Emperor geese exhibit a monogamous mating system, and will bond with a different goose only if the previous mate dies. Little is known about the formation of pair bonds but they are formed by the time breeding grounds are reached. Because copulation is rarely seen, it is assumed that mating occurs either just prior to or during migration to breeding grounds.
Mating System: monogamous
Upon arriving at the nesting grounds, females begin building nests. Eggs are normally laid within the first 10 days after arrival. Clutch size ranges from 3 to 8 eggs with an average of 5. Females incubate the eggs for 24 or 25 days, after which the goslings hatch over a 10 day period. Goslings hatched earlier in the breeding season have a much higher likelihood of surviving the following winter. Most goslings are fully feathered 30 to 47 days after hatching and are able to fly at 50 to 60 days old. Goslings remain close to their parents through the first winter and have even been noted to return to breeding ground with parents the following spring. Though young geese may return to breeding grounds early in life, most females do not lay eggs of their own until 3 to 4 years of age.
Emperor geese nests are commonly parasitized by other birds. A study conducted in the Yukon Delta breeding ground found that an average of 62% of nests had parasitic eggs. The majority of these eggs had been laid not by other species, but by other emperor geese. Females and males actively guard their nests but if a parasitic egg is successfully laid in or very close to a nest, the pair is highly likely to accept the egg.
Breeding interval: Emperor geese breed once yearly.
Breeding season: The breeding season occurs from late May to August.
Range eggs per season: 3 to 8.
Average eggs per season: 5.
Average time to hatching: 24 to 25 days.
Range birth mass: 74 to 81 g.
Average birth mass: 87 g.
Range fledging age: 50 to 60 days.
Average time to independence: 50 to 60 days.
Range age at sexual or reproductive maturity (female): 3 to 4 years.
Range age at sexual or reproductive maturity (male): 3 to 4 years.
Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate)
Females are solely responsible for selecting a nest site, building a nest, and incubating the eggs. The same nest site may be used multiple years. Pairs chase rival goose species, members of their own species, as well as juveniles from previous seasons, away from their nesting sites. While both males and females defend the nesting area, males are much more aggressive than females. While the female is incubating, males remain close to the nest to fend off predators and other geese. By the end of the incubation period, females lose an average of 20.7% of their body mass due to energy being focused solely on incubating and protecting the eggs.
Once goslings hatch, they are brooded by females for the first 2 to 3 weeks. Goslings are able to forage within 24 hours of hatching. While the young feed, they stand between their parents who are both highly protective. If terrestrial predators approach and a body of water is nearby, the entire group flees to water. But if water is not close, parents surround their offspring while hissing and flapping their wings at the approaching predator. When an aerial predator strikes, juveniles hide beneath their mother’s wings as the father jumps at and attacks the predator. Males are even known to act as decoys, allowing the goslings and their mother to escape. Juveniles leave breeding grounds around the end of August but remain with their parents until the following spring, when they are chased away from the breeding grounds.
Parental Investment: precocial ; male parental care ; female parental care ; pre-fertilization (Provisioning, Protecting: Male, Female); pre-hatching/birth (Provisioning: Female, Protecting: Male, Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Male, Female); pre-independence (Provisioning: Male, Female, Protecting: Male, Female); post-independence association with parents