Deer are omnivorous opportunists and will feed catholically on grasses, heather, lichen, shoots, bark, leaves, herbs, rushes, buds, nuts, fungi, fruit and berries; even holly and bramble. They are typically mixed concentrate feeders, which means they select young shoots, young foliage, fruits and other high quality foods from which they can extract bone-building nutrients; “mixed” comes from their ability to switch between grazing and browsing. Muntjac appear particularly partial to ivy and the wild arum known as lords and ladies (Arum maculatum). The type of food consumed depends as much on location and season as on species.
Carnivorous tendencies have been documented in some species. Indeed, the Red deer was assigned the title “Most bloodthirsty ungulate” by the 2007 Guinness Book of Records in response to observations of some individuals feeding on shearwater chicks on Rum. I have come across sproadic reports of other deer eating small vertebrates, including a captive muntjac buck that caught and ate a sparrow on two occasions. Additionally, between February and August 2008 and 2009, Paul Doloman led a study of nigtjar (Caprimulgus caprimulgus) nests in Thetford Forest on the Norfolk/Suffolk border. Remote cameras monitoring the nests recorded three instances where a deer flushed a bird off eggs; in one case, during 2008, the Fallow deer (Dama dama) proceeded to eat the eggs. It remains unknown how common such nest depreation by deer is.
Along with the more customary items in the diet, a range of inedible objects have also been recovered from deer digestive tracts; these include polythene bags, balloons, string and even a pair of disposable underwear. Unfortunately, these kinds of objects can easily get stuck and cause a blockage. In her 1991 book Deer, Norma Chapman notes that a study of more than 80 Fallow deer stomachs collected in Essex found that they all contained at least one foreign object.