Factors Affecting Foraging by Breeding Farmland Birds.

Abstract

This thesis investigates nesting success, foraging behaviour, chick diet and growth of three ecologically contrasting species (skylark Alauda arvensis, yellowhammer Emberiza citrinella and song thrush Turdus philomelos) on farmland, in relation to microhabitat and the availability of invertebrates. Fieldwork was conducted on a lowland mixed arable farm in Leicestershire over the breeding seasons 2000 - 2002. Telemetry was used to monitor adult song thrushes and fledgling skylarks. Skylark nest density was highest in set-aside and lowest in crops, yellowhammer nest density was greatest in herbaceous vegetation and field margins whilst song thrush nest density was greatest in hedgerows and woodland. Birds did not utilise the available habitat at random for nesting. Provisioning skylarks and yellowhammers utilised managed set-aside strips more than would be expected, although both foraged in cereals. Song thrushes foraged primarily in woodland and boundary habitats, avoiding cereal crops, permanent pasture and set-aside. The largest cause of nest failure for each study species was predation. Skylark nest survival improved with increasing vegetation density at the nest site. Yellowhammer nest survival was affected by temperature and rainfall. Song thrush nest survival was affected by predator control, nest site location on the farm and habitat. Skylarks selected sparse vegetation and bare ground when foraging in winter cereals. Each species selected some invertebrate taxa over others, independently of chick age. Skylarks and yellowhammers did not forage in sites with greater invertebrate abundance than others, but song thrushes tended to forage in habitats with higher invertebrate biomass. Skylark chick growth rates were positively influenced by set-aside and negatively by spiders in the diet and the proportion of 'other habitats' around the nest. Skylark chicks with higher growth rates were more likely to fledge. No habitat effects or environmental factors were found to affect song thrush chick growth rates. Tagged skylark chicks exhibited a low post-fledgling survival rate, with most predated whilst flightless. This thesis identified actual and potential conservation benefits for nationally declining skylark, yellowhammer and song thrush populations arising from the management of cropped and non-cropped habitats.