mergansers are migratory waterfowl that breed in northern North America and winter farther south. In the fall, they undergo an extensive migration to reach their wintering grounds. So where exactly do common mergansers go when cold weather arrives?
Quick Facts on Common Merganser Migration
Here are some key points about the winter migration of common mergansers:
- Common mergansers breed across Canada, Alaska, and the northern United States.
- They migrate south in the fall to spend the winter along the Atlantic, Pacific, and Gulf coasts.
- Some will winter as far north as the Great Lakes if open water is available.
- Their winter range extends south through Mexico and into Central America.
- Males and females often winter separately in different locations.
- Young birds may not migrate as far south as older, more experienced birds.
- Spring migration back to the breeding grounds begins in March.
Breeding Range
During the summer breeding season, common mergansers inhabit areas across northern North America. Their breeding range includes:
- Alaska
- Canada
- Northeastern and northwestern United States
- Around the Great Lakes region
- Northern New England and the Canadian Maritimes
They nest near rivers, lakes, ponds, and coastal areas that provide ample food supplies. The female builds the nest in tree cavities, rock crevices, or under vegetation. Once the young hatch in early summer, the female leads them to water where they forage for small fish and aquatic insects.
Fall Migration
As winter approaches, common mergansers start moving south in search of open waters with abundant food. Their fall migration begins in September or October and continues through November. Several factors influence the timing and distance of their migration:
- Age – Older birds migrate earlier and farther than juveniles
- Sex – Males precede females during fall migration
- Weather – Early winter storms can spur earlier migrations
- Food availability – Lack of food may force them south sooner
They generally migrate in small flocks, traveling mainly at night. Males and females often migrate in separate groups. Their flights average 20 to 40 miles per day following rivers and coastlines towards their wintering grounds.
Wintering Range
Common mergansers winter in coastal areas and inland waters along migration routes across North America. Their winter range extends from southern Canada down through the United States into Mexico and Central America. Here are some details on where they spend the winter months:
- Great Lakes – If waters remain open, some will winter as far north as the Great Lakes
- Eastern Coast – Chesapeake Bay, Carolinas, Gulf Coast, Florida
- Western Coast – Pacific Coast from British Columbia to Baja California
- Interior Lakes & Rivers – Throughout the southern United States
- Mexico – Coastal regions and inland lakes of northern and central Mexico
- Central America – As far south as Panama
Males and females often occupy different wintering habitats. Males tend to prefer coastal marine waters. Females are more likely to winter on inland freshwater lakes and rivers. Their distribution depends on the availability of open water areas with enough food to sustain them until spring.
Spring Migration
As winter ends, common mergansers begin migrating back to their northern breeding grounds. They start heading north again in March and April. Spring migration occurs over a broader time period than fall migration. The males depart first, followed by females and juveniles. They travel along similar routes as in the fall, including major rivers, the Atlantic coast, Mississippi Flyway, and Pacific Coast.
Again, weather impacts the exact timing of migration. Warmer conditions and open waters in the north spur earlier departures from the wintering grounds. Younger birds usually return later than older, more experienced mergansers. Most arrive on the breeding areas by May to establish territories and seek out mates for the upcoming nesting season.
Threats During Migration
Common mergansers face certain threats and challenges during their seasonal migrations between breeding and wintering areas. Some of the key hazards they encounter along migration routes include:
- Habitat loss – Draining of wetlands diminishes feeding and resting places
- Pollution – Oil spills, chemical runoff, garbage impact water quality
- Overhunting – Legal hunting combined with poaching takes a toll
- Power lines – Collisions with power lines cause injury and death
- Predators – Eagles, owls, snapping turtles prey on mergansers
Conservation measures aimed at protecting wetland habitats and regulating hunting pressure can help reduce these threats. Promoting sustainable development along migration routes is also important for safeguarding common merganser populations as they journey between their northern breeding and southern wintering grounds.
Winter Diet
Common mergansers rely on finding adequate food throughout the winter months while on their southern wintering grounds. Their diet consists mainly of small fish, aquatic insects, crustaceans, mollusks, and amphibians. Here is more on the types of prey they consume during winter:
- Small fish – Minnows, sticklebacks, smelt, trout, herring, eels
- Insects – Caddisfly larvae, dragonfly nymphs
- Crustaceans – Shrimp, crayfish, crabs
- Mollusks – Snails, mussels, clams
- Amphibians – Salamanders, frogs, tadpoles
They find food by diving below the water’s surface. Their slender, serrated bill is well-adapted for grasping slippery prey. Males and females employ somewhat different foraging strategies. Males are more likely to dive into deeper waters offshore. Females stick closer to shore in shallows and along the water’s edge.
