I saw this on the Oregon birdmail site and thought it
might be of interest to some of you.
Big-Eyed Birds Sing Early Songs: Dawn chorus explained
John Pickrell
As dawn breaks on a misty Welsh morning, the earliest
birds to break
into song are likely to include European robins,
followed by
blackbirds and song thrushes and then a plethora of
other species as
sunlight crowns the horizon. The last to join the
chorus, such as
chaffinches and blue tits, may not chime in until 100
minutes after
the first crooners began. > SUNRISE SERENADE.
Big-eyed birds such
as
the European robin (above) start singing much earlier
than neighbors
like the house sparrow (below), whose eye is 80
percent as large.
RSPB
Images
This pattern is repeated worldwide, and ornithologists
have often
pondered what determines when a particular species
begins its morning
singing. Now, scientists say that they've found the
explanation: The
larger a bird's eyes, the earlier it starts to sing.
The staggering
of
avian choruses was first documented 70 years ago but
has remained
unexplained. Now, researchers have revisited an idea
first proposed
in
the 1960s but never tested. It's that visual
acuityâdetermined by
eye sizeâgoverns when birds start to sing. Robert J.
Thomas at the
University of Bristol in England and his coworkers
recorded the time
and intensity of light at the moment when each species
broke into
song
at seven European woodland sites. The researchers then
temporarily
captured individuals from 57 of these species and used
calipers to
take rough measurements of eye-surface diameter.
In the April 22 Proceedings of the Royal Society B,
the researchers
report a strong relationship between the light
intensity at which
birds start to sing and eye size. "Birds with big eyes
do start to
sing earlier," says Thomas. The relationship remained
even after the
scientists used statistics to discount other factors
such as body
size. "Even though the result is expected, it's an
excellent example
of the way in which visual capabilities affect
communications
behavior," says Sandra Vehrencamp of Cornell
University. Birds
singing
at dawn and dusk attract mates and defend their
territories. However,
this musical interlude can be hazardous since it can
draw the
attention of predators, such as tawny owls, to a
bird's location. The
singing also makes it difficult for a bird to hear
danger coming,
says
Thomas. It therefore makes sense, he adds, that
singing not begin
until the bird can be on the lookout for trouble. >
References:
Thomas, R.J., et al. 2002. Eye size in birds and the
timing of song
at
dawn. Proceedings of the Royal Society of London B
269(April
22):831-837. Abstract available at
http://dx.doi.org/10.1
098/rspb.2001.1941.Sources: Robert J. Thomas Centre
for
Behavioral
Biology School of Biological Sciences University of
Bristol Bristol
BS8 1UG United Kingdom Sandra Vehrencamp Department of
Neurobiology
and Behavior Cornell University Ithaca, NY 14853
From Science News,HREF="http://www.sciencenews.org/20020420/toc.asp";>Vol.
16, April 20, 2002, p. 245.
