- Top of page
- REVIEW OF EVIDENCE
- RECENT EMPIRICAL EVIDENCE
The effects of deer in woodlands are known to result in habitat changes which can be detrimental to songbirds. In the first part of the paper we review the effects that deer may have on critical resources for woodland birds. The principal mechanism by which deer may affect habitat quality is through the reduction of low woody vegetation, which forms a key element of the preferred habitat of several species – this may be associated with loss of nest-sites, increased exposure to predators and reduction of food. The second part of the paper presents new evidence for the impacts of deer on vegetation structure, and how they may be contributing to the declines of some woodland songbirds in Britain. Results from extensive studies of deer at 13 woodland sites in England and Wales reveal that understorey foliage density decreases significantly with increasing deer density. Reduction of foliage was greatest at below 1.5 m above ground. We also report preliminary results of an experimental comparison of songbird densities between deer-fenced and unfenced areas of young coppice woodland in eastern England. In this study, deer browsing caused extensive impacts on vegetation structure in young coppice, though there was considerable variation between plots. Deer browsing resulted in reduction of canopy cover, reduction in density and cover of understorey vegetation, and an increase in grass cover. Abundance of bird species using the understorey, including all migrants, was significantly higher in coppice where deer were excluded. These results lend support to the hypothesis that deer are at least partly responsible for causing declines in some British bird populations, but they do not eliminate the possibility that increased shading is also responsible for changes in woodland structure. At present there is much spatial variation in deer densities in Britain so that impacts on low vegetation and habitat quality for birds are not expected to be the same in all regions. However, with continuing increases in deer numbers it is to be expected that such impacts will become more widespread.
Results from recent monitoring surveys have indicated that populations of several species of woodland birds are declining (Fuller et al. 2005, Hewson et al. 2007). The reasons for the declines are not entirely clear, and it is possible that a combination of factors is involved. However, there are increasing indications that deer may be responsible for causing a deterioration in habitat conditions for some woodland birds and Fuller et al. (2005) suggested that intensified deer browsing could potentially affect nearly a half of the declining species. Deer populations continue to increase in Britain (Gill 1990, Ward 2005) and they can cause substantial changes to woodland vegetation. Several of the woodland bird species about which there has been recent conservation concern depend on the understorey (defined here as all low woody and herbaceous vegetation within 2 m of the ground), the vegetation zone most directly affected by ungulate browsing. These species include Hedge Accentor Prunella modularis, Common Nightingale Luscinia megarhynchos, Song Thrush Turdus philomelos, Garden Warbler Sylvia borin, Willow Warbler Phylloscopus trochilus, Willow Tit Poecile montanus, Marsh Tit Poecile palustris and Common Bullfinch Pyrrhula pyrrhula. In contrast to these species, Wood Warbler Phylloscopus sibilatrix and Common Redstart Phoenicurus phoenicurus tend to be more abundant in open grazed woodland and may benefit from the impact of deer.
The effects of deer on woodlands in lowland Britain was the subject of a conference in 2000, which resulted in several reviews covering the effects of deer on woodland vegetation and fauna (Fuller & Gill 2001, Fuller 2001). It was clear from these accounts that deer browsing on vegetation has potential effects on the habitats, and consequently the populations, of a wide range of fauna, including invertebrates, small mammals and birds (Feber et al. 2001, Flowerdew & Ellwood 2001, Fuller 2001, Perrins & Overall 2001, Stewart 2001). However, there was a lack of direct evidence linking changes in British bird populations to deer as well as for the potential mechanisms (reductions in food sources or increased nest predation) involved. Since then, more evidence has become available, both in published sources and from our own work.
This paper is presented in two parts. First, we review evidence for the effects of deer on habitat structure and resources for birds. We also summarize evidence linking deer directly to bird abundance and breeding success. Secondly, we present new information, revealing evidence for the relationship between deer density and understorey vegetation density, as well as for the effects on songbirds of exclusion of deer from coppiced woodland. The emphasis of this paper is on woodland in the lowland parts of Britain where the typical deer species undergoing population increases are Roe Deer Capreolus capreolus, Fallow Deer Dama dama and Muntjac Deer Muntiacus reevesi.
- Top of page
- REVIEW OF EVIDENCE
- RECENT EMPIRICAL EVIDENCE
The recent evidence obtained on deer impacts gives increased support to the hypothesis that deer are capable of causing deterioration in habitat conditions for several woodland songbirds. There is widespread evidence that understorey vegetation is depleted by deer and comparisons between fenced and unfenced blocks of coppice woodland have revealed significant differences in densities of both migrants and understorey-dependent species. Further, recently reported studies based in British Columbia have also established links between the effects of deer browsing on vegetation, invertebrate populations, nest predation and songbird densities (Martin & Joron 2003, Allombert et al. 2005a, 2005b).
However, there is much that remains unclear about how deer are affecting British woodland bird populations. Apart from our results using fenced enclosures in Bradfield Woods, direct evidence of a link between deer numbers and songbirds is still lacking. It is still not entirely clear to what extent recent declines in woodland songbird populations have been caused by deer or whether they simply coincide temporally with a general increase in deer numbers. Nonetheless, the Bradfield Woods results do suggest that, at least at a local scale, reductions in some species may be associated with increased pressure from deer. Across Britain there is currently much regional variation in the abundance of different deer species and in total deer density. It should be noted that Bradfield Woods lies in a part of eastern England that holds one of the highest deer densities in the country. It cannot be assumed therefore that such impacts will be evident in coppiced woods in all parts of lowland Britain. However, deer are spreading and numbers continue to increase in many regions so it is to be expected that such impacts will become more widespread.
Increased shade from the woodland canopy may confound the impact of browsing. Both browsing and shading have similar effects, namely to reduce understorey vegetation. Many mature high forest stands in Britain have been simultaneously affected by both processes in recent decades. It is therefore difficult to distinguish the effects of each of these factors using survey methods. It would, however, be possible to separate the effects in an experimental investigation.
Evidence from investigations of browsing by deer in woodland suggest that increasing the level of shade appears to compound the effect of browsing by reducing understorey vegetation still further.
The more light-demanding understorey species appear to respond to increased light levels by recovering even where browsing pressure remains unchanged and there is evidence that bird species using the understorey can increase after the woodland canopy is thinned (DeGraaf et al. 1991, Bell & Whitmore 1997, Matsuoka et al. 2001). This has led to the suggestion from a North American study that effects of deer browsing may be offset by intentional thinning (DeGraaf et al. 1991). However, thinning at the intensity carried out for conventional woodland management in Britain is usually followed by rapid canopy closure, and it may therefore be necessary to apply a very heavy thinning to achieve a sustained increase in understorey cover. Furthermore, increases in the understorey cover may benefit deer or attract an increased browsing pressure. Efforts to improve understorey cover for birds may therefore be most successful if carried out in combination with deer management. There is clearly a need to understand more about how browsing and shading interact to affect understorey structures within different management systems.