Information on the mating system of these animals is not available.
After returning to its colonies between late February and late March, L. ridibundus lays its eggs in late April and May. Females lay from one to three eggs and incubate them for 22 to 26 days. Larus ridibundus is strongly inclined to nest near vegetation. At the very beginning of the breeding season, male L. ridibundus congregate in larges areas close to the nesting colony before females arrive. In these areas, termed "clubs," each bird is semi-territorial in that it does not stay in one particular area for a considerable amount of time. A male first regards visiting females as a threat, showing his aggressive oblique display and making long calls. In response, the female extends her neck upward and faces away, a display that makes her sex and potential as a mate known. The male then mitigates his response. The mating process continues when females keep coming back to a selected male, staying with him for progressively longer periods of time. The potential pair exchanges mutual displays. Their courtship culminates in the male regurgitating food for the female, an act followed by copulation.
As soon the pair of L. ridibundus settle into their colony area, they become territorial and defend their area against intruders (primarily conspecifics). They clearly mark their pairing territories, which range in size from 9 to 11 square meters. Their territorial boundaries are subject to slight changes as dictated by daily disputes with neighbors or intruders.
Colonies of L. ridibundus are made up of 11 to 100 breeding pairs; however, a few colonies reach numbers greater than 10,000 birds. These colonies are loosely divided: a breeding bird will eventually become accustomed to its neighbors, but will become very aggressive towards birds that it cannot recognize.
Research has shown that L. ridibundus is a philopatric species in which individuals are inclined to return to breed in the subcolony in which they were born. This tendency is especially evident within a large colony located in a relatively unstable habitat. Philopatry encourages the establishment of kin groups and eventually heightened cooperation among neighbors (Howard and Moore, 1991; Moynihan, 1955; Prevot-Julliard et al., 1998; Gill, 1995).
Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization
Average time to hatching: 22 days.
Average eggs per season: 3.