Caspian terns are monogamous, with pairs staying together at least for the breeding season. Some pairs remain together for many years, but only 25% of pairs mate again the next year in some populations. Pairs are formed soon after birds arrive on the breeding grounds, although some pairs form during migration or on the wintering range. Males attract mates with a "fish flight." Males capture a fish and then fly with it over a group of terns. Females and males join in the display, flying with the male as he repeatedly passes over the group. He then lands near a female and makes bowing movements with his head. Females may ignore the male, try to steal the fish, or beg for the fish. This display is usually repeated several times before the male will feed the fish to the female, at which point the bond is formed and they copulate. Pairs perform a "high flight" display together to cement the bond, ascending and diving together as they vocalize. Symbolic construction of nest scrapes is also part of the courtship ritual.
Mating System: monogamous
Caspian terns arrive on the breeding grounds from late March to late May. Pairs begin to form nest scrapes soon after they arrive on the breeding grounds. They breed in late May and early June, laying from 1 to 3 buffy, splotched eggs. Eggs are laid every 2 to 3 days in a simple scrape nest and incubation begins immediately with the first egg. Caspian terns have 1 brood yearly. Incubation is from 25 to 28 days and fledging occurs 37 days after hatching. Most individuals don't breed until they are 3 years old, although some attempt breeding in their 2nd year.
Breeding interval: Caspian terns breed once yearly.
Breeding season: Caspian terns breed in late May or June.
Range eggs per season: 1 to 3.
Range time to hatching: 25 to 28 days.
Average fledging age: 37 days.
Range age at sexual or reproductive maturity (female): 2 (low) years.
Average age at sexual or reproductive maturity (female): 3 years.
Range age at sexual or reproductive maturity (male): 2 (low) years.
Average age at sexual or reproductive maturity (male): 3 years.
Both parents incubate the eggs and alternate frequently throughout the day. A parent will relieve the other by bringing and offering a fish. Then the incubating parent takes the fish and moves off the nest. Females tend to spend more time caring for eggs and young. Parents protect their young from heat by standing above them to provide shade. Young hatch in the order they were laid and are semiprecocial when hatched, with downy feathers and reliant on the parents for feeding. They remain in or near the nest for about a week after hatching. They are fed fish by parents soon after hatching and they begin to accompany parents on foraging trips within a week or so of fledging. Caspian terns have the longest period of dependency of any tern species.
Parental Investment: precocial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Male, Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Male, Female)