By Robyn Bailey, NestWatch Project Leader.
On June 12, 2019, NestWatcher Edie Wieder reported an albino nestling Tree Swallow in a nest box in Needham, Massachusetts. Edie monitors 20 bluebird nest boxes as a volunteer with the Trustees of the Reservation in Massachusetts, and this is her second year participating. Albinism is a genetic mutation that prevents the body from producing melanin. Tree Swallow nestlings typically have sooty gray upper parts, like the sibling pictured below, and that gray coloration is due to melanin. Notice also how the bill is pinkish in this first photo of the rare youngster.
Edie reports that both parents had normal coloration, as did all four of its siblings. The finding was especially surprising because this particular nest was one in which the female preferred to stay on the nest during nest checks, even after the young had hatched. Edie wasn’t sure what she would find when she checked the nest on June 12th, but she certainly wasn’t expecting to find a snow-white nestling. At first, she was confused, thinking another bird might have laid an egg in this nest…except she knew that there aren’t any pure white cavity-nesting species in her area. She snapped a photo so she could leave the birds in peace and examine the evidence later. It wasn’t until later that she realized how rare her sighting was.
Other Types of Color
Not all color is derived from pigment. As adults, Tree Swallows get their iridescent blue-green plumage from the structure of their feathers, which scatter light. The feathers have a base of melanin, so if you were to destroy the structure of the feather, you would see only gray. This albino nestling will not obtain the iridescent teal plumage as an adult because there is no base of melanin, which plays a role in the scattering of light.
By comparing these two photos, we can see that the bill changes from pink to yellow; however, this is no trick of the light! Albinism is a lack of melanin, which is made by the body; however, other pigments may still be present in the body. Carotenoids are the class of pigments responsible for oranges and yellows, and albinistic birds can have them in spades (check out this yellow albino American Goldfinch). Unlike melanin, carotenoids are obtained from the diet. From the change in bill color, we can deduce that by the time the second photo was taken, the little nestling had obtained enough carotenoids through its diet to change the bill color to yellow. Therefore, the bills of typical Tree Swallow nestlings must contain both melanin and carotenoids, with the black hues masking the underlying yellow. In the albinistic nestling, there is nothing to mask the yellow color.
The Eyes Have It
Although difficult to see in the photos, Edie reports that the eye was red rather than the typical dark brown. The eye, containing no other pigments, appears as red due to the underlying blood vessels. Pink or red eyes are a good indicator that a bird is truly an albino, rather than a species that just happens to be all white (like the White Tern).
Once In A Lifetime
Albinism is rare, with estimates ranging from 0.05% to 0.1% frequency in birds. A sighting this unique might come only once in a NestWatcher’s lifetime, so keep your eyes out for unusual birds. Thanks to Edie’s observations, we can all share in the joy of discovering something unexpected.