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

许多鸟类和哺乳动物利用树洞筑巢。次级洞穴使用者争相寻找由啄木鸟等挖掘机挖出的筑巢洞，这种挖掘机被称为主要洞穴挖掘机。已经很好地描述了不同种类的洞巢鸟所使用的资源类型(树基质)和洞穴的特征。然而，对于种内和种间对筑巢地点的竞争如何影响栖息地的格局、景观上的巢间距以及森林中洞巢鸟类的繁殖成功，人们了解得很少。研究繁殖密度的后果很重要，因为许多栖息地正变得更加分散，例如森林采伐的结果。因此，剩余栖息地的密度可能会增加，鸟类可能会被迫更多地相互作用。 本研究的主要目的是研究繁殖密度和对筑巢地点的竞争对洞巢鸟类行为和繁殖成功的影响。我将利用不列颠哥伦比亚省Riske Creek使用天然树洞的不同洞穴筑巢者社区的数据来寻找巢间距和繁殖成功之间的相关性。然而，我将集中在两个常见的洞穴筑巢物种的野外实验：山青鸟和树燕，它们相互竞争筑巢地点，并随时使用巢箱。通过控制巢的密度，我将研究在保卫巢穴和觅食以及最终繁殖成功之间是否存在行为和生理上的权衡。 高密度的潜在巢穴竞争者可能会增加繁殖成本，但也可能有好处，如集体围捕捕食者。通过对巢密度和捕食者(模型猫头鹰和松鼠)在燕子和蓝鸟对巢箱中的呈现进行实验操作，我将测试相同或其他物种的邻居是否有助于群居。具体地说，我将检验这样一种假设，即丛生巢比孤立巢更安全，不受捕食者的攻击。总而言之，我的研究将增加我们对鸟类在一个日益受到碎片化影响的景观中所经历的繁殖挑战和好处的理解，并可能有助于发展森林收获模式，以减轻繁殖鸟类的压力。

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.