Gulls exhibit sexual monomorphism. This means that male and female gulls possess nearly identical plumage, coloring, and feather patterns, making them visually indistinguishable to the casual observer. The sexes of gulls share the same white, gray, and black coloration throughout their lives. Consequently, identifying a gull’s sex requires careful observation of minute physical differences or, more reliably, specific actions performed primarily during the breeding season.
Subtle Physical Distinctions
While both sexes appear similar, slight differences in size and structure exist. In most gull species, the male tends to be marginally larger and heavier than the female. For example, in a mated pair standing side-by-side, the male will typically present a slightly greater body mass and a longer overall body length.
The head and bill structure can also offer subtle clues. Males often feature a bill that is slightly thicker and more robust. This heavier bill, combined with a tendency toward a squarer head shape, can give the male a more dominant appearance. Conversely, females may exhibit a somewhat more refined bill and a rounder head profile. These differences, however, are highly unreliable for sexing a single, isolated bird, as the size range of an individual female can easily overlap with that of a smaller male.
Indicators Based on Behavior
Behavioral observation, particularly during the courtship and nesting phases, offers the most definitive evidence for distinguishing sex in the field. One of the most reliable field signs occurs during copulation, where the male is consistently seen mounting the female. This act provides a momentary, unambiguous confirmation of each bird’s sex within the pair.
Another strong indicator is the ritual of courtship feeding, a behavior crucial for strengthening the pair bond and providing the female with the nutritional resources needed for egg production. In this display, the male will regurgitate food directly to the female, who often solicits the meal with begging calls and postures similar to those of a chick. The bird providing the meal is the male, and the recipient is the female.
Furthermore, distinctions in parental roles and territorial defense are often observable. Males typically engage in more frequent and intense agonistic behaviors, such as aggressive posturing and chasing intruders away from the nesting territory. They also tend to give more of the loud, territorial vocalizations, such as the “long-call” and “yelp.” While both parents share incubation and chick-rearing duties, the male often takes on the bulk of the intense territorial protection and provides the initial significant food loads.
The Importance of Observation Context
Successful sex identification in gulls relies heavily on context; single-bird observations are prone to error. The subtle physical differences become meaningful only when two birds are seen interacting as a bonded pair, allowing for a direct comparison of relative size and robustness. Without this side-by-side contrast, it is nearly impossible to make an accurate determination based on morphology alone.
The degree of sexual size dimorphism can vary significantly between the numerous gull species. For the strongest behavioral cues, observation should be concentrated during the spring breeding season, as courtship feeding and mating displays are largely absent at other times of the year.