|
BNA Guide to Practical Field Work With Small
Mammals
By Roger Tabor
See also Of Mice
and Men - An introduction to studying small mammals and
their ecology
Without doubt far more useful information
on the normal activities of small mammals in Britain has been
obtained by Longworth live-trapping than by any other single
technique. (See BNA Live Trapping leaflet.) However, any technique
will have a selective bias, and a pattern of field catches
does not necessarily reflect the "real" picture. Gathering
further information on small mammals by other means will help
gain better overall perspective.
The clearest example of this is seen in
the distribution of the harvest mouse. Our smallest British
rodent was scientifically overlooked until recorded by Gilbert
White in 1789. Sporadic records were accumulated by the Victorians.
However, the introduction of a new generation of reaping machines
meant that by the formation of the BNA in 1905, naturalists
in the absence of any positive evidence assumed the mouse
to have been almost eradicated. The advent of live trapping
with Longworth traps did little to change the assessment of
their declined status.
It was only during the 1970’s when Stephen
Harris emphasised that recording harvest mouse nests would
establish a pattern of distribution that a substantial number
of records materialised. Stephen Harris’ survey with the Mammal
Society established observations for 23 vice-counties where
it was previously unrecorded. The findings led Harris to state
that there was no evidence of any decline since the animal’s
discovery, just that it had been unrecorded. Recorders look
for the characteristic ball shaped harvest mouse nest suspended
from woven leaves at stalk height (around 10 – 60 cm above
ground). However, this technique also has it’s bias for the
nests are easiest to see from September to January. The mouse
longitudinally frays the leaves and weaves in long grass leaves
while still leaving them attached to the plant, so the nest
colour changes with the season making them hard to spot in
summer. The weaving in of leaves from around a clump, however,
keeps the nest upright and visible in the autumn and winter
when other grasses collapse. Consequently, and not surprisingly,
only under 5% of the recorded nest locations were in growing
cereals. Dormice can also be detected by their characteristically
different spherical breeding nests, particularly of shredded
honey- suckle bark that are also above ground.
Although harvest mouse nest sightings
were numerically the most productive, live trappings did produce
8.5% of the survey results. A similar proportion (almost 10%)
derived from regurgitated pellets from birds of prey, of which
most came from Barn Owls. Harvest mice, unless superabundant
in an area, can hardly be said to be a major food item of
Barn Owls, as they rarely occur in their diet at much over
1% by weight (of invertebrate prey.)
Relatively few harvest mice can be caught
by Longworth live trapping in the spring and summer, while
the population seems to peak during autumn and winter. Where
live trapping and investigation of Barn Owl pellets have been
carried out over the same area there has been the same relative
lack of summer harvest mice in the owl’s diet. This mutual
support of two techniques lends greater confidence in interpreting
the results. The Barn Owl method of hunting has a different
bias to that of Longworth live-trapping. Even so, interpretation
must be cautious for both techniques might be revealing that
the mice may be more mobile on the ground and so more catchable
in the late autumn, rather than just an increase in numbers.
Indirect techniques not using trapping
often require detective fieldcraft for identification from
bone fragments, hair, droppings, food remains and so on. The
distribution of the common dormouse was also established during
the 1970’s by Elaine Hurrell and Gillian MacIntosh’s Mammal
Society survey that recorded dormice from their distinctive
way of opening hazel nuts with a smooth-edged hole. Owls could
almost seem to have been purpose-built for naturalists interested
in small mammals. Whilst most diurnal raptors, like kestrels,
shatter many of the bones of their prey when they tear at
it with their beaks, owls generally swallow prey whole. The
woodmouse and bank vole are such a significant part of Tawny
Owl diet, that if they are low in numbers then the owls’ breeding
success is reduced. Skulls and lower jawbones of small mammals
survive both avian and mammalian carnivore digestion better
that most longbones. Similarly the last evidence of a long
decomposed corpse is usually the skull and jawbone rather
than the longbones. Consequently, whether examining owl pellets,
or fox, cat or other carnivore droppings, or bones in discarded
bottles, it is worth developing an understanding of basic
skull differences. For example, both bats and insectivores
have a continuous row of teeth along the jaw, while rodents
have a large gap between their incisors and their cheek teeth.
Shrews, unlike moles or bats have long procumbent lower incisors.
Voles have worn-flat cheek teeth with sides of columnar waved
ridges, while the more omnivorous mice have crowned and constant
rooted teeth. That the woodmouse has 4 roots to the first
upper molar, while the house mouse has 3 roots, allows an
easy distinction to be drawn between otherwise fairly similar
skulls.
Additional useful information can be
drawn from teeth for age structure of a population. For example,
at the base of the long wavy teeth of bank voles, trailing
roots develop with age as the columnar teeth force up, replacing
worn material. The cusps of the procumbent incisors of shrews
are also worn with age, but the whole body of shrews are easier
to age and are normally more available than those of other
small mammals. Shrew tails are invariably bald by their second
summer, and the coat on the back , particularly of common
shrews, is much darker in their second year. Shrews have fairly
distasteful flank glands, so although readily killed by cats,
are rarely eaten. As Longworth traps are rarely specifically
baited for shrews, the sample provided by cats may be a more
reliable index of numbers.
If bones are beyond recognition, even
a few remaining hairs can be of great value for identification.
Hair is not readily digested and consequently remains in disgorged
owl pellets and carnivore droppings – even when well weathered.
Viewed directly through a light microscope air spaces of the
medulla within then hair can be diagnostic. However, the surface
pattern of cuticular scales on hair is particularly indicative,
and gelatine or thermoplastic casts can be easily made on
a microscope slide and examined. Although the scales covering
the hair are very fine (0.0005mm thick), being only about
one hundredth of the width of the hair, none-the-less they
can be felt. Take a hair and pull it first one way and then
in the other direction between your fingertips. As the scales
overlap with exposed edges towards the tip only, pulling from
the root towards the tip gives little resistance . However,
pulling in the other direction the hair drags as it catches
on the scale edges. The patterns of the surface scales can
be seen even more clearly under Scanning Electron Microscopy.
Not only can species be identified, but with people individual
differences have been detected. When making comparisons between
species, with hair, don’t forget the longer guard hairs are
different to the under fur hairs, and that hairs can be different
along their length in colour, medulla cells, cross section
shape and in the surface cuticular scale patterns. For practical
purposes it is useful to make up a reference set of microscope
slides, for a positive identification from a few hairs can
be invaluable in fieldwork interpretation.
|
