The Michigan Natural Features Inventory entry for Great Blue Heron Rookeries explains:
The great blue herons in Michigan are largely migratory, with almost all leaving the state during the winter months. Most leave by end of October and return in early to mid-March.
The great blue heron is mostly a colonial nester, occasionally they nest in single pairs. Colonies are typically found in lowland swamps, islands, upland hardwoods and forests adjacent to lakes, ponds and rivers. Nests are usually in trees and may be as high as 98 ft. (30 m) or more from the ground. The platform like nests are constructed out of medium-sized sticks and materials may be added throughout the nesting cycle. Nests are usually lined with finer twigs, leaves, grass, pine needles, moss, reeds, or dry gras. The same nests are refurbished and used year after year. Nest size varies; newer nests may be 1.5 ft. (0.5 m) in diameter with older nests reaching up to 4 ft (1.2 m) in diameter (Andrle 1988). Nests can also be used by Canada geese (Branta canadensis), house sparrows (Passer domesticus), and great-horned owls (Bubo virginianus)…
Most great blue herons return to southern Michigan heronries in mid-March although a few may remain through the winter if there are areas of open water. Courtship and nest building commences from early April in southern Michigan to early May in the extreme northern portions of the state. Both sexes are involved in the nest building process with males primarily gathering sticks from the ground, nearby trees, or ungarded nearby nests. Males pass sticks to females who then place them on the nests. Between 3 and 7 (usually 4) greenish blue eggs are laid in April and May in Michigan. Both sexes take a turn at incubation with females incubating mostly at night and males during the day. The incubation period lasts from 25-29 days. In Michigan hatching occurs in the first week of May in the south while parents are still incubating nests in the far northern part of the state. For the first 3-4 weeks post hatching, one parent remains on the nest with the young.