256
THE ESSEX NATURALIST.
Although the Song Thrush is so close to the Redwing in many
ways, one difference can be noted. Flocks of Thrushes are al-
ways "loose," the birds hanging together in a disjointed flock.
The Redwing frequently flocks as close as Starlings, and will
often perform elaborate aerial manoeuvres after the manner
of Starlings ; in my experience these large movements, wit-
nessed, of course, in daylight, are carried out silently. When in
small flocks the birds often utter their "seep" call.
On closely comparing Thrush with Redwing a difference in
the eye becomes noticeable. The Thrush has by far the bigger
eye. This difference cannot easily be shown in figures, but my
measurements are 5 mm. for Redwing and 6 mm. for Thrush.
The difference is more apparent in the bones of the skull, where
the rule can be placed on the frontal bone between the orbits.
The space is 4 mm. in the Thrush, but 6 mm. in the Redwing.
I do not think that size of eye is generally a safe guide to the
habits of a bird ; it may be in this case.
The Song Thrush is much more often noticed on migration
than is the Redwing, although apparently this curious point
has never attracted consideration. My impression is that for
every hundred Thrushes met with at lighthouses or lightships
there cannot be more than about thirty Redwings. The figures
I have used have been drawn from such schedules as those pub-
lished by the Committee on the Migration of Birds, those by
Barrington on Migration in Ireland, Herluf Winge on Danish
Lighthouses, Gaetke on Heligoland, and Dr. Eagle Clarke on
British and other stations. For example, when Dr. Clarke spent
a month on the Kentish Knock Lightship, off the Essex coast
(17th September to 18th October, 1923) he did not record a single.
Redwing, although Song Thrushes were abundant. The date, of
course, is not late enough for safe deductions to be made ; but
Gaetke, in his "Heligoland" speaks clearly of the comparative
rarity of the Redwing and the extreme abundance of Song
Thrushes.
In my 1911 paper I expressed the opinion that nocturnal
Redwings flew only in fine weather—that is, on nights fairly
free from wind. I would modify this opinion ; I consider that,
roughly speaking, the birds may be heard any night, without
reference either to the weather or the state of the moon. On
a stormy night the conditions are not suitable for outdoor vigils ;