The common loon (Gavia immer) is a large diving bird found across North America. It breeds on freshwater lakes and migrates south to coastal areas for the winter. The common loon goes through a dramatic molt between its breeding plumage and its winter plumage. This allows it to be camouflaged for the different environments it inhabits. In its winter plumage, the common loon has silvery gray feathers on its head, neck, and back with white underparts. Its change in plumage between seasons has long fascinated birdwatchers.
Breeding Plumage
During the breeding season, the common loon has striking black and white plumage. Its head and neck are black with a green iridescent sheen. There is a distinctive black and white checkerboard pattern on its back. The chest is solid white. This bold contrast makes the loon easy to identify in summer. The black coloration likely serves to regulate body temperature as the loon spends long hours sitting on its nest in the hot sun. The sharp pattern helps camouflage the loon when it is swimming in the water amid reflections of shoreline plants.
Winter Plumage
In preparation for winter, the common loon molts into its duller gray and white plumage. By early November, its breeding plumage is completely replaced. The head and neck become smoky gray on the back and pure white on the underside and around the eyes. The iridescent sheen is gone from the head and neck. The back loses its checkerboard pattern and becomes a more uniform gray-brown color. The chest remains white. The gray provides camouflage amongst the gloomy winter waters of the loon’s coastal habitat.
Interestingly, the common loon molts earlier than other loon species in North America. For example, the yellow-billed loon retains its breeding plumage through December. The rapid molt of the common loon may be an adaptation to allow more time in its camouflaged winter colors. Its silvery winter plumage likely helps it avoid detection from predators and prey alike.
Male vs Female Winter Plumage
The winter plumage of male and female common loons appears identical to human observers. However, studies suggest loons themselves can determine sex from subtle differences imperceptible to our eyes. One study found male loons may have darker plumage around the eyes while females have lighter plumage in this area. More research is needed to conclusively identify any sexual dimorphism in common loon winter plumage. Banding studies show male and female loons often migrate together and reunite year after year, so they likely have ways to recognize their long-term mates even in similar winter garb.
Juvenile Winter Plumage
Juvenile common loons from the previous breeding season initially resemble adults in winter plumage. However, juveniles can be distinguished by darker plumage overall with more mottling on the head and neck. Their feathers also tend to be more ragged and worn since juveniles do not complete a full molt prior to their first winter. Juveniles molt partially but will still retain some juvenile flight feathers and coverts.
By their second winter, most juvenile common loons are indistinguishable from adult birds in winter plumage. The juvenile’s worn feathers are replaced with the sleek silvery feathers of mature adults. Banding studies show juvenile loons have significantly lower survival rates through their first two winters. The juvenile’s partial molt may impede flight or thermal regulation enough to reduce their survival during migration and overwintering.
Molting Sequence
In late summer, common loons begin their molt into winter plumage, which proceeds gradually over a period of several weeks. Studies of banded loons and collected specimens detail the typical sequence of the molt:
Stage 1
The molt starts with replacement of the tertial flight feathers and innermost secondaries on the wings. At the same time, the bird sheds feathers on the throat and chest. Some black feathers remain on the head and neck at this early stage.
Stage 2
Next to molt are the wing coverts and remainder of secondaries, along with feathers on the side and back of the neck. The head begins to show a salt-and-pepper mix of gray and black feathers.
Stage 3
In the final stage, the old primaries and outer tail feathers are replaced last. At the same time, the final black feathers on the cheeks and crown are replaced with gray. When the primary molt is complete, the loon’s winter plumage is full and complete.
This molting sequence likely maximizes flight capability throughout the process. The loon keeps its old active flight primaries and tail feathers longest before replacing them last.
Color Variations
Most common loons transition from bold black and white breeding plumage to muted gray and white winter plumage. However, subtle variations exist. Some loons retain dusky flecks on their back or throat all winter. A few white winter plumage loons have been documented with no gray tones. One theory proposes that loons which overwinter furthest north may benefit from whiter plumage that blends best with snow and ice. Banding data could shed more light on whether wintering grounds correlate to winter color variations.
Rarer color morphs like leucistic loons missing some pigment have been observed in wintering flocks. Reports of wintering loons exist showing piebald plumage mixing black and white feathers normally seen only in summer. These aberrant color patterns likely persist year-round and are not strictly related to winter molting.
Conclusion
The common loon’s striking seasonal transformations have long captivated bird enthusiasts. Its breeding finery vanishes in favor of camouflaged winter garb. Careful observations allow documentation of the precise molting sequence over two to three months as summer coloration fully transitions to the loon’s silvery gray and white winter outfit. Slight variations in winter plumage likely reflect both individual differences and adaptations to local conditions across the loon’s vast migratory range. Further study would deepen understanding of how subtle distinctions in winter plumage correlate to sex, age, and wintering geography of this widespread waterbird.