THE SMALL MAMMALS OF ESSEX 47
is not necessarily associated with the relative abundance of the
prey species.
The majority of the records are 'casual' records, largely nest
records. These records refer to the breeding nest, which is a
compact structure made out of shredded grass leaves and is
about the size of a cricket ball, as first described by Gilbert
White (1789). It is not generally realised that a smaller non-
breeding nest is also built, this being about the size of a golf ball,
of much less solid construction than the breeding nest. One can
virtually see through these non-breeding nests and they are pro-
bably very ephemeral structures. Often these nests are as
numerous as the breeding nests, but are easily overlooked. Nests
can be very abundant; in a strip of reeds in a ditch on Bower's
Marshes over 60 nests were found within a distance of 200 yards
(although very few mice were caught in the same area). Nest
density cannot be taken as an absolute measure of abundance
since more than one nest is built by a pair of mice for each litter
raised, and if a litter is disturbed it is quickly moved to another
nest. The Bower's Marshes site is of interest because it demon-
strates the very local distribution of Harvest Mice; this species
has been known in the reeds bordering only one field for 30
years and is found nowhere else in the area. This phenomenon
adds to the difficulty in locating populations of this species.
Occasionally 'swarms' of Harvest Mice occur. Very locally,
often in an area of only one or two fields, this species becomes
so abundant that several mice can be seen running about in the
herbage and can easily be caught by hand. Large numbers can
be caught by a line of people moving slowly across the area.
These 'swarms' of mice have been seen in Essex at Flatford Mill
and at High Beech. The Harvest Mouse in Britain is near the
northern limit of its range and the nest is probably an important
device to protect the young from cold during their early develop-
ment. Studies at Sewardstone have shown that heavy rain soon
destroys the nest, and presumably the young therein die of cold
and exposure. The effect of rain on the distribution of the Har-
vest Mouse has been discussed in detail by Adams (1913) who has
shown that the northern and western spread of this species in
England is limited by the amount of summer rainfall, and any
county that receives more than three inches of rain in July and
August is not colonised by the Harvest Mouse. The 'swarms'
seem to occur after a dry summer, the weather having permitted
a successful breeding season.
Teagle (1964) has published a detailed survey of the distri-
bution of the Harvest Mouse in the London area in which he has
shown that this species is associated with what he refers to as the
'stalk zone'. Any area of tall monocotyledonous growth is an
ideal habitat for Harvest Mice. In Essex records refer to cereal
fields, Phragmites, areas of waste ground colonised by rank mixed
herbs, vegetation bordering sewage farms, rank vegetation in