Nest webs: A community-wide approach to the management and conservation of cavity-nesting forest birds

Abstract

We propose that cavity-nesting bird communities are structured in nest webs analogous to food webs, where interspecific and intraspecific interactions are centered around nest site availability. Our sites of mixed deciduous coniferous forest in north-central BC, Canada, support a rich diversity of eight species of primary cavity nesters (PCNs) (woodpeckers), four species of weak cavity nesters (nuthatch, chickadees) and 20 secondary cavity-nesting species (ducks, passerines and birds of prey). Richness varied across forest-types with some plots being `hotspots' and others being depauperate. Across a range of forest-types, we measured species richness and abundance of birds and squirrels using point count census techniques, playbacks of taped calls of woodpeckers and owls, and by searching for active nests. We also measured resource use (trees able to support cavities, or existing cavities) in relation to availability (tree species, abundance and habitat characteristics such as edge-type and degree of fragmentation). In both 1995 and 1996, we found significant positive correlations between the abundance of primary and secondary cavity-nesting birds, and negative correlations between both of these groups and weak cavity excavators. None of the three cavity-nesting groups was positively correlated with open-nesting (non-cavity) forest birds, rejecting our null hypothesis of common habitat quality. In contrast, the abundance of secondary cavity-nesting species was significantly negatively correlated with non-cavity species, possibly because these species compete for resources other than nest sites, such as invertebrate prey. Using these data, we construct a nest web for the cavity-nesting community in north-central British Columbia. This approach demonstrated strong and weak links among species in the web and identified key species whose presence may be critical to the integrity of the community. We plan to test how nest web structure shifts in response to different forest-cutting regimes, and whether changes in species richness and abundance can be ameliorated with selective harvesting.