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 population size. 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.

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The environments in which avian species exist fluctuate widely in space and time. In the grasslands of southeastern Arizona, there are annual cycles of rainfall which distribute rainfall unevenly throughout the year. Two peaks of rainfall usually occur, one in December and January, and a second in July and August. The temporal pattern and magnitude of rainfall can vary from year to year. There are also significant patterns of rainfall variation across the landscape, both long and short term. The end result of this variation is that the environment in which birds in southestern Arizona breed varies on a number of spatial and temporal scales. Several sites on the Santa Rita Experimental Range and the Research Ranch were censused during the summers of 1982 and 1983 to study the response of the avian community to heterogeneity in their environment. In the Santa Rita Experimental Range, two habitat types were studied: mesquite savannah and grassland. Assuming no temporal dynamics in community structure, mesquite habitats appeared to have higher total densities than grasslands in 1982 and 1983. However, this was true only during April-June. In July and August densities appeared to be higher in the grassland habitats. Patterns of species richness and eveness also appeared to be different when temporal dynamics were considered than when they were ignored. The timing of individual species appears to be responsible for the differences in community structure obtained by the two methods, and this suggests that the assumption of no temporal dynamics in community structure during the season is invalid. The densities of eight species of emberizids were correlated to characteristics of the vegetation on the Research Ranch and Santa Rita sites. One group of species appeared to be associated with open grassland habitats, while a second group appeared to be associated with mesquite habitats. There were a large number of nonlinear relationships of species with certain habitat measurements. While the associations of birds with habitat features were statistically significant, the causal relationships between habitat variables and avian densities were not elucidated by the correlations.

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