Certainly I would agree (though it's been many years since I've been in a
position to watch them, alas) that Yellow Robins use vertical perches to
keep an eye on the ground (though I suspect that they wouldn't pass up a
trunk insect if one presented itself). I doubt, though, that they are
descended from birds of treecreeping habit - hanging on to a trunk is a lot
different from moving around on one, and accomplished creepers like
Climacteris have a number of specialized adaptations for doing so
(including very large and strong feet).
As to why a vertical rather than a horizontal low perch for scoping the
ground - perhaps it gives a slightly different perspective for scanning and
locating ground prey, since the bird looks from a slightly different angle?
Possibly this may make spotting certain kinds of insects easier - do YR's
eat different sorts of insects, or take them from different areas, than
birds that hawk to the ground from a horizontal perch? Do they make more
strikes, or more successful ones?
My guess, though, is that this habit may first have evolved in
Tregellasia-type robins - there are simply a lot more vertical perches near
the ground than horizontal ones in primary rain forest - and that the
Yellow Robins may have been "stuck" with the habit once they moved out into
other types of habitat as rain forests withdrew.
--
Ronald I. Orenstein Phone: (905) 820-7886
International Wildlife Coalition Fax/Modem: (905) 569-0116
1825 Shady Creek Court
Mississauga, Ontario, Canada L5L 3W2 Internet:
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