Found over a restricted range in Britain, the yellow-necked mouse (Apodemus flavicollis) may be locally common but nationally its status is unclear. It is thought to favour mature deciduous woodland but it is a regular visitor to houses in some areas. Its national range may be limited by climatic factors, possibly showing an aversion to wetter, colder areas. Its distribution is also associated with that of long established woodland sites. The current status of this species is also less than clear, despite being common in some areas.
Yellow-necked mice are largely nocturnal and are expert climbers. They make full use of the woodland floor and canopy when moving. Home range sizes are generally slightly larger than those of wood mice and home ranges overlap between and within the sexes. Generally their ranges are less than 0.5 hectares, shrinking in the non-breeding season and increasing for males in the spring. They have dark brown fur on the back with a white underside and a complete band of yellow fur across the neck area, protruding eyes, large ears and a long tail. Head and body length (95-120mm). Tail length (77-118mm). Weight (14-45gms).
Feeding primarily on tree seed, fruits, some green plants and invertebrates, their diet is very similar to their close relative the wood mouse (A. sylvaticus). They may specialise on eating tree seeds, possibly selecting those species of seed with the highest energy value. Food may be stored in their complicated underground burrow systems. Burrows are often constructed amongst root systems and contain nests furnished with plant material as bedding. The tunnel system can be extensive, covering a wide area and having several entrances. Nests may also be found above ground in tree holes and in houses. They have successive pregnancies from February to October producing litters of 2-11 young. Males become reproductively active first in the spring and the onset of breeding falls 2-8 weeks earlier than for wood mice where the two species occur together. The pups are born naked and blind weighing about 2.8g. Their eyes open after 13-16 days and their distinctive yellow collar of fur is discernible by then. Animals born in spring reach sexual maturity in 2-3 months while animals born later in the year develop more slowly and do not breed until the following year. In years of high tree seed abundance the breeding season may be prolonged into the winter. Few mice survive more than a year and the average life expectancy of juveniles is only 3-4 months. The population peaks in the autumn then declines over winter into spring before increasing again.
Yellow-necked mice are rarely seen except where they enter property. It is quite common for some houses in rural settings to experience a regular seasonal problem of invasion by this species where they can become a pest. They are unlikely to cause much damage to field crops due to their close affinity with woodland.