Bluebird Bulletin 4: Nestlings

Eastern bluebirds leave the hatchling days behind around day 8. They enter the nestling stage, a period named for their close-knit huddle within the nest. This is an astonishing chapter of their lives that lasts until they take their momentous first flight. A whirlwind of intense growth occurs within the cozy confines of their nest. It’s a time of physical development. They transform from vulnerable hatchlings into sleek, agile young birds. They get ready to face the world. Join us as we explore their remarkable journey. We’ll witness their rapid physical changes. There’s also the intricate development of their iconic blue plumage to observe. All this leads up to their exciting debut into the sky.

To truly appreciate the incredible speed of a bluebird’s early life, observe the stark differences in their physical development between day 8 and day 15, as highlighted by NASB in the following growth chart.

The Nestling Nursery: A Whirlwind of Growth

Roughly one week after hatching, eastern bluebird nestlings transition into a bustling stage. Life continues to revolve around essential tasks: eating, sleeping, pooping, and growing. Their early days are a whirlwind of activity, fueled by the relentless dedication of their parents. Their tiny chirps evolve into insistent calls for attention, especially when a meal is imminent, signaling their ever-growing appetites. Both parents become tireless providers, orchestrating a veritable food delivery service. The early morning hours often witness a flurry of activity, a “bluebird rush hour” as parents shuttle back and forth with provisions. While the tempo may slow a bit in the afternoon, feeding continues, a relentless pursuit to fuel the remarkable transformation occurring within the nest. This stage is truly a masterclass in development, as their bodies become miniature factories, preparing them for their eventual launch into the world. As the nestlings grow, they become more mobile, transitioning from a sleepy heap to active, standing figures, often performing a “half-stand” of anticipation as their parents arrive with another delectable morsel.

Here’s a peek into the nursery!

Warming Up to Independence: A Nestling’s Thermal Control

A crucial physiological leap for eastern bluebird nestlings is the development of thermoregulation. Initially, these tiny hatchlings are completely reliant on their mother’s body heat to maintain a stable temperature, much like fragile embers needing constant warmth. During the nestling stage, their internal thermostats begin to kick in. This crucial ability to maintain their own body temperature frees the female parent from constant brooding. While she may still snuggle with her brood overnight, her days become less tethered to the nest, allowing her to dedicate more time to foraging. This newfound independence is a vital step in their journey towards self-sufficiency, preparing them for a life outside the protected nest.

Physical Milestones: The Countdown to Takeoff

The physical changes during the nestling stage are truly astounding. Around day 8, a significant milestone occurs as their eyes, once sealed, begin to open, revealing a world of new sights. At this tender age, they’re feather-light, weighing around .8 ounces —about the same as an AA battery—a testament to their delicate beginnings. Weight gain stabilizes at about an ounce. From then on, the focus shifts dramatically to perfecting their plumage and refining their bodies for flight.

Feather Formation: Unfurling Beauty

During the nestling stage, their bodies become miniature feather factories as their feathers rapidly develop.  These tiny birds, initially sparsely covered in down, quickly sprout “pin feathers”—the beginning forms of their future plumage. You can think of these as miniature, tightly rolled-up scrolls, each one a promise of the vibrant blues to come.

These pin feathers are encased in a protective layer called a feather sheath. It’s made of keratin, the same tough material as our fingernails. This sheath acts like a secure casing, safeguarding the delicate growing feather and its vital blood supply within. As the feather matures, the sheath begins to break down, typically starting at the tip. It’s a gradual process, as if a scroll is slowly unfurling, revealing the fully formed feather beneath. This disintegration leaves behind a fine, white, powdery dust. The nestlings themselves contribute to this process through preening. Their tiny beaks to meticulously clean and further break down the sheaths. Once the sheaths are completely gone, the feather is fully developed and ready for action. This feather metamorphosis is a crucial marker of the nestling’s progress, signaling that they are nearing the exciting milestone of fledging—that momentous first flight into the open sky.

Fledging Prep: Inside the Nest Box

Around day 11, the nest box transforms into a miniature training ground. The nestlings begin to crawl, then stand, and even engage in vigorous stretching and wing-flapping exercises, like tiny avian athletes building strength for their grand debut. They also start preening, meticulously pulling at the sheaths of their emerging feathers, a vital grooming behavior. The once cozy nest becomes increasingly crowded, a bustling apartment filled with growing bodies. By day 12, they are almost entirely feathered, and by day 13, you might even catch them sleeping with their heads tucked into their new shoulders, mimicking adult behavior. The young males begin to show the tell-tale bright blue on their tail feathers, while the females display delicate white edging.

On the Cusp of Flight: Nestling’s Last Days

As they approach fledging, the parents’ feeding method shifts; by day 13 or 14, it’s less about entering the nest and more about quick “drive-by” deliveries, dipping only their heads into the now-squishy confines. Around days 16 or 17, a restless energy fills the nest. The nestlings, though still with rather short tails, begin to peek out of the cavity entrance, their curiosity and the crowded conditions a powerful magnet to the outside world. This is a critical window; if they fledge too early, they may land on the ground, vulnerable to predators, though their diligent parents will continue to feed them there. The optimal window for departure, when they are best equipped for sustained flight, is typically around days 19 and 20. Those who linger until day 22 or even longer are often the less developed, needing a bit more time to transform from fluffy balls into streamlined, nearly four-inch-long fledglings, perfectly poised for their soaring adventure.

Ready for Takeoff: The Threshold of Flight

As the final feather sheath crumbles and the last stretches are performed, the eastern bluebird nestling stands at the precipice of a new chapter: fledging. This period of intense development within the nest is a testament to nature’s efficiency, transforming a fragile hatchling into a capable young bird in a remarkably short span of time. From their first chirps to their final preening before departure, every moment in the nest is a critical step in preparing these vibrant creatures for their vibrant lives in the wild.

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References

What is a Pin Feather
NatureChest.com

NABS Fact Sheet: Eastern Bluebird Nesting Growth Chart
North American Bluebird Society

Growth Stages of Nestling Bluebirds
Avian Report