The ecology and conservation of seed-eating birds on farmland.
1. 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 ofthese species through an understanding of their behaviour.
2. 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.
3. Three species of seed-eating passerine are studied in detail, skylark Alauda
arvensis, yellowhammer Emberiza citrinelia and com bunting Miliaria calandra. In
winter, yellowhammers and com 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.
4. The functional responses of skylarks, yellowhammers and com 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.
5. 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 com buntings preferred to forage close to them, skylarks did not.
6. 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
7. 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.