. On each occasion the vocal effort is infused
with much feeling tone, and it would be impossible to point to any one
feature which is peculiar to only one occasion. The question therefore
arises as to whether the emotional outburst which we are attributing to
the arrival of a female may not after all be due to the presence of an
intruding male. It may be so. But although I can recall no single
instance in which the presence of an intruder could be definitely
excluded, yet I should hesitate to base upon this any broad
generalisation.

When the normal course of the song is not interrupted by the arrival of
a female, when, that is to say, the male still pursues the routine to
which he has all along been accustomed, and still sings at stated
intervals in stated places with a voice that betrays no heightened
emotional tone, even though the song may convey some meaning to the
delicate perceptual powers of the female, we have nothing to lay hold
upon which can be construed as an indication of direct relationship
between the song and the presence of the female.

The partial or complete suspension of the song after pairing has taken
place is the most interesting, as it is the most noticeable, feature.
Not that it is by any means universal--if it were so, some of the
difficulties that beset the path of interpretation would be removed, but
it is sufficiently widespread to demand explanation. In nearly every
case it is, however, only temporary, the period during which the male is
silent varying from a few days to a few weeks. The male
Grasshopper-Warbler, when it first reaches us, sings persistently, but
when it is joined by a female a change becomes apparent; instead of the
incessant trill, there are spasmodic outbursts of short duration, and in
the course of a few days the bird lapses into a silence which may be
broken for a short while at dawn, or late in the evening, but is often
complete. More striking still is the change in the case of the
Marsh-Warbler, and the sudden deterioration, or even suspension, of
strains so beautiful and so varied, at a moment, too, when it might
least be expected, at once arrests the attention. The Reed-Warbler that
had its headquarters in a willow sang vigorously from the middle of May
until a female arrived on the 20th June, when its voice was hushed,
except for occasional outbursts which lacked force and were of short
duration. When the Wood-Warbler secures a territory it repeats its
sibilant