How to tell if a young bluebird is female or male by its plumage

Young eastern bluebirds can be identified by sex on the basis of their plumage. Young bluebirds wear the juvenile plumage for only a few months before beginning to replace it for their first adult plumage.

This article points out the key plumage elements used to determine the sex of young bluebirds as well as the plumage changes young birds go through before acquiring their adult plumage.

The wing and tail feathers are key elements to determine the sex of young eastern bluebirds.

The following are the extended wings of male and female young bluebirds identifying the important feather groups. The terminology used on the extended wings here is used throughout this article.

The wings of a male and female juvenile eastern bluebird

Scientific Name: Calothorax lucifer

Wing of a Juvenile Male

Wing of a Juvenile Female

Eastern bluebirds have a total of 19 wing feathers and each wing feather has a corresponding wing covert. Wing feathers are the long visible feathers while the wing converts are located at the base of each wing feather. Wing feathers are also known as flight feathers.

Each wing has a set of 10 primary and a set of 9 secondary wing feathers with their respective set of wing coverts. The set of primary wing coverts are hidden when a bird folds its wings.

Eastern bluebirds have 12 tail feathers, which are also known as rectrices.

Wing feathers, wing coverts, and tail feathers are essential in determining the sex of young bluebirds. In general, these groups of feathers are often used to identify birds.

Age 13-16 days old: In the nest

Scientific Name: Calothorax lucifer

Nestling Male, view from Above

  • The tail feathers are blue when they begin to show.
  • The primary and secondary wing feathers also show blue but one would need to hold the nestling and expand the wing to notice the extend of the blue.
  • The wing feathers closest to the body (tertial feathers) are edged with rusty. The intensity or saturation of the rusty edging is variable among nestlings.

Nestling Female, view from Above

  • The tail feathers are gray and continue gray until the day young birds fledge.
  • The primary wing feathers are bluish while the secondaries mostly gray with hints of blue. However, this is difficult to notice in the nest, unless one holds the nestling and manually expands the wing.
  • The tertial feathers are edged with buffy-brown.

Notes: Nestling eastern bluebirds begin to show feathers by day 10-11 after hatching; feather colors are noticeable. The blue tail in males and gray in females become apparent. However, according to Pinkowsky (1974d) some individual variation can occur and it is not until day 13 that nestling eastern bluebirds can be assigned to a sex reliably.

Age 19 to 24 days old: Just fledged

Scientific Name: Calothorax lucifer

Fledgling Male

  • The rather short tail at the time of fledging is bright blue.
  • The primary and secondary wing feathers are blue. Colors are now easy to notice.
  • The wing coverts are blue with some gray.

Fledgling Female

  • The tail at the moment of fledging is still gray but begins to show blue at the base. As the tail grows longer, the blue base becomes more apparent.
  • The outer tail feathers show a noticeable pale edging.
  • The primary wing feathers are bluish but this may be difficult to notice when the bird is perched and has the wing folded.
  • The secondary wing feathers and coverts are mostly gray with a variable amount of blue.

Notes: The coloring of fledging young bluebirds is variable. Some young bluebirds males and females may show more saturated colors than others. Studies on bluebird molt have shown that the amount and quality of the food while molting has an effect on plumage coloration.

Age approximately 5 weeks old: Following their parents

Scientific Name: Calothorax lucifer

Dependent Fledgling Male

  • The blue wing feathers and wing coverts contrast with the gray-spotted back and streaked breast.
  • All wing ant tail feathers appear fully grown without pin or blood feathers at the base.

Dependent Fledgling Female

  • The wing color blends in with the back’s color. Females have some blue in the wing feathers and wing coverts but are difficult to notice when the bird has its the wings folded.
  • The tail has grown in length and now is easier to see how the base of the tail is bluer than the outer half. The extent of the gray in the tail is variable among individuals. Some females have gray only on the tip of the tail while others have a larger gray portion of the tail.

Age 6 weeks or older: Nutritionally Independent

Scientific Name: Calothorax lucifer

Independent Male

  • The sharp contrast between the blue wing and the back and breast is more accentuated.
  • The tail is fully grown and bright blue.

Independent Female

  • The gray secondary wing feathers and wing coverts, when the wing is folded, show little contrast with the concolor back and breast.
  • The tail is fully grown. The young female female in this photo shows gray only near the tip of the tail.

Notes: At about the age of 6 weeks, young bluebirds no longer depend on their parents for food. The siblings tend to forage in close proximity until they join flocks of young birds and begin to move about. Some young birds stay in their natal territory for a longer period while others even help raise the next brood if the parents have a double brood in the season.

Young eastern bluebirds in North American wear the juvenile plumage between the months of June through August.

Period of July through October: Acquiring the first adult plumage

Scientific Name: Calothorax lucifer

Sub-adult Male

  • Between July and October young bluebirds of the year begin to replace their juvenile plumage for their first adult plumage.
  • Blue and red-brick feathers begin to appear.
  • The wing feathers an coverts are also gradually replaced for pure blue feathers.
  • Birds show a haphazard looking plumage.
  • The first adult plumage, also called the first basic plumage, has a grayish cast to it and it is not as saturated as the plumage of older bluebirds.
  • The onset of molting depends on the hatching time. Some birds start molting earlier than others.

Sub-adult Female

  • The change in colors from a young bird to its first adult plumage is not as dramatic as in males. The juvenile gray with whitish spots on the back turn into a lighter and uniform gray without the whitish dots.
  • The wings feather also turn into a more uniform and lighter gray without the pale spots of juvenile birds.
  • The female in the photo has temporarily lost her secondary wing coverts leaving the base of the wing feathers exposed.

Notes: Molting into the first adult plumage take place during the months of July through October, in North America.

Some individuals retain wing and tail juvenile feathers into the winter. However, the rate at which juvenile birds completely replace their plumage and the amount of juvenile feathers they retain varies among individuals.

By mid to late October, all juveniles have finished molting into their first adult plumage.

Most juvenile bluebirds complete their molt while still in the breeding grounds, before undergoing migrations or leaving the natal grounds. Other individuals complete their molt after leaving their natal ground or during migration.

Does the eastern bluebird plumage change color in the winter? This is a common question asked by bird enthusiasts. No, young bluebirds wear the first adult plumage for a year. This first plumage is dull and has a grayish tone to it, compared to older birds. Bluebirds then replace the same adult plumage for a fresh one every year during the month of July through October. Review some of the reasons why bluebirds may vary in color; please see: Why are Some Bluebird Bluer than others?

Conclusions:

The sex of young eastern bluebirds can be reliably determined as soon as day 13 after hatching. After fledging, the amount of blue in the wing feathers, wing coverts, and tail reveal the sex of fledglings bluebirds. Young bluebirds acquire their first adult plumage by the end of the breeding season in which they were born. From then on, bluebirds replace the same plumage every season.

References:

  • Pinkowski, B. C. (1974d). Criteria for sexing eastern bluebirds in juvenile plumage. Inland Bird Banding News 46:88-91.
  • Patricia A. Gowaty and Jonathan H. Plissner. Birds of the world, Published March 4, 2020.

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