Decaying trees improve nesting opportunities for cavity-nesting birds in temperate and boreal forests: A meta-analysis and implications for retention forestry

Abstract

Many studies have dealt with the habitat requirements of cavity-nesting birds, butthere is no meta-analysis on the subject and individual study results remain vague orcontradictory. We conducted a meta-analysis to increase the available evidence fornest-site selection of cavity-nesting birds. Literature was searched in Web of Scienceand Google Scholar and included studies that provide data on the habitat requirements of cavity-nesting birds in temperate and boreal forests of varying naturalness.To compare nest and non-nest-tree characