Issues in corvid behaviour and ecology

In part one, the winter use of habitats available to four corvids (carrion crow, jackdaw, magpie and rook), sympatric in the mixed agricultural landscape of Keele, Staffordshire, is examined. Grassland, especially per-" manent pasture, was the dominant crop, and a majority of each species was found there. Selection between the different habitats available occurred. Carrion crows and magpies were found mostly on permanent pasture or grazed temporary leys, while jackdaws and rooks occurred mostly on permanent pasture. Rooks foraged mostly for below surface invertebrates, especially earthworms. Jackdaws took small invertebrates from the surface, and beneath surface litter and dung. Carrion crows took mainly medium invertebrates and large earthworms above the soil surface, and medium invertebrates from dung. Magpies took both small and medium invertebrates above the soil surface, and from litter or dung. The overlap of the four species in space, time, foraging microhabitat and prey types taken is examined. Discriminant function analysis showed that each species was significantly separated from every other on at least one of the functions derived, but magpies were found to occupy little unique niche space. Based on the observed overlaps and some negative or facilitative effects of other species' absence or presence on short-term prey intake rate, predictions are made about the possible behavioural mechanisms which might occur to reduce the impact of these effects. Computer simulation indicated that in general overlap between the four species was less than would occur by a chance utilisation of the available resources, suggesting that the species did differ in their use of the habitats sufficiently to cause some partitioning of resources. In addition certain species avoided foraging on a site when other species were present. Some interspecific aggression was seen, mainly by carrion crows against the other species. Data suggested that this was probably due to the presence of other birds disturbing large earthworms down their burrows, thus making them unavailable to foraging carrion crows, rather than to true 'interference' competition directed against species competing for the same resources. In part two, functional interpretations of avian grouping are reviewed, and field observations of rook flocks foraging for earthworms described. The data suggested that social attraction to larger flocks increased the efficiency with which patches of earthworms were located, since larger flocks built up on the denser prey patches. This effect may also have occurred within flocks foraging on a single field. A field experiment was conducted in which levels of prey density were manipulated. Wild rooks showed changes in behaviour in response to variation in prey densities which were consistent with the field observations. However, in addition to showing social attraction after an individual bird's 'giving-up time' on a patch had been exceeded, birds may also have monitored the size of other flocks available in the vicinity, and moved on before their 'giving-up time' was exceeded if a larger flock were foraging elsewhere. Since larger flocks tended to occur on the densest prey patches, this mechanism may further increase the efficiency with which prey patches are located