94
THE ESSEX NATURALIST.
several Continental ornithologists have pointed out an "in-
fallible" test in the wing formula, and one that I have used in
doubtful specimens during the past few years. In the closed
wing of the Rook the second primary is longer than the sixth,
and in the Crow the second primary is shorter than the sixth.
In 1918 my friend, Mr. H. G. Taylor, pointed out that the young
Rook, as shot for the table (when of course it is most liable to
be mistaken for a Crow), has the wing formula of the Carrion
Crow. I soon proved this to be quite correct; for, it appears,
when all the other primaries are well-grown, the critical second
primary of the Rook is only half-developed. This little detail
seems well worth further and closer study.
On the 19th May 1918, while Mr. Taylor and I were watching
the Nightjars in the Forest at Theydon Bois, we saw a male
bird "churring" on a dead branch; and after a few moments,
it flew down and settled on the ground near its mate, calling
without a break during the whole of its journey. While on the
wing the note was soft and musical, a bubble rather than a
rattle (reminiscent, perhaps, of the voice of the female Cuckoo),
changing to a dull churr while the bird was on the bare ground.
This was my first experience of a Nightjar churring in flight;
and, indeed, from a considerable experience of the bird, I had
doubted if it ever did occur. In the present instance we had an
excellent view of the whole performance, and saw it repeated,
in almost full daylight—the time being 9.0 p.m. (8.0 Greenwich
time). Is it possible that this bird calls both with the inspiration
and the expiration of its breath? The cadence of the note is
connected with obvious movements in the trachea, visible by
the "swelling" of the throat: the trachea, of course, is a rigid
tube, and cannot swell, but I use the common expression de-
noting the alteration in length of the windpipe.
It is now well-known that the male Cuckoo utters his familiar
call with closed mandibles—sometimes, however, I have seen
them separated by about 1/8 of an inch. In 1912, near Albyns, I
was lucky enough to have a female bird in the field of my glass
when it uttered its note, and I saw that she called with wide-
open beak, as does the young Cuckoo, but although I cannot
imagine any possibility of error, I would like again to see the note
given, or to hear of the observations of others who have seen
the female Cuckoo in song. If established, it will be a curious.