The difference between male and female seagulls is not very noticeable to the human eye. The only main difference is that male seagulls tend to have brighter plumage than their female counterparts. However, humans are often unable to pick up on this difference because it is so subtle.
Although the difference between male and female seagulls is not very noticeable to humans, seagulls can tell the difference right way. Seagulls see more colors than a human can because the human eye has three cones that detect red, green and blue. By contrast, the seagull eye has four cones. The fourth cone allows seagulls to see infrared color, which humans cannot see. Because of this difference, the male seagulls appear much brighter, and the female seagulls seem to be dull.
The birds that tend to have the most variation in color are species wherein only the female takes care of the young. In those cases, the brighter plumage helps attract predators to the male birds, which helps to keep the predators away from the female and her young. However, seagulls are very attentive parents. Male and female seagulls pair for life, and they both take turns incubating the eggs and feeding the chicks after they are hatched.