The great blue heron is the largest heron species in North America. It is a large, slate-grey bird with white and black accents on the head and neck. Male and female blue herons look identical from a distance and are generally indistinguishable unless seen in a breeding pair. However, when viewed up close or examined in pairs, there are some significant and interesting gender differences.
The most obvious difference between the male and female great blue heron is their size. Male herons are visibly larger than their female counterparts, typically weighing between 2.72 and 3.63kg. A female typically weights between 4.5 and 6lbs. The male heron's bill is also typically longer than the female's. This difference is typically most visible when a mating pair is seen sitting side by side.
The males and females play very different and distinctive roles in the heron mating rituals. Herons form mating colonies between May and June. Within the colony, herons break off in breeding pairs that will remain together and monogamous during the breeding season and while sharing the responsibility of rearing young. Males in the colony perform for the females, flying large 360-degree circles over the nesting ground, calling loudly, and fighting with other males who challenge their courtship for the lady of their choice. Females tend to remain in a single location during the mating ritual, calling the males to her with her song and waiting for the right partner to come to her.
The male goes in search of and selects a nesting site before the female's arrival at the nesting location. After selecting a site the male begins to gather the resources needed to build a nest. He presents the twigs to the female with great pomp, laying them at her feet and shrieking. The female then takes responsibility for the creation of the nest. She takes the gifts the male brings her and either builds a new nest or uses the supplies to repair an old one. The female then lines the nest with leaves and plants to create a padding for her to lay the eggs in. She lays three to six eggs in the finished nest.
The great blue heron does not reach full physical maturity until 3 years of age. Often male and female herons can be sexed at different points in their physical development according to their stage of plumage development. Fledgling herons who have just left the nest still have slate-grey down, or soft hairlike feathers, along their plume. In the spring, as yearlings, the male heron will develop a pendant crest and white feathering on the head before their female counterparts. The third spring, when the heron is mature enough to breed their legs will turn a bright orange and the skin around their beak will turn a bright blue colour. Males' legs are typically slightly darker than females, though from an observational distance, this distinction may not be visible.