Aging Nestling Bluebirds Through Growth Stages

Aging of Nestling Bluebirds

Young bluebirds are hard to age. Food, weather, temperature, and the number of birds in a nest determine how large, heavy, and quickly nestlings grow and mature. Even so, specific characteristics can help you estimate the age of nestlings, regardless of these variations.

Things to consider when aging baby bluebirds 

Bluebirds hatch in different sizes 

This article is based on Benedict Pinkowski’s 5-year study of the breeding biology of eastern bluebirds in Michigan. Benedict Pinkowski observed, monitored, and measured as many as 184 young bluebirds during his studies. Other studies and personal experiences with bluebirds are also added in this article.

B. Pinkowsky found that larger eggs in the clutch resulted in larger hatchlings. He separated eggs in the study of wild nesting bluebirds into three categories: small-sized eggs, medium (normal-sized), and large eggs. Youngs hatched from small eggs had an average weight of 1.8 g (0.06 oz). Young hatched from medium-sized eggs 2.4 g (0.08 oz). Finally, those hatched from large eggs had an average weight of 2.8 g (0.09 oz). 

 Studies on nesting birds, other than bluebirds, found that larger eggs not only result in larger hatchlings but also larger hatchlings tend to develop faster.

Female eastern bluebirds lay small eggs at the beginning of their clutches, but egg size increases as she completes her clutch. Consequently, it is expected that siblings hatch with minor differences in size.

Baby bluebirds vary in their development

Bluebird nestlings differ considerably in their development, according to ornithologists who studied the subject. Even among nestlings that hatch on the same day, such variation in development is noticeable.

Variation in nestling development is expected in all birds.

Nestling bluebirds studied showed that some individuals were up to two days ahead of others in development despite having hatched the same day. A possible reason for this is that those nestlings were better at getting fed by their parents.

Nestling weight gain and growth

Nestling bluebirds do not grow steadily from the day they hatch to the day they leave the nest. Instead, as they grow, they go through two distinct stages. 

These stages may be hard to notice, but nestlings grow rapidly from the day they hatch until day 5, when they weigh an average of 14,1 g (0.5 oz).

After day 5, nestlings continue to grow but at a slower rate than during the first five days.  After reaching an average of 27.2 g (0.95 oz) in weight by day 12, growth levels off.

Bluebird grow stages

Stages of growth are divided into two-day categories, which I believe reflect differences in development. The use of two-day grow stages makes it easier to place a young bluebird in one of the 10 stages of growth below.

Terminology used to describe feather development

Pin feathers sometimes referred to as “blood feathers,” are feathers in the process of developing. Pin feathers appear for the first time during the early development of a nestling. Adult birds can also grow pin feathers to replace feathers that fall out during molting.

Feather sheaths protect the feather growing as a pin. A feather sheath maintains the pin feather’s cylindrical shape until the sheath breaks down, beginning near the tip, allowing the mature feather to emerge. Feather’s growth process is complete when the sheath disintegrates down to the base of the feather.

Days: 0 & 1

Appearance: Big head with large dark eyeballs; eyes are closed; blackish wispy down on head and spine; skin is orange/red; lay down in awkward positions, even belly up; able to raise the head and open mouth.

Weight: 0.1 oz
Body Length:
1.4 inches

Notes: Hatching generally happens in the morning hours and takes 24 to 50 hours for all eggs to hatch. Notice the presence of an egg indicating that these nestlings are between 24-50 hours old or less. Also, notice the wing without any dark spots (feathers) under the skin. Some eggs may take longer to hatch, but if an egg remains in the nest for more than three days after all other eggs have hatched, it can be assumed that it won’t hatch. This is normal. The growing nestlings are likely to break it with their weight, and the parents remove the shell.

Days: 2 & 3

Appearance: Pin feathers begin to appear under the skin as tiny dark dots along the wing, spine, shoulders (scapular) area, and belly; by the third day, the tiny dark dots become dark rows of pin feathers under the skin; nestlings may still lay down in awkward positions; skin tone more fleshy; eyes closed.

Weight: 0.2 oz
Body Length:
1.8 inches

Notes: By the second day, the tiny dots under the skin begin to look like dark rows of pin feathers. Those pin feathers quickly increase in numbers and become dark rows where pins under the skin are no longer discernable. The photo shown above corresponds to nestlings on the third day.

Days: 4 & 5

Appearance: Wings and spine turn darker; pin feathers on the wing begin to break out the skin; pin feather appear in the head; pin feathers begin to break out the skin on wings and spine; rows o pin feathers noticeable on belly and throat; may still lay in awkward positions; nestlings begin a rapid increment in size.

Weight: 0.4 oz
Body Length:
2.1 inches

Notes: The fourth and fifth days mark the beginning of rapid development. The nestlings begin to gain body weight and size at a faster rate than previous days. By the fifth day, some nestlings show more pin feather growth than others. The female continues brooding the nestling during the day and night as nestlings cannot thermoregulate their body temperature.

