Cornwall Mammal Group

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!

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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

own records.

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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.

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Where and when to look

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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.

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How to look

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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.

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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.

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Report results via our contact form here.