THE SMALL MAMMALS OF ESSEX 37
species react differently to bottles, some species are able to
escape from bottles more easily than others and this is a further
source of bias. Live-traps are not baited with shrew food but
bottles soon attract insect fauna (especially once an animal
has died in the bottle) and this may be a further attraction to
shrews.
Owl pellets. Barn owls (which produced most of the pellets
analysed) and Short-eared Owls tend to hunt over open grass-
land. Also a high proportion of the pellets were collected near
the Essex coast where the grassland habitat is especially fre-
quent. Both these factors help to explain the bias towards the
Field Vole (Microtus agrestis).
Long and short-term trapping. It is well known that small
mammal numbers fluctuate through the year. One night's trap-
ping might, for this reason alone, give different results from the
longer-term studies. In fact the two sets of trapping results are
fairly similar in terms of relative frequency of the various
species. Most of the differences can be accounted for by the
wider habitat range covered in the short-term trappings. The
Coptfold areas included no rough grassland (hence the Microtus
and Micromys results) but was especially favourable to Apodemus
flavicollis. The low shrew proportions recorded from the inten-
sive trapping surveys result from the death of most of the shrews
the first time they are caught. The rodents are released and any
recaptures added to the total captures but a shrew is only
counted once.
The bottle-hunting and extensive trapping survey gave re-
sults which consisted of small samples from many different
Table 5
Distribution and abundance of small animals
A comparison of the percentage of localities sampled in which the species
was found; and the percentage of total catch in those areas where the
species was definitely present. In the live trapping 67 localities were
sampled but from only 52 of these was sufficient information available to
calculate the "percentage catch", (see appendix 1). The bottles were col-
lected from 155 localities although from only 33 localities were the
Apodemus remains identified to species