Days: 6 & 7 (First Week)

Appearance: Noticeable size increment; nestlings look dark as contour and head pin feathers rapidly emerge; chin and throat sparsely covered by pin feathers; lay down in normal position; eyes begin to open like slits.

Weight: 0.6 oz
Body Length:
2.3 inches

Notes: By the first week, the nestlings have grown in size faster than the feathers. Their legs and feet are well developed, and nestlings can make a half stand when the parents arrive to deliver food. Although the nestlings still have plenty of bare areas on their body, they can thermoregulate their body temperature. The female slows down brooding during the day but still broods the nestlings during the night.

Days: 8 & 9

Appearance: The sheath around the tips of the wing feathers begins to break down. Tail pin feathers are present but undeveloped.; eyes are more open; hatchling huddle together,  pin feathers around the base of the bill, and chin become denser. Able to keep their temperature without the parents’ help; show fear if handled.

Weight: 0.8 oz
Body Length:
2.5 inches

Notes: The rate at which nestlings develop varies among individuals. Those differences are perhaps more noticeable during this period, where some nestlings have mostly pin feathers while others have unsheathed feather tips.

Days: 10 & 11

Appearance: Contour, head, and wing feathers are open, and the nestling look like they have a feather cover; all feathers have a protective sheath at the base; belly, flanks, and areas under the wing still bare; legs and feet are well developed; nestlings can stand, stretch their wings and preen themselves.

Weight: 0.9 oz
Body Length:
3.1 inches

Notes: As nestlings acquire a layer of feathers, they are better able to thermoregulate their temperature. The exposed or visible parts of their bodies are covered in feathers while the belly, flanks, and areas under the wings have growing pin feathers, but these areas are not exposed. The female does little brooding anymore, even at night.

Days: 12 & 13

Appearance: Visibly feathered except for the areas around the base of the beak; tail is still short; pale eyering becomes noticeable; nestling orient themselves towards the nest entrance and stand up to receive food; feathers cover the ears. Body weight levels off. Sex can be assigned reliably.

Weight: 0.9 oz
Body Length:
3.4 inches

Notes: Nestlings are restless in the nest. They can preen themselves and stretch their wings. Day 12 and 13 marks the period when weight gain stabilizes. From this point on, most changes occur in the feathers. Blue secondary wing and tail feathers correspond to male bluebirds. Gray secondary and tail feathers, which show a white edging, correspond to female bluebirds.

Days: 14 & 15 (Second Week)

Appearance: Nestlings are completely feathered without visible bare areas; the bare areas under the wing and belly have feathers with a sheathed base; differences from day to day are more difficult to notice; tail feathers are noticeable yet remain short.

Weight: 0.9 oz
Body Length:
3.8 inches

Notes: This is the period when nestling bluebirds look feathered without bare areas.  Differences in feather color between male and female nestlings become more noticeable.

Days: 16 & 17

Appearance: Nestlings are completely feathered, at least the parts one can see without handling the bird; pale spots in the bird’s plumage increase in density; nestlings begin to peek out of the cavity entrance; some nestlings fledge as early as day 16.

Weight: 0.9 oz
Body Length:
4.3 inches

Notes: This is the period when nestlings become restless  inside the nest. Nestlings appear fully feathered but the tail is still rather short; the cavity is crowded with several large nestlings inside. Nestlings that fledge before day 16  often land on the ground and hide near the ground where parents continue feeding them until they are capable of sustained flight.

Days: 18 through 22

Appearance: Based only on appearance, it is difficult to tell the age of nestlings. From this point on, their plumage looks smoother and more complete; the sheathed feathers around the base of the beak are now open and look smooth; all tail feathers and some wing and contour feathers still have a protective sheath on the base.

Weight: 0.9 oz
Body Length:
4.3 inches

Notes: Although some nestling bluebirds fledge as early as day 16, it is days 19 and 20 when most nestlings leave the nest. Some less developed nestlings stay in the nest until day 22 or longer. Nestling leaving the nest as 19 and 20-days old are less likely to fall to the ground and be vulnerable to fall prey to cats, foxes, raccoons, and other ground predators.

References:

Breeding Adaptations in the Eastern Bluebird. Benedict C. Pinkowski, The Condor, Vol. 79, No. 3 (Autumn, 1977), pp. 289-302 (14 pages) Published By: Oxford University Press.

Davis, W. H., P. J. Kalisz and R. J. Wells. (1994b). Eastern bluebirds prefer boxes containing old nests. Journal of Field Ornithology 65:250-253.

Peakall, D. B. (1970). The eastern bluebird: its breeding season, clutch size, and nesting success. Living Bird 9:239-256.

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