Effects of breeding density on the behaviour and reproductive success of cavity nesting birds

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*** Many species of birds and mammals use tree holes for nesting. Secondary cavity users compete for nesting holes created by excavators such as woodpeckers which are the called the primary cavity excavators. The types of resources (tree substrates) and characteristics of cavities used by different species of cavity-nesters have been well described. However, there is a poor understanding of how intra- and inter-specific competition for nest sites influence the settlement patterns, nest spacing on the landscape, and the reproductive success of cavity-nesting birds in forests. Studying the consequences of breeding density is important because many habitats are becoming more fragmented, for example as a result of forest harvesting. As a result, densities may increase in the remaining habitat patches, and birds may be forced to interact more with one another. *** The main objective of my research is to study the effects of breeding density and competition for nesting sites on the behaviour and reproductive success of cavity-nesting birds. I will use data from a diverse community of cavity-nesters using natural tree holes at Riske Creek, B.C to search for correlations between nest spacing and reproductive success. However, I will focus on field experiments involving two common cavity-nesting species: Mountain Bluebirds and Tree Swallows which compete with each other for nesting sites and readily use nest boxes. By manipulating nest density, I will study whether there are behavioural and physiological trade-offs between defending a nest site and foraging, and ultimately reproductive success. *** High densities of potential nest competitors may increase reproductive costs but there may also be benefits such as communal mobbing of predators. Using experimental manipulations of nest density and presentation of predators (model owls and squirrels) at the nest boxes of swallow and bluebird pairs, I will test whether neighbours of the same, or of other species, contribute to group mobbing. Specifically, I will test the hypothesis that clumped nests are safer from predators than isolated nests. In sum, my research will increase our understanding of reproductive challenges and benefits experienced by birds in a landscape increasingly affected by fragmentation, and may aid in developing forest harvest patterns that reduce the stress for breeding birds.**