THE SMALL MAMMALS OF ESSEX
53
based on owl pellet analysis alone would underestimate the
abundance of this species. A. C. Wheeler identified Bank Vole
remains from owl pellets on Foulness Island in the late 1950s
(Seear 1964) but the much larger collection of owl pellets from
D. Hunford's survey on Foulness has failed to produce any Cleth-
rionomys at all. It is possible that Bank Voles do not live on
Foulness and that the early record results from an owl with a
full stomach flying to Foulness. Only trapping could resolve
this mystery.
Field Vole (Microtus agrestis (Linn.)) Map 10
Laver: "This Vole abounds sometimes to such an extent as to
entirely destroy the herbage ... Its insatiable appetite compels
it to be abroad at all seasons of the year and at all hours of the
day . . ." Laver also includes early (16th century) accounts of
mouse plagues and concludes that these were Field Vole plagues.
Field Voles have very precise habitat requirements: tussocky
grass and similar dense herbage. They do not live in woodlands
or growing arable crops. They make runs at ground level under
a cover of grass and their diet is almost exclusively grass.
Table 4 indicates the great discrepancies between the main
survey results for this species. The main reason for their rarity
in bottle and trapping results is that Field Voles live mainly in
habitats not adequately sampled by these methods. If the traps
are set in the runs of this species, in rough grassland, they can
be caught. The owls clearly hunted in such areas and found it
easy to catch Field Voles.
There is no reason to say that the Field Vole is not still a com-
mon species. It is very common in its habitats, which are still
widespread in Essex. However, there has been a move away from
dairy farming towards arable crops since the end of the 19th
century. This has led to a decrease in rough pasture land and it
seems certain that there is a smaller area of Field Vole habitat
today. This, perhaps, explains the absence of spectacular 'vole
plagues' in recent times. The last reported plague years were in
the 1930's on the Langenhoe marshes and on Potton and Wallasea
Islands (Day 1956).
General Discussion
Many of the apparent changes between the end of the last
century and the 1960s, in the known pattern of small mammal
distribution and relative abundance, have resulted from improved
study techniques and do not represent real changes.
The most obvious real changes are in the extent of suitable
habitats for Dormice and Harvest Mice. Both have suffered as
a result of changing habitats during this century, and the Harvest
Mouse may also suffer a further decline if the reed-beds and
waste-land habitats which are now this species' main stronghold
are not effectively conserved. Those species which do not live in