202 The Bird 
a ptarmigan, while yet in the brown garb of summer, 
is exposed against a hillside of snow, it becomes very 
wary. 
It is interesting, in this connection, to observe how a 
Nighthawk carries out its colour resemblance to a knot 
or a rough piece of bark, by perchiyg, not crosswise, but 
lengthwise, along a branch or fallen tree-trunk. 
A volume might easily be written of the various ways 
in which protective coloration works out among birds, 
but there is so great a difference of opinion, and indeed 
so many exceptions to every theory which may be ad- 
vanced, that it is better, for the most part, to go to Nature 
without @ priori theories, and putting ourselves as nearly 
as possible in the position of the creatures themselves, 
to hope for better ability to see with their eyes. And 
it is right along this line that we most need fresh data 
and experiments, namely, the actual ability of birds 
and insects to distinguish shades, colours, forms, and 
motion,—whether efficient in certain ways or not. We 
know that many men cannot distinguish a scarlet ball 
lying upon green grass; that is, they are partly colour- 
blind. If this were the case with certain hawks, a male 
Scarlet Tanager would be forever safe from them among 
the green foliage. 
An important fact, which for years had been appar- 
ent to me, but unexpressed until Mr. Abbott Thayer put it 
into words, is that colours which we would ordinarily term 
conspicuous are often exactly the opposite when found in the 
plumage of a bird. Writing of the Motmot in my volume 
““Two Bird-lovers in Mexico,” I say: “I have often 
