Cornwall Mammal Group

Update: After 3 years of hard work, we have collected 754 nest records and a further 60 records from barn owl pellets. We have carried out 245 field surveys covering 51 hectads (10 x 10km squares) across the county and found harvest mouse evidence in all of them. Over three years we have devoted over 1000 person-hours to surveys and another 150 hours to analysing over 1500 barn owl pellets. We now have evidence that harvest mice occur widely across Cornwall and the next two years will focus on getting a better understanding of habitat associations and also looking at higher moorland sites. Many thanks to everyone who volunteered their time!

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The Mammal Society have begun 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.