Parasitic jaegers usually breed every year, but about 3% of experienced breeders will not breed in a given year. Non-breeding is often due to death of a mate or divorce, though environmental features (such as lack of food) can cause the birds to defer breeding until conditions improve. Parasitic jaegers are monogamous, and no polygamy have been recorded. They reach maturity in their fourth or fifth year, at which time they travel to breeding areas and establish territories. Juveniles often do not return to breeding colonies but, if they do, they form clubs with other juveniles.
Parasitic jaegers breed in the Arctic tundra between May and September, after migrating there from the Southern Hemisphere. They demonstrate site fidelity, with pairs returning year after year to the same territory. Parasitic jaegers defend their territory with vocalizations, chases, and distractions. Occasionally, three birds will defend a territory, the third bird is usually a resident on the territory rather than a visitor.
When both dark and light color morphs are available, parasitic jaegers seem to choose mates of the opposite color morph. Females of either color morph seem to prefer dark males. Dark males form pairs sooner in the year and breed earlier than pale males, possibly because pale birds are more aggressive. There is some evidence that pale morphs will eventually be eliminated by selective breeding in parasitic jaegers.
Mating System: monogamous
Parasitic jaegers exhibit reversed sexual dimorphism, in which females are larger than males. Larger females produce larger eggs and they have more body heat and surface area for incubating eggs. Females lose weight after their chicks hatch, so larger females are better adapted to stresses encountered post-hatching. Larger females can also defend better defend nests from predators and compete better for mates than smaller females. Smaller males, on the other hand, are more efficient at foraging. Kleptoparasitism requires agile flight, which is easier for birds with shorter wings and less momentum. Long distance foraging is more efficient for smaller birds. Smaller, more agile males are also better at territorial displays and courtship rituals.
When parasitic jaegers reach one year old, they begin to visit breeding colonies looking for potential territories. Once a pair has established their territory, usually within a colony, they will continue to use that spot for the remainder of their breeding lives, unless a divorce or death occurs. Breeding pairs arrive at their territories in late May or June and lay their eggs before the end of July. Pairs that are particularly dimorphic (females are much larger than males) lay earlier in the year than pairs that are more closely matched in size. Young birds (5 to 8 years old) lay their eggs the latest. Eggs are 58 mm long and 40 mm wide and weigh about 48 grams, 6% of which is the shell's weight. Two eggs are laid per nest, and only one brood is attempted per pair per year. Both parents incubate the eggs, which hatch in 25 to 28 days. Hatching success is from 70 to 80%. Hatchlings are altricial and covered with downy feathers. About 85 to 95% survive to one week of age. Hatchlings fledge in 25 to 30 days. The average number of chicks raised by a pair of birds in a particular colony ranges from 0.13 to 0.97. The average number of chicks per pair is dependent on several factors, including food availability and how many pairs attempt to breed.
Breeding interval: Parasitic jaegers breed once a year.
Breeding season: Parasitic jaegers breed from May to September.
Range eggs per season: 1 to 2.
Range time to hatching: 25 to 28 days.
Average birth mass: 48 g.
Range fledging age: 25 to 30 days.
Range age at sexual or reproductive maturity (female): 4 to 5 years.
Range age at sexual or reproductive maturity (male): 4 to 5 years.
Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate)
Parasitic jaegers only attempt to raise a brood if environmental conditions are good enough to support both parents and offspring. If conditions are poor, parasitic jaegers delay breeding until the next year. Parasitic jaegers have long lifespans and high adult survival rates, so their likelihood of surviving to attempt breeding the next year is high.
Parasitic jaegers invest heavily in raising offspring. Males forage more than females during courtship and incubation. Females do most of the incubation, though males do help with incubation. After hatching, females begin to help with foraging to provision hatchlings. Near the end of the breeding season both males and females forage for their young equally. Parasitic jaegers defend territories and young vigorously; hatchlings are almost constantly watched by at least one parent. Males defend nests more than females, but females grow more aggressive after the eggs are laid. Sometimes an extra-pair jaeger will assist with defense. This third jaeger can be a resident on the territory or a visitor.
Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: 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)