Millions of mirrors - Iridescent coloration in birds

Submitted by Johan on 9 December 2015.

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Like the rainbow shimmer on a soap bubble, the blue color of this male tree swallow (Tachycineta bicolor) feather is iridescent. Light reaching the feather is reflected by its microstructure like a myriad of small mirrors, making it look very bright.

Why do males tree swallow display this vivid color? The answer could originate from the sexual selection theory developed by Darwin. Because the “millions of mirrors” of tree swallow feathers need to be in very good state to keep their shiny look, the brightest males could be in the best overall health. Females would therefore prefer these males, as they would provide 'good genes' and possibly the best care to young.

Male tree swallow (Tachycineta bicolor)

Does sexual selection theory hold in tree swallows? In order to answer to the question, iridescent coloration from male tree swallow rump feathers collected in southern Quebec, Canada, was measured in a black room using spectrophotometry. A precise and unbiased measurement is a crucial step in the research, and is particularly challenging to get with iridescent coloration. Indeed, as iridescence implies a drastic change in brightness with a slight movement of the animal, the position of the feather when measuring its color is critical to control.

A two-year study revealed that male tree swallows have individual differences in a unique property of iridescent coloration, the angular breath of color reflectance. Angular breadth has the potential to greatly influence the male’s detectability by females in his environment, and could convey information about male’s condition. Females may therefore use the information from this angular property to choose males.

Length of the iridescent region : 5 mm

Technique: A camera Olympus SC30 integrated in a stereomicroscope Olympus SZX7 was used. The only modification is the removal of dust on the background velvet.


Sonia Van Wijk (Author)


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