The harvest mouse

The word "wetlands" conjures up various images. For you, it may be extensive boggy grasslands, beds of rushes, or waders and overwintering ducks. Less romantically, cold feet while standing with binoculars vainly searching for the variety that a 'friend" assured you "was there yesterday". A harvest mouse would probably not be high on the list in the word-association game. But signs of harvest mice have been found at Nansinellyn Marsh Reserve, near Perranporth, as a result of a survey in which 50 tennis balLs on canes were plan ted by three intrepid volunteers during August this year.

The harvest mouse is our smallest rodent, weighing in at between five and ten grams - about the weight of a 20p piece. (The pygmy shrew is lighter but that is an insectivore. It lives in tall, dense vegetation. including cereal fields, reedbeds. sedge beds, grassy hedgerows and bramble patches. In shape it is, as you would expect, mouse-like, but with small ears. I has a long prehensile tail. Wood mice and yellow-necked mice have an escape mechanism whereby, if the tail is seized, the skin strips off. Harvest mice do not have this option. The colour is a russet orange on the dorsal side and pure white underneath. Hearing is acute. eyesight is poor. although they can detect sudden changes in silhouette several metres away.

Harvest Mouse nest
Tennis ball cut open after summer to show harvest mouse nest. Photos: Ron Evenden

The harvest mouse is the only British mammal to build a nest of woven grass above ground. To find this nest is proof of its presence in the area. A breeding nest is about ten centimetres across and is made by the female. This is a clever construction. She sits on a stem and uses her incisor teeth to split the ends of the grass leaves and weave them into a flamework. This is then lined with more leaves pulled thfl)ugh the framework. The centre has a lining of grass or thistle down. Non-breeding nests are also built, being only half the diameter.

The breeding season is late May to October with August/September as a peak period. The average litter is five and there can be more than one brood. The female abandons the young at 15- 16 days. The offspring may continue to use the nest for a couple of days or so after this.

Harvest Mouse on Grass
Harvest mouse at work. Photos: Ron Evenden

The diet is mainly insects and seeds but may be supplemented with fungi, moss and root material. However, not all that much is known about their diet. With mammals, the ratio of surface area to body volume is important. The higher this ratio, the greater the heat loss. In small mammals this is hich. so consequently they need a comparatively higher intake of food than large mammals. In the case of the harvest mouse. this means consumiuc a third of its body weight each day. Even so. this is only about two grams. It is thought that this small rodent can reach densities of 200 per hectare, but even with those numbers they are not considered to be a threat to food stocks in this country. Plagues of harvest mice in Russia and Europe have caused extensive damace.

Their distribution is mainly flom Yorkshire southwards, with only isolated pockets in Wales. Perhaps the latest survey will show this is not correct. Who knows?! Surveys have thrown up surprises both good and bad. An example of good news came from the 1970 harvest mouse survey. It was thought up until then that it had become rare since the nineteenth century, but this survey showed it was more widely spread and locally common. Another case of under- recording - let us hope that this is still trile.

This handsome. clever creature deserves our protection and is worth more than its weight in coinage.

Ron Evenden Retirement from the teaching profession has allowed Ron Evenden more time to persue his interest in nature. An active member of the mammal society an the Otter Group, he has also been involved in the dormouse survey, led the Trust's harvest mouse project, and has kindly offered to lead our new Mammal Group.

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