Eastern Bluebird Sialia sialis
Order: PASSERIFORMES
Family: TURDIDAE
Because bluebirds adapt to human habitats quickly, scientists have studied them extensively. They are an ideal species for us all to observe and learn from as they use nest boxes, forage in the open, gather in flocks during winter, and chatter together, revealing where they are. Unlike most bird species, juvenile gender is easily distinguishable as juvenile males have blue tails, juvenile females have brown tails while both have speckled breasts.
Why are male birds often so colorful? Female mates select males with brighter colors more often. Why? A study of Eastern Bluebirds demonstrated that males with larger reddish breast areas more frequently fed their nestlings, had heavier offspring, and paired and nested earlier. This adaptation must be true for the evolution of other species as there are some brilliantly colored male birds out there! Unlike humans, many birds detect color on the ultraviolet spectrum also. A study of Bluebirds showed that males with more ultraviolet hues had more offspring. A Bluebird’s coloration indicates he’s macho and fit so is more likely to reproduce successfully and pass on healthy genes!
Did you know that there is no blue pigment in bird feathers? We see blue because the microstructure of the feather reflects the blue while absorbing all other colors. Thus, when the light strikes the bird just right, we see dazzling blues. Females are tanner and blend in more with the background while nesting to protect herself and her young. A drawback to the male’s bright colors is that he is more visible to predators. Nature seeks a balance between attracting mates and vulnerability to predators. In some species in which hormones during courtship cause body parts to become brilliant, hormone levels decline after mating, so their colors diminish. Colors also fade in the sun, or vivid feather tips wear off, better protecting the male. Eastern Bluebirds are fascinating Florida birds.
Juanita Baker, Ph.D.
Florida Bird of the Month
Pelican Island Audubon Society
References:
Gowaty, P. A. and J. H. Plissner (2015). Eastern Bluebird (Sialia sialis), version 2.0. In The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.381
Lynn Siefferman, Geoffrey E. Hill (2003). Structural and melanin coloration indicate parental effort and reproductive success in male eastern bluebirds. Behavioral Ecology, 14, 6, 855–861. https://doi.org/10.1093/beheco/arg063