BRANTA — Robert A. Robinson
Ecology and conservation of seed-eating birds on farmland
Institution: University of East Anglia, Norwich, UK
Supervisors: WJ Sutherland
Details: PhD 1997 (Completed)
Address: British Trust for Ornithology, Thetford, IP24 2PU, UK (Oct 2012) Email
Subject Keywords: agricultural change, foraging, functional response, conservation, depletion models, seeds
Species Keywords: farmland passerines, larks, finches, buntings, Sky Lark Alauda arvensis, Yellowhammer Emberiza citrinella, Corn Bunting Miliaria calandra
Many species of bird breeding in Britain have declined in recent decades. However, only in the farmland ecosystem are the majority of species, primarily seed-eating passerines, declining. Similar declines in these species are also occurring elsewhere in Europe and North America. This thesis aims to model the effects of habitat change on population size of these species through an understanding of their behaviour.
A wide range of factors, primarily relating to the intensification of agricultural management, have been implicated in these declines, and these are reviewed in detail. There is little evidence for consistent impacts on the breeding biology, but considerable circumstantial evidence that declines in winter food supply have been a major cause of the declines.
Three species of seed-eating passerine are studied in detail, skylark Alauda arvensis, yellowhammer Emberiza citrinella and corn bunting Miliaria calandra. In winter, yellowhammers and corn buntings used stubble fields exclusively; skylarks much preferred stubble fields. All avoided winter cereal fields. This is related to seed density, stubble fields held virtually all the cereal grain found in the soil and many more weed seeds than winter cereal fields.
The functional responses of skylarks, yellowhammers and corn buntings are described. Seed availability plays an important role in determining intake rate, with seeds buried beneath the surface harder to obtain, particularly for yellowhammers. Seed availability has more general implications for the nature of functional responses.
Bird numbers were related to seed density, the aggregative response. The scale at which aggregative responses were observed varied between species. The availability of seeds in the soil column affected the birds use of fields, with the number of seeds on the surface important. The location of hedgerows was also important, yellowhammers and corn buntings preferred to forage close to them, skylarks did not.
The general effects of overwinter mortality on population size are modelled and the effects of two causes of winter mortality, starvation and predation are analysed. Individuals traded-off these two risks which had an interactive effect on mortality. Density-dependence in the breeding season interacts with winter mortality to set population size.
The results presented in this thesis are used to model the effects of habitat changes on populations of seed-eating birds. The predicted population, on the basis of winter food supply, was close to the current actual population size. These models are used to assess the effectiveness of different management options, with unharvested conservation headlands being best for species such as the yellowhammer and tree sparrow Passer montanus, while set-aside is best for species such as the skylark.
Robinson, R.A. & Sutherland, W.J. 2002. Post-war changes in arable farming and biodiversity in Great Britain.J. Appl. Ecol. 39:157-176.
Robinson, R.A. 2001. Feeding ecology of skylarks in winter – a possible mechanism for population decline? In Donald, P.F. & Vickery, J.A. The ecology and conservation of skylarks: 129-138, RSPB, Sandy.
Watkinson, A.R., Freckleton, R.P., Robinson, R.A. & Sutherland, W.J. 2000. Predictions of biodiversity response to genetically modified herbicide-resistant crops. Science 289:1554-1556.
Robinson, R.A. & Sutherland, W.J. 1999. Food resources and winter habitat selection of seed-eating birds.Ecography 22:447-454.