Discussion here on birds' use of shadows has thus far focused on prey
using shadows in order to detect (and even identify as
harmful or harmless (i.e., like a vulture-like shadow) predators. I
will discuss here one anecdote from personal experience about prey
use of shadows and one paper I came across recently based on predator
use of shadows. One poster mentioned that shorebirds in Jamaica Bay,
NY seem nervier and jumpier on cloudy days (when shadows would be
harder to detect). I noticed the same on the Cabo Rojo Salt Flats in
Puerto Rico where more than 2,000 peeps winter every year in a system
involving large, open lagoons. Attacks by peregrines and merlins
occur almost daily during the winter and often several times a day.
I did a 3-week informal study which was cut short on shorebird
behavioral responses to raptor attacks. (I was a teaching assistant
for the Manomet Bird Observatory's Field Biology Training Program at
the time, and I spent most of my time helping students with their
research.) I never examined my data for a sunny vs. cloudy trend;
this is just a gut feeling based on memories of data collected six
months ago. But maybe someday I will get a chance to look at that
aspect of the problem; if I do, I will post a note. Of course, even
if there is a difference it might have nothing to do with shadows.

Another interesting case of birds using shadows is a study done
several years ago with Burrowing Owls in captivity. Many small
mammals that Burrowing Owls prey upon are less active in the middle
of the night (when shadows from moonlight are longer) and less active
at dawn and dusk (when diffuse light means that shadows are generally
shorter and less distinct). In this study (the reference escapes me,
but I can post it if nec.), the authors wanted to see whether the
mammals might be responding to an aspect of owl hunting behavior,
i.e., do the owls use shadows to locate prey? They put an owl in one
box with a window the bird could see through into another box. A
bolt would swing by on a string and stop. This was done under two
light conditions; one in which four red light bulbs lit up, one in
each corner of the prey box; in the other light condition, only one
light bulb in the ceiling of the box was lit. Naive observers
recorded when they saw the owl fixate on the swinging bolt. The
authors found that the owl was more interested in the one light
condition (where longer, more visible shadows were cast) and less
interested in the four-light condition (where virtually no shadows
were cast but the "prey" was actually better illuminated).

I am studying visual perception in raptors (although not shadow
perception specifically), and I would be interested in any other
reports of birds "using" shadows. As for the comment that Gray Jays
do not seem to use them, the author is quite correct that it would be
pointless for jays living in woods and fields to rely on shadows to
detect predators. However, in the open desert (where the original
example came from with zone-tailed hawks) or in large, open lagoons,
it might make more sense for birds that are potential prey to use
any and all cues at their disposal.

Peter L. Whitlock
Raptor Research Center
Boise State University
Boise, Idaho
e-mail: whit0522 at varney.idbsu.edu