The bird communities of three disused quarries on the Magnesian limestone of Durham county

Abstract

In order to assess the ornithological importance of three disused quarries on the Magnesian Limestone escarpment in Durham county the distribution and abundance of the breeding birds were examined using the CBC technique. Thirty-nine bird species were recorded on the sites, of which twenty-seven species were considered to be holding territories. The habitat was also sampled at 137 randomly located 0.02 ha sample units at each site. Variables recorded were those that were thought to be the most important in influencing the distribution and abundance of the birds on the sites. In addition to the census, the behaviour of all birds recorded on the sites during the same period was quantified. Correlations between the breeding bird communities at the three sites and the habitat variables were explored using numerical classification (two-way indicator species analysis), indirect gradient analysis (detrended correspondence analysis) and direct gradient analysis (canonical correspondence analysis).Ordination of the combined data from the three quarries illustrated a gradient of bird species composition from open grassland to enclosed canopy scrub. When the ordination was constrained by the environmental variables two enviromnental gradients became apparent. Both were associated with the increasing cover relating to the successional sequence at the quarries. The first gradient was that of uninterrupted succession from bare quarry to enclosed canopy ash woodland. The second gradient was from a well developed field layer to low scrub, which was interpreted as a form of sub-climax. The behavioural analysis indicated that although broad patterns in habitat selection could be determined from the coarse scale approach, the more subtle patterns in habitat utilisation were lost at this level of analysis. The importance of specific aspects of the habitat were elucidated using chi-square analysis. This analysis further indicated the importance of the hawthorn scrub to breeding and foraging birds. The grassland was also found to be important for foraging birds. The implications of these findings in view of the importance of die quarries as examples of limestone grassland is discussed. The potential benefits to the common farmland avifauna of sympathetic management of these sites is also discussed. The preliminary results of this study indicate that the appropriate management of the successional process at these sites could benefit both the avifauna and flora.