[CT Birds] The eyes of Eagles
dennisvz at optonline.net
Thu Jul 16 00:14:35 EDT 2009
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
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.
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