This is a fascinating question, as greylag geese and Canada geese are closely related species within the same genus (Branta). However, there are some key differences between them that may prevent successful mating in most cases.
Taxonomy and Physical Differences
Greylag geese (Anser anser) and Canada geese (Branta canadensis) both belong to the family Anatidae and genus Branta. However, they are separate species. Some key physical differences between them include:
- Size – Greylags are smaller, weighing 7-9 lbs on average, compared to 10-20 lbs for Canada geese.
- Plumage – Greylags have more gray and brown plumage compared to Canada geese which are darker black and white.
- Bill – Greylags have a shorter, stubbier bill that is pink on the bottom. Canada geese have longer black bills.
- Voice – Greylags have a soft, musical call compared to the louder honking of Canada geese.
These differences, especially in size, plumage color, and courtship behaviors, likely help the geese identify their own species and choose the correct mates.
Breeding and Nesting Habits
In terms of breeding, greylag geese and Canada geese have some overlap but also distinct differences:
- Greylags breed earlier, starting as early as February. Canada geese breed later, typically April to May.
- Greylags nest on the ground, often on islands. Canada geese nest on the ground or in elevated sites like tree cavities.
- Clutch sizes differ – greylags lay 3-5 eggs while Canada geese lay 4-8.
- Greylags have a shorter incubation period of 27-28 days compared to 28-30 days for Canada geese.
- Greylag goslings are lighter in color than Canada goslings.
These variations in timing, nest sites, egg traits, and gosling appearance help ensure they pair up with their own species.
Distribution and Geographic Overlap
In addition, the native ranges of greylag geese and Canada geese have only partial overlap:
- Native greylags are found across parts of Europe and Asia.
- Canada geese are native to North America.
- There are some introduced populations of greylags in North America, mostly in parks and private collections.
- Introduced Canada geese populations have become established in some parts of Europe.
So while their ranges now overlap in some areas, they evolved in distinct regions and did not intermix historically when their mating cues would have developed.
Behavioral Differences
Greylag geese and Canada geese show some behavioral variations that likely contribute to mating selectivity:
- Greylags are more vegetarian, feeding on grasses, herbs, and agricultural crops. Canada geese are more omnivorous, eating aquatic vegetation, fish, insects, and human garbage.
- Greylags migrate shorter distances between distinct wintering and breeding grounds. Some Canada goose populations no longer migrate.
- Canada geese are more adaptable to human-altered habitats like golf courses and city parks. Greylags prefer more natural wetland habitats.
- Greylags have more complex family structures, often staying with both parents for longer. Canada geese families break up sooner.
These differences in diet, migration, habitat use, and family structure place the two species in distinct social settings for finding and choosing mates.
Compatibility of Hybrids
There are occasional reports of greylag-Canada goose hybrids where their ranges now overlap. However, multiple factors likely limit the fertility and viability of hybrid offspring:
- The two species have different chromosome numbers – greylags have 80 chromosomes while Canada geese have 68-86. This makes successful meiosis and production of fertile eggs less likely.
- Size differences may create developmental problems in hybrid embryos.
- Imprinting on parent species would influence hybrid mating choices later, limiting chances to mate.
- Hybrids would likely be poorly adapted to the niche of either parental species.
For these reasons, greylag-Canada goose hybrids likely occur only rarely and have lower fitness. This maintains reproductive isolation between the species.
Conclusion
In summary, while greylag geese and Canada geese are closely related and have some potential for hybridization where introduced, several key factors limit interbreeding between the species:
- Physical differences in size, plumage, bill shape, and voice that distinguish species.
- Behavioral isolation mechanisms including differences in breeding season, nest sites, clutch sizes, and gosling coloration.
- Geographic and habitat isolation between their native ranges.
- Ecological differences in diet, migration, social structure, and habitat preferences.
- Low fertility and poor viability of hybrid offspring due to chromosomal and developmental incompatibilities.
These isolating mechanisms maintain the genetic integrity of the two species and account for why mixed greylag-Canada goose pairs are uncommon in the wild. Overall, the evidence suggests greylags and Canada geese generally mate true to their own species even where their ranges now overlap.