Cranes, belonging to the family Gruidae, are among the largest flying birds. Like all bird species, cranes reproduce by laying eggs, but they invest significant time and energy into ensuring the survival of their young. The reproductive cycle, from nest construction to raising the young, involves dedication from both parents. This process begins with selecting and building a suitable nesting site, followed by laying eggs and a long period of parental care.
Selecting and Building the Nest
Crane pairs establish their breeding territory and begin constructing a nest several weeks before the female lays eggs. They typically choose locations near water, such as marshes, wet meadows, or shallow wetlands, where the water offers protection from mammalian predators. The nest is a simple, mound-like platform built directly on the ground or in shallow water. Both the male and female participate in gathering materials, constructing the foundation from available plant matter, including reeds, cattails, grasses, and weeds.
Nest Structure
The final structure can be quite substantial, sometimes reaching four to five feet in diameter. The female arranges the materials on the mound to form a slight depression or cup for the eggs. Throughout the nesting period, the pair continues to add and rearrange materials, maintaining the integrity and height of the platform against rising water levels.
Crane Egg Characteristics and Clutch Size
The clutch size is small compared to many other bird species, reflecting the high level of parental investment. Most crane species typically lay a clutch of two eggs, though one or three eggs are sometimes observed. This small clutch size is common among long-lived birds. The female usually lays the eggs about 48 hours apart.
Egg Appearance
The eggs are relatively large, supporting the development of a precocial chick. For example, Common Crane eggs measure around 95 to 97 millimeters in length and weigh up to 190 grams. Crane eggs feature coloration that provides camouflage against the earthy and vegetative backdrop of the nest. They are generally oval-shaped with a smooth, non-glossy surface. The color varies by species, often ranging from olive-brown to pale brownish buff, typically covered in dark spots, blotches, or speckles.
Incubation Period and Parental Roles
Once the first egg is laid, incubation begins immediately, lasting approximately 28 to 32 days. Since incubation starts with the first egg, the chicks often hatch a day or two apart. Both the male and female crane share the duties of sitting on the nest.
Incubation Duties
The parents possess specialized bare patches of skin on their lower abdomen, called brood patches, which facilitate the efficient transfer of body heat to the eggs. The pair rotates turns frequently, sometimes exchanging every two hours during daylight. The female often takes the majority of the incubation duty at night, while the male usually remains nearby to guard the territory. The non-incubating parent forages for food and stands watch, protecting the nest from predators. The adult on the nest turns the eggs several times daily to ensure even heating and proper embryonic development until the chick, called a colt, is ready to emerge.
Raising the Colt to Independence
Crane colts are precocial, meaning they hatch with their eyes open, covered in downy feathers, and can leave the nest within a day. This ability allows the family to move away from the vulnerable nest site almost immediately. Initially, the colts stay close to the parents, often seeking shelter under their wings for warmth and protection, as they cannot yet fully regulate their body temperature.
Development and Dispersal
The parents feed the young colts for the first few weeks, presenting food items directly to the chicks, though the colts begin foraging for themselves soon after hatching. Their diet consists of various small items found in the marsh and surrounding fields, such as insects, snails, tubers, and seeds. If two colts hatch, the parents may separate, with each adult taking primary responsibility for one chick to reduce sibling competition. The young crane grows rapidly, fledging (taking its first sustained flight) typically between 60 and 70 days after hatching. Even after fledging, the colt remains dependent on its parents for an extended period, learning foraging techniques, migration routes, and how to avoid danger before finally dispersing to join a flock of non-breeding cranes, often until the following spring migration.