The Hidden Glow

Okay, this is probably one of the coolest things I’ve learned about any bird mating in the past three years.

Now that I’ve raised the bar far to high for myself, enter: the Eurasian Blue Tit. A bird easily identifiable due to their bright yellow and blue coloration. The sexes of this species have always thought to be indistinguishable from one another, much like the Albatross, without genetic sexing. Only the most experienced handles are able to tell the difference, and even then only with the bird in hand.

A Eurasian Blue Tit

However, recent work done by a group of researchers in Sweden decided to look outside the box, and from the bird’s point of view. To be specific: in the UV scale. There are several groups of bird which view light within the UV spectrum, one of which is the passerines (which includes the Eurasian Blue Tit). Andersson, Örnborg, and Anderson (1998) thought to examine the markings of this species at a UV scale, which is a while new spectrum of color and light that humans cannot see without the assistance of special cameras, lenses, and other technological aid.

Turns out, the Blue Tit is in fact sexually dimorphic, simply on a wavelength that we are unable to see. They found that the males have a marked difference of the chroma on their brilliant crown patches. And oh how I wish they had taken pictures of the marking! It was even proven to stand out against both brown-grey and bright green backgrounds, translating into contrast with either tree foliage or even the dead leaf litter on the forest floor.

This find could potentially be the explanation for several behaviors and displays these birds are known to exhibit. For instance, this species is known to be polygamous, with females seeking extra-pair copulations with healthier males. The scientific community were unsure as to how the females decided which males were healthiest, with theories including certain vocalizations in song and even leg length, as it was the only determinable trait they could find between mate choices. Now it seems that the UV coloration is what is selected for, which is supported by the fact that during mating and territorial displays the males display their crown patches to females and other males.

Just goes to show there is more than meets the eye when it comes to bird behavior. 

Bonus: This is a direct quote from the journal article.

“Although hardly perceivable in the field (except when directly compared in the hand) to the UV-blind and yellow-biased primates that write ornithological handbooks…”

References

Andersson, S., Örnborg, J., Andersson, M., 1998. ‘Ultraviolet sexual dimorphism and assortative mating in blue tits’, Proc. R. Soc. Lond. B., Vol 265, p. 445-450.

Picture: http://www.raywilsonbirdphotography.co.uk/Diary/2010/December_2010.html