The abundance and suitability of tree cavities and their impact on hole-nesting bird populations in beech forests of NE Iberian Peninsula

Abstract

Abstract. The availability and suitability of tree cavities for hole-nesting birds were surveyed in three beech wood types (mature, cleared and coppice forests) in the North east of the Iberian Peninsula. We sampled the occupation of cavities and the abundance of hole-nesting birds. We also tested experimentally with tit nest boxes whether the lack of suitable nest-holes may limit the abundance of secondary cavity-nesters. We surveyed hole-nesting birds before and after nest box provision. Trunk and branch cavities (25.9%) were significantly more abundant in mature woods, and are correlated with the density of secondary occupants. Stump and root cavities (74.1%) were more abundant in coppice forests. Shortage of big diameter's (> 45 cm DBH) and good bearing trees explained the lack of cavities in managed forests. Only small proportion of available cavities was used by birds (5.5%). All occupied cavities were placed in trunk (5.5%) and presented smaller diameter entrances than the whole availability of cavities. Nest boxes occupation rate was higher in the plots where suitable nest holes were scarce (managed woods), and consequently it brought an increase on both Great Tit and Blue Tit populations. These two species populations were favoured from the next breeding season after the provision of nest boxes, but not in mature stands nor in control sites (with no nest boxes). Therefore, results show that suitability of cavities rather than availability determines secondary hole-nesting bird abundance in managed forests.