The snow goose (Anser caerulescens) is a species of goose that exhibits strikingly different plumage color phases. The two main color phases are white and blue morphs. There are also intermediate morphs that show a mix of white and gray feathers. The different color morphs of the snow goose have long fascinated bird watchers and scientists alike.
White Morph
The white morph snow goose has pure white plumage with black wingtips. The head can have small dark spots or streaks. The bill is pink with a dark “grinning patch.” The legs and feet are also pink.
White morph snow geese make up approximately 95-98% of the population. The widespread white color provides excellent camouflage against snow and ice in the arctic breeding grounds. White feathers likely evolved as an adaptation to hide from predators in the tundra environment.
Juvenile Phase
Juvenile white morph geese have a gray body and white head during their first autumn and winter. Their plumage gradually transitions to pure white as they molt into adulthood.
Blue Morph
The blue morph snow goose has gray-brown plumage replacing the white. They have white head markings like the white morph but their bill is darker gray. Blue morph feet and legs remain pink.
Blue morphs comprise around 1-5% of the population. Their darker color is thought to be controlled by a single gene. It provides camouflage against darker substrates like mud and vegetation.
Juvenile Phase
Juvenile blue morph geese have pale gray plumage with darker wing feathers. The head is white like adults. Their gray juvenile plumage helps blend in during their first migration and winter.
Intermediate or Heterozygous Morph
Intermediate snow geese show a mix of white and gray plumage. They frequently have a “salt and pepper” appearance with interspersed white and dark feathers.
The intermediate morph results from a heterozygous genotype. This means the goose carries one gene for white color and one for blue. The mixed white-gray appearance is the intermediate phenotypic result.
Juvenile Phase
Young intermediate geese tend to appear mostly gray-brown with some white feathering. Their appearance can vary from very dark to almost completely white.
Morph Ratios
The ratio of white to blue snow geese can vary across different nesting colonies. However, white birds consistently make up the vast majority of the population.
| Location | White Morph | Blue Morph |
|---|---|---|
| Bylot Island, Nunavut | 95% | 5% |
| Southampton Island, Nunavut | 87% | 13% |
| Queen Maud Gulf | 98% | 2% |
| Western Canadian Arctic | 97% | 3% |
| Hudson Bay Lowlands | 99% | 1% |
These surveys indicate the blue morph remains relatively rare across the snow goose’s arctic breeding range. However, blue morph frequency can be higher in some areas like Southampton Island. Even there, the white phase still predominates by a wide margin.
Origin and Genetics
The genetic basis of the blue and white morphs has been studied since the 1960s. Early research using captive breeding and hybridization showed a single gene controls the morph phenotypes. The blue allele is incompletely dominant over the white allele.
The blue morph likely originated from a very old ancestral mutation. This mutation originally occurred thousands of years ago in an isolated population. Birds carrying the novel blue allele were geographically separated from other populations.
Over time, interbreeding of blue phase geese back into main populations introduced the recessive blue allele more widely. Blue morphs remained relatively rare but were sustained at low frequencies by continued gene flow between isolated groups.
Blue Allele Advantages
The persistence of the blue morph in wild populations suggests there must be some advantage to the blue gene. Possible benefits include:
- Enhanced camouflage against certain dark Arctic backgrounds
- Increased visual communication among blue phase geese
- Positive sexual selection for rare blue morph mates
These potential benefits may help explain why the blue allele has been maintained instead of disappearing entirely.
Differences Between Morphs
Most research indicates there are very few substantial differences between white and blue snow geese besides feather coloration. However, some studies suggest possible differences in behavior and biology:
Nesting Behavior
One study in Nunavut found blue phase breeding pairs initiated nests around 2 days earlier on average than white geese. Earlier breeding may confer advantages like earlier gosling hatching.
Migration Timing
Observations during migration indicate white geese tend to precede blue geese by 1-2 days on average. Reasons for this slight delay in blue geese remain unclear.
Dominance and Aggression
Observations of captive flocks suggest blue morph geese may be more dominant and aggressive. The blue color confers an intimidating appearance that influences behavior.
Mate Choice
Blue geese frequently mate with other blues when possible. However, mixed blue-white pairings are common in areas where blue morphs are rare. No evidence indicates mating preference affects reproductive success.
Morph Segregation
Some studies propose blue geese may segregate themselves into discrete groups while migrating and wintering. This voluntary segregation could enhance group identification and communication. However, mixed flocks are also very frequently observed.
Conservation and Population Trends
Snow goose populations have grown rapidly over the past 50 years. Liberalized harvest regulations have aimed to control overabundant goose flocks damaging Arctic habitats. Despite control efforts, snow geese continue to thrive across North America.
The blue morph remains a relatively constant minority across the snow goose’s range. There are no indications the blue allele frequency is substantially increasing or decreasing in wild populations.
More research is needed to understand if shifting environmental conditions in the Arctic could selectively favor one color morph over the other. For now, the rare blue phase persists as an unusual variant of the common white snow goose.
Conclusion
The snow goose provides a fascinating case study of plumage polymorphism. The white morph enjoys near-universal predominance, while the blue morph is sustained as a rare but stable minority. The genetic basis of the color phases is well-understood, yet questions remain about possible behavioral and ecological differences between the morphs.
Continued field studies and observations will help elucidate whether natural or sexual selection may favor one morph over the other. Tracking color ratios across different breeding colonies can also reveal whether the blue allele frequency is rising or declining. For now, the unlikely persistence of the blue goose remains evolutionarily enlightening and visually striking to observe in the wild.
