Characteristics of black woodpecker cavities chosen and not chosen by honeybee swarms

The black woodpecker ( Dryocopus martius ) is an important excavator of tree cavities across much of the Palearctic. Honeybees ( Apis mellifera ) are frequent secondary occupants of black woodpecker cavities in managed forests dominated by beech ( Fagus sylvatica ) in Germany; however, it is unclear how well the tree holes fulfil the bees’ requirements and whether honeybees discriminate among black woodpecker cavities according to their characteristics. Here we report on the tree diameter, cavity height above the ground, cavity entrance direction, cavity entrance size, cavity volume, and relative entrance position of N = 20 black woodpecker cavities, half of which were used at least once by honeybees within a previous 2 to 4-year monitoring period. Black woodpecker cavities chosen by honeybee swarms had higher volumes (median volume 41 L vs. 19 L) and smaller entrances (44 cm 2 vs. 67 cm 2 ) than cavities not recently chosen by bees, and entrances were positioned at the bottom rather than in the middle or top of the cavity body. We conclude that black woodpecker cavities can serve as suitable nest sites for honeybees, provided they have time to enlarge through secondary decay. The mapping, labelling, and long-term preservation of black woodpecker cavity trees can promote populations of wild-living honeybees in managed forests, which are otherwise largely depleted of suitable tree cavities.