Letter-winged Kite (Elanus scriptus)
A. Juvenile Letter-winged kites (Elanus scriptus)
B. Mature male below.
This is the only nocturnal kite. It can be compared directly with the
sympatric sister taxon, Elanus notatus (Black-shouldered kite) which
is diurnal. The available evidence indicates that the letter-wnged kite
has evolved its nocturnal abilities only recently (<1 MA) to take advantage
of the arrival in Australia of the Long-Haired rat (Rattus villosissimus)
which plagues the Channel Country at irregular intervals. The recent and
limited nature of the nocturnal shift is evident in:- the eye (which
still has two well-developed foveas, like diurnal but unlike nocturnal,
raptors),; the fact that it needs moonlight to hunt; the kite even synchronises
its hatching to coincide with the full moon.
C. Bifoveate fundus of the sister species, Elanus notatus,
black-shouldered kite.
I was astonished to find that letter-winged kites have a bifoveate
retina like their sister species, the black-shouldered kite. I do not yet
have a good fundus photograph of the letter wing that shows both foveas
in the same shot, but it looks very similar to this next fundus photo from
the black shouldered kite except that there is less blueish sheen and the
foveas are slightly further apart. The circular field is 30 deg across.
Eye-cup preparation showing the proximity of the two foveal pits in
Elanus notatus. The close proximity of the pits in the elanid kites
and the true "booted" eagles (15-16 deg) emphasises the puzzle of having
two foveas. If the foveas are so close together, why not have just a single
fovea that can be moved over this short distance? The answer may lie with
the completely different central connectivity of each fovea. The folded
pigmented structure rising off the white optic nerve head is the pecten
oculi.
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