Deer (Overview) Interaction with Humans - Decline in Bird Species

Deer are grazers and as such affect vegetation in the habitats where they feed. Given that both Red and Roe are part of our native fauna (i.e. have been here since the last glacial retreat) and that Fallow have been in the UK for the last thousand years or more, it might be argued that they have evolved with their habitats. We have, however, seen a steep rise in deer numbers in recent years (an estimate published in 2013 suggests there are 1.5 million deer in Britain and this is likely a significant underestimate) and many biologists think that this is putting increased pressure on many of our most valuable ecosystems. The government’s POSTnote points out that:

“Lowland ancient woodland, upland heath and blanket bog can suffer particularly from deer over-grazing, excessive browsing and trampling”

and notes how:

“4,000 hectares [almost 10,000 acres] (about 4%) of woodland with SSSI [Site of Special Scientific Interest] status is currently in ‘unfavourable’ condition due to deer impacts”.

Any change to an ecosystem invariably has consequences for its inhabitants. There have been many studies looking at the impact deer may have on bird populations, but the results are inconclusive – some have found an association between deer removing understorey vegetation and a decline in ground-nesting birds, while others have failed to link the two. Some nest monitoring studies have observed deer depredating nests (i.e. eating the eggs), although it is unknown how often this happens. Nonetheless, high deer densities do have a demonstrable impact on both the composition and structure of plants in a given habitat and this change has been linked to declines in various invertebrate species. Logically, given that invertebrates are a staple food source for birds (especially songbirds), a decline in invertebrate numbers is likely to adversely impact bird numbers.

A study looking at the impact of deer on songbird populations on the archipelago of Haida Gwaii, off the coast of British Columbia, published in 2005 concluded that:

“deer [in this case, Black-tailed deer, Odocoileus hemionus sitkensis] overabundance results in a decrease in songbird habitat quality through decreased food resources and nest site quality and may explain part of the continental-scale decreases in songbird populations”.

Closer to home, the situation is considered similar. According to the Forestry Commission for England’s Woodland Improvement Grant 80 (March 2009) appraisal, the decline of the woodcock (Scolopax rusticola) in the Yorkshire and Humber region can be at least partly attributed to “deer/sheep browsing leading to loss of shrub layer”. There is also the question of what impact these changes to vegetation structure and invertebrate communities have on small mammal populations, but I know of no data on this.

Fortunately, the situation is not hopeless and there are measures that can be taken. The exclusion of deer from areas of wood and grassland by fencing can be highly effective. Nonetheless, fencing is costly to install and maintain and deer are good jumpers. Fencing can also be counterproductive if deer manage to get into the exclosure, because they may be unable to get out and can decimate the vegetation within before starving to death. Culling can be implemented in order to reduce the local deer population and, if the meat is sold as venison, this can help offset the costs of deer management. In the end, it is likely that a combination of fencing and culling will be necessary in any given area.

It should not be assumed that all deer-related impacts on woodlands are detrimental. By removing shrubby growth and bramble, the deer open up the forest floor and allow colonisation of species that are otherwise rapidly out-competed. Similarly, deer can help disperse plants and may play a key role in the regeneration of fragmented woodlands. At the 12th Annual Conference of the International Association for Landscape Ecology, held in Cirencester in the Cotswolds during June 2004, Forestry Commission ecologist Amy Eycott and two colleagues presented data on how deer disperse plant seeds in their droppings. Eycott and her co-workers report that large-bodied grazing deer had the greatest number of seeds and highest number of seed species in their pellets. Studies in America have found a similar situation with White-tailed deer (Odocoileus virginianus).