Harvest Mice in Cornwall
Citizen nest survey
Update: We've now reached the end of the 2022-2023 harvest mouse season with
over 200 records collected. Many thanks to everyone who volunteered their time!
The Mammal Society are about to begin a nationwide survey of harvest mice using
citizen scientists to gather records of abandoned harvest mouse nests. Cornwall is
ahead of the game here and we have been setting up survey training to improve our
Harvest mice numbers are dwindling across the UK probably as a result of habitat loss
and changes in agricultural practices. However it is difficult to assess any decline
without good baseline data. In Cornwall, harvest mice have been recorded from most
areas over the years but in the recent past records have tailed off. This is probably from
lack of recording effort – nobody is looking for their distinctive grass nests. Only 21
records have been submitted to ERCCIS in the last 5 years. It is worthwhile looking
anywhere in the county.
Where and when to look
Although harvest mice were associated with cereal crops they are found in many other
habitats with tall grasses, reeds or sedges: marshes, rough grassland, bramble, ditches
and gardens, road verges and field margins. During the spring and summer months
harvest mice build nests in the grass at heights of 0.3 to 1 m above ground level. The
smallest nests, used during the day, are golf ball-sized whilst the breeding nests may
be as big as a tennis ball. The nests are woven from strips of leaves 1-2 mm wide, and
still attached to the plant.
When in use there is no obvious entrance hole but abandoned nests have a conspicuous
hole. The nests are abandoned in the autumn when the animals move down to spend the
winter at ground level. Nests are built in stronger grass and reed species, especially
Cocksfoot (Dactylis glomerata), tufted hair grass (Deschampsia cespitosa), Purple moor
grass (Molinia caerulea) and reed canary-grass (Phalaris arundinacea). Nests are often
about half way up the stem of reeds and cereal but in tussock-forming grasses such as
cocksfoot the nests are often lower, just at or below the top of the tussock itself.
How to look
Always ask for permission if looking on private land and always check for potential
hazards – especially deep water in reed beds etc. – your personal safety far outweighs
any records you might collect! Use your hands to open up the vegetation, concentrating
on tussocky clumps, larger clumps of vegetation may be worth investigating even if they
have slumped over in the winter weather.
Negative results are just as valuable as positives – so let us know! For any site tell us the
site name and location (grid reference) and your contact details. Describe the site – what
sort of plants are there, is it arable or pasture, reed bed or marsh etc. If you do find nests
(or something you think may be a nest) take photographs of the nest and surrounding
vegetation. If you are sure it is empty (there is an entrance hole and it is winter) you can
remove the nest and we can arrange to collect it.
Report results via our contact form here.