Breeding Ecology of the Farmland Yellowhammer (Emberiza citrinella): A Scottish Case Study.

Author Macleod, C.J.
Citation Macleod, C.J. (2001). Breeding Ecology of the Farmland Yellowhammer (Emberiza citrinella): A Scottish Case Study. Unpublished Ph.D. thesis. University of Dundee, Dundee.

Abstract

Farmland songbird populations have undergone significant declines over the last three decades. These declines have been linked to the intensification of agricultural practices. Direct mortality as a consequence of farming operations, loss of nesting habitats and diminished food resources have been implicated as the main causes of farmland bird declines. However, the response of individual species to the different changes in farming practices tends to be varied. In order to understand further the actual mechanisms causing these declines it is necessary to investigate the ecology of each species in relation to farmland management. This study, therefore, focuses on the farmland yellowhammer, Emberiza citrinella, a species that started to decline significantly later than most other declining farmland birds. Patterns in yellowhammer habitat selection in relation to different measures of habitat quality are investigated to determine the habitat requirements of this species during the breeding season. The fieldwork was carried out on two Scottish lowland farms in an area that is generally regarded to be less intensively farmed than the south of England where previous studies have been based.
Changes in farming practices associated with agricultural intensification have resulted in a transformation of the farming landscape, therefore, it is crucial to understand the habitat preferences of the species of conservation concern to ensure that management plans meet the requirements of the species. The first chapter investigates the distribution of territories of the yellowhammer and two other species, the blackbird, Turdus merula, and the chaffinch, Fringilla coelebs, in relation to boundary characteristics and adjacent crop type. Boundary habitat features rather than crop type were the main determinants of the territory density of all three species in the present study. This appeared to reflect the nesting habitat preferences of each species. The maintenance of hedgerow, ditch and woodland patches in farmland would be beneficial to the focal species.
Agricultural intensification has also resulted in a loss of plant and invertebrate diversity and abundance in farmland habitats. Consequently, a reduction in the food resources of farmland birds has been hypothesised as one of the mechanisms causing bird population declines. The second chapter investigates the composition of plant and invertebrate communities in relation to habitat management and pesticide inputs. Each type of non-crop and crop habitat was found to support a unique plant and invertebrate community. Crops with higher plant diversity and species richness also sustained significantly higher invertebrate abundance, richness and diversity. Game crops appear to provide a practical means of enhancing plant and invertebrate diversity and abundance, as well potential bird food resources, in the farming landscape.
However, any management plans aimed at enhancing the food resources of the yellowhammer also need to understand the dietary preferences of this species as well as temporal and spatial patterns in food supplies. An investigation of nestling diet composition in relation to invertebrate food resources around the nest, in chapter three, indicated that coleoptera, arachnida, lepidoptera and, possibly, mollusca, were the preferred prey. The proportion of grain in nestling diet was found to increase through the breeding season. Trends in invertebrate abundance in relation to habitat type indicate that the yellowhammer would benefit from management practices that enhance heterogeneity in the farming landscapes as different habitat types supported high numbers of each of the preferred prey items.
The foraging habitat preferences and behaviour of adults feeding nestlings were examined, in the fourth chapter. These were related to habitat availability, invertebrate abundance and the rate of short -term weight gain of chicks to determine foraging habitat requirements of the yellowhammer for breeding. Trends in foraging habitat preferences provided support for national trends with a preference for cereals, in particular winter wheat, and an avoidance of grassland habitats. The mean distance travelled by adults feeding chicks at the nest was negatively correlated with the mean invertebrate abundance around the nest and the rate of short-term weight gain was positively correlated with invertebrate abundance, especially the preferred prey. This suggests that the yellowhammer may suffer increased breeding costs in areas where invertebrate abundance has decreased. The role of predation in causing farmland bird declines is still under debate therefore the final chapter assessed patterns in nest predation in relation to nesting habitat and mammal activity. Although nests in conifer trees and grassy field margins were most susceptible to predation and nests in hedgerows were least likely to be predated, these trends were generally not significant. However, artificial nest experiments suffered similar levels in predation in relation to nesting habitat type and marks left on artificial eggs indicated that rodents were the main predators. Rodent activity was highest under hedgerows and lowest in ditches. Mustelids, which predated more frequently on chicks than eggs, were more active later in the season, although only low levels of activity were detected.
The yellowhammer would be most likely to benefit from holistic farmland management practices, as recommended in previous studies. Patterns in territory distribution, nesting habitat preferences and nest predation indicate that non-crop habitat features such as hedgerows and ditches are important for the yellowhammer. Similarly, invertebrate food supplies, in particular the preferred prey, coleoptera, lepidoptera and arachnida, are significantly correlated with travel costs of foraging adults and the rate of weight gain of chicks. Management practices that enhance weed diversity and species richness will help to enhance invertebrate abundance, richness and diversity in farmland habitats.