Having access to sufficient food resources is critical so birds can build up energy reserves before the long return migration. Coastal estuaries, marshes, streams, and lakes that harbor abundant small fish and aquatic invertebrates provide important wintering habitat.
Wintering Behavior
During the winter months, common mergansers display some behavioral adaptations for surviving cold conditions and finding adequate food:
- Floating in sheltered groups on open water to conserve body heat
- Synchronized diving in groups to herd fish
- Females with broods remain together through the first winter
- Males establish winter territories to secure food resources
- Roosting on shoreline rocks or logs when not feeding
- Migrating short distances during winter to find open water
Their social behavior shifts compared to the breeding season. Males tend to remain solitary or form loose flocks. Females with juvenile birds form larger flocks that forage cooperatively. They remain gregarious throughout winter for better protection from predators.
Major Wintering Areas
Some of the major wintering sites and concentration areas for common mergansers include:
- Salton Sea in California
- Chesapeake Bay
- North Carolina coast and inland reservoirs
- Texas Gulf Coast
- Florida Everglades
- Great Lakes – if ice-free areas persist
- Rivers of the Central Valley of California
- Colorado River basin lakes and reservoirs
- Northern Gulf of Mexico coastline
- Pacific coast from Washington to Baja California
These regions provide the combination of shallow sheltered waters, abundant food sources, and protection from severe weather that common mergansers need to overwinter successfully. Conservation of these important habitats ensures they have sufficient places to rest, feed, and wait out the cold months until the spring migration.
Population Status and Trends
According to the North American Breeding Bird Survey, common merganser populations are on the rise across most of their breeding range. From 1966 to 2015, their numbers increased by 2.1% per year on average. The current total breeding population is estimated at 460,000 birds. Here are some trends in different regions:
- Increasing in central Canada and Alaska
- Stable or increasing across northern United States
- Declining in areas of eastern Canada
This overall upward trend can be attributed to several factors:
- Ongoing recovery from overhunting in the early 1900s
- Increased use of nest boxes providing greater nesting sites
- Establishment of protected wetland habitats
- Adaptability to human-altered waterways
While populations remain healthy, continued monitoring is needed. Threats like wetland loss, contaminants, and climate change impacts could potentially shift trends. Managing habitats and regulating harvest will be important for long-term conservation.
Research Techniques
Scientists use various research methods to study the migration patterns and winter ecology of common mergansers:
- Leg banding – Attaching numbered metal bands allows tracking migration routes and survival rates when birds are recaptured or found
- Radio telemetry – Implanting radio transmitters enables tracking movements of individual birds
- Satellite tracking – Using satellite tags provides real-time monitoring of migration routes
- Stable isotope analysis – Comparing ratios of elements in feathers identifies wintering origin
- Genetic analysis – Detecting genetic differences reveals connectivity between breeding & wintering populations
- Field surveys – Conducting counts at wintering sites identifies population trends
Combining these techniques provides fuller understanding of migration patterns, winter distribution, habitat use, and other conservation-relevant aspects of common merganser winter ecology.
Importance of Wintering Habitat
Why is conserving wintering habitat so crucial for common mergansers? Here are some key reasons:
- Provides enough food resources for them to survive until spring migration
- Allows them to rebuild energy reserves needed to migrate and breed
- Supports molting into alternate plumage during February-April
- Offers shelter from harsh weather conditions
- Reduces mortality risks from predators, storms, and lack of food
Loss of wetlands, pollution, drainage projects, and water diversions in wintering areas would have detrimental effects. Managing adequate habitat is also important to support both wintering and migrating merganser populations at sustainable levels.
Conservation Status
The common merganser has an extensive range and large total population size. As a result, it is classified as a species of Least Concern on the IUCN Red List of Threatened Species. Here is a summary of its current conservation status:
- Global population estimate – 1.5 to 1.7 million birds
- Population trend – Increasing
- Threats – Habitat loss, contaminants, climate change
- IUCN status – Least Concern
- Legally protected – Migratory Bird Treaty Act in U.S. and Canada
While not currently threatened with extinction, habitat protection and responsible management practices are still needed to maintain healthy common merganser populations into the future.
Conclusions
In conclusion, common mergansers undergo an extensive migration each year between their northern breeding grounds and southern wintering areas. They winter along coasts and inland waters across the United States, Mexico, and Central America. Their distribution is driven by the availability of open water, ample food resources, and suitable shelter. A variety of research methods help reveal their migratory patterns, winter ecology, and habitat needs. Conserving wetland habitats across their breeding and wintering range remains key to ensuring the long-term viability of common merganser populations.
