Deer are bad news for birds

No.:  2011-46
November 2011

Deer may be affecting some bird species on far larger scales than previously appreciated, new research has found. The study, published this week in the British Ecological Society's Journal of Applied Ecology, provides evidence that the populations of several woodland bird populations fare worse in areas that have high, rather than low, numbers of deer.

Muntjac by Neil Calbrade 

Browsing by deer tends to reduce
vegetation

The research led by Dr Stuart Newson from the British Trust for Ornithology (BTO) applied new methods of analysis to large national bird and deer monitoring data. The study focused on eleven woodland bird species in lowland England and their relationships with three widespread and abundant deer species: muntjac (Muntiacus reevesi), roe deer (Capreolus capreolus) and fallow deer (Dama dama). The eleven bird species depend on low dense vegetation in woodland and scrub, and are therefore potentially vulnerable to browsing by deer which tends to reduce this vegetation.

There was evidence that increases in deer have been associated with large-scale population declines for five of the eleven bird species. Of these, it was suggested that the impacts of deer are likely to have been greatest for two species of conservation concern, the amber listed Nightingale and the red listed Willow Tit. These two birds have declined by 54% and 65% respectively over the last ten years.

Currently deer management aimed at reducing the impacts of deer typically takes the form of excluding deer through the use of various types of fencing and / or culling of deer. According to Dr Newson, “Our results emphasise the importance of developing co-ordinated national strategies for minimising deer impacts. With numbers and ranges of deer predicted to expand even further, it is suggested that such strategies should be targeted on areas that continue to support concentrations of species that are especially vulnerable to over-browsing by deer.”

This study is not suggesting that deer are the only, or even the main, factor driving woodland bird declines; many other factors are potentially implicated. Nonetheless, these findings build on earlier experimental work carried out on nightingales by the BTO that has showed that deer can reduce habitat quality for this species. "More widely, growing evidence from other parts of the world suggests that increases in deer abundance may be depressing population levels of breeding woodland birds that are associated with dense understorey habitats." This study is important because it indicates that deer browsing may affect some bird populations on a large scale.

Stuart Newson, Alison Johnston, Anna Renwick, Stephen Baillie and Robert Fuller (2011), ‘Modelling large-scale relationships between changes in woodland deer and bird populations’, doi: 10.1111/j.1365-2664.2011.02077.x is published in the Journal of Applied Ecology on 4 Nov 2011.

View the abstract.

Notes for Editors

  1. Copies of the paper are available from Becky Allen, British Ecological Society Press Officer, tel: +44 (0)1223 570016, mob: +44 (0)7949 804317, email: beckyallen [at] ntlworld.com
  2. The eleven bird species include, Dunnock, Nightingale, Song Thrush, Willow Warbler, Willow Tit, Marsh Tit, Bullfinch, Blackcap, Chiffchaff and Blackbird. All eleven depend on dense understorey for nesting and / or foraging.
  3. The paper uses 12 years of Breeding Bird Survey (BBS) bird and mammal data (1995-2006) collected each year from about 2000 sites spread across the whole of England. BBS data were collected and this work funded as part of the British Trust for Ornithology / Joint Nature Conservation Committee (on behalf of CCW, NE, CNCC and SNH) / Royal Society for the Protection of Birds partnership.
  4. The BTO is the UK’s leading bird research organisation. Over thirty thousand volunteers contribute to the BTO’s surveys. They collect information that forms the basis of conservation action in the UK. The BTO maintains a staff of 100 at its offices in Norfolk, Bangor and Stirling, who analyse and publicise the results of project work. The BTO’s investigations are funded by government, industry and conservation organisations.
  5. Journal of Applied Ecology is published by Wiley-Blackwell for the British Ecological Society. Contents lists are available at www.journalofappliedecology.org
  6. The British Ecological Society is a learned society, a registered charity and a company limited by guarantee. Established in 1913 by academics to promote and foster the study of ecology in its widest sense, the Society has 4,000 members in the UK and abroad. Further information is available at www.britishecologicalsociety.org 

Contact Information

Dr Stuart Newson
(BTO Senior Research Ecologist)
Office: 01842 750050 (9am-5.30pm)
Email: stuart.newson [at] bto.org

Paul Stancliffe
(BTO Press Officer)
Office: 01842 750050 (9am-5.30pm)
Mobile: 07585 440910 (anytime)
Email: press [at] bto.org

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