Eastern Bluebirds often use human-provided nest boxes during their breeding season. Raising young is a swift cycle that culminates when the young birds leave the nest, an event known as fledging. Fledging is the act of a young bird developing the feathers and strength needed for its first flight out of the nest cavity. The time it takes for a bluebird to reach this milestone requires rapid development within the safety of the nest.
The Nestling Stage Timeline
The period from hatching until the young bluebirds exit their nest is remarkably brief, reflecting an intense period of growth. For Eastern Bluebirds, the nestling stage typically lasts between 16 and 22 days after the eggs hatch. A large study of these birds found the average nest departure age to be around 19 days, though many fledge closer to the 18-day mark.
The young must develop quickly to survive outside the nest environment. The exact length of the nestling period is influenced by external conditions. Broods raised earlier in the spring, when insect food is less abundant, may take longer to fledge than those raised during peak summer. Weather and the availability of protein-rich insects, which parents constantly deliver, directly affect the speed of the nestlings’ growth.
Key Developmental Changes During the Nestling Stage
The first physical changes are observable just a couple of days after the nestlings hatch, when they are initially blind and nearly naked. Dark pin feathers, the sheaths that contain the growing flight feathers, begin to appear along the wings and spine by day two or three. Around day five or six, the nestlings’ eyes open, allowing them to become aware of the parents entering the box with food.
By day seven to eight, the young birds can regulate their own body temperature. This means the female parent no longer needs to continuously sit on them for warmth, allowing both parents to focus entirely on foraging. The young gain weight rapidly during the first 12 days, sometimes reaching a mass greater than their parents before their growth rate slows.
As the feathers continue to emerge from their sheaths, the nestlings become fully feathered by day 15 or 16, though their tails remain short. This stage is marked by pre-fledging behaviors, such as restlessness and wing-flapping exercises, which strengthen their flight muscles. The young birds may begin to peek out of the entrance hole, signaling their readiness to leave the cavity and take their first, somewhat clumsy, flight.
Life Immediately After Fledging
The moment a bluebird leaves the nest box is a major event, but it is not the beginning of independence. Fledging marks the transition from being confined to the nest cavity to being mobile in the outside world. The young birds’ first flights are often short and direct, taking them to a nearby tree, shrub, or other cover where they remain hidden.
During the first week, these fledglings are still completely dependent on their parents for food and safety. They remain clustered together in dense cover, making it easier for the adults to locate and feed them. Over the next two to three weeks, the parents continue to feed and guide the young birds while they gain strength and hone their skills.
The parents gradually reduce the amount of food they deliver as the fledglings begin to practice the sit-wait-and-drop strategy. They learn to catch their own insects and develop the strong, maneuverable flight necessary for adult life. Once the young can reliably feed themselves, typically by 30 to 40 days of age, they achieve nutritional independence and disperse from the family unit.