This species is active throughout the 24-hour period, but in areas disturbed by humans they tend to be more active under the cover of darkness (2). They may occur either solitarily, or in single-sex groups where population densities are higher (2), and large herds may gather during autumn and winter (3). Sika deer browse on trees and shrubs, and also feed on grasses, sedges, holly, conifers, fungi, acorns, bark, heather and ivy (3). This species causes a great amount of damage, being a serious pest of woodland and farmland (4).
During the breeding season, or 'rut', which occurs between late August and October, males occupy territories and compete for access to females (3). These contests involve vocalisations such as screaming, parallel walking and eventually fighting, which can result in serious injury and even death (2). The successful stags then mate, and hinds (females) give birth, usually to a single calf, in May and June (3).
Introduced sika deer crossbreed with native red deer, causing hybridisation where the two species occupy the same range. This poses a threat to the red deer as it dilutes the gene pool (3).