[CT Birds] The eyes of Eagles

Dennis Varza dennisvz at optonline.net
Thu Jul 16 00:14:35 EDT 2009

Hi Folks

I just ran across a special issue on bird vision. A lot of the papers  
are technical, but there are some interesting points.

Avian Color Vision and Coloration: Multidisciplinary Evolutionary  
American Naturalist Volume 169 Special Supplement

In short:
With the development of small and inexpensive spetrometers scientists  
have been able to learn a great deal about how birds see. They found  
that birds possess  the most richly endowed visual system of any  
vertebrate. In addition to what humans can see (trichromic visual  
system) they have a tetrachromic visual system and can see  
Ultraviolet (A ) light in the region of 300 to 700 nanometers.  
Therefore birds see color in a different way than humans do.

One paper I thought of general interest and comprehensible follows.  
It show how interpreting what birds do may not be so obvious.

Morphology and Ornamentation in Male Magnificent Frigatebirds:  
Variation with Age Class and Mating Status
American Naturalist Vol. 169 pp. S93 to S111

Male Magnificent Frigatebird ornamentation includes bright iridescent  
plumage and a red inflatable gular pouch. These signals are displayed  
during courtship along with a drumming sound produced through  
specialized beak clackings resonating in the gular pouch. The extent  
of white in the plumage identifies three age classes of non juvenile  
male. Here we investigate how morphological and secondary sexual  
traits correlate with age class and mating status. Even though  
several age class-related differences in morphology and visual  
appearance can be identified, the only features that significantly  
predict mating success are acoustic components of courtship display.  
Specifically, males that mate drum at a lower fundamental frequencies— 
that is they have larger gular pouches — and have a quicker more  
consistent drumming cadence than unsuccessful males. The fundamental  
frequency decreases with age class, reflecting an increase in gular  
pouch size. This implies that females prefer older or possibly more  
experienced or viable males. Drumming cadence speed and stability  
might reflect male stamina. Apart from the acoustic differences with  
mating status, there is a non significant tendency for back feather  
iridescence to be of shorter reflectance wavelength spectra in mated  
than in unmated males, which, when combined with acoustic variables,  
improves prediction of age class and mating status.

Dennis Varza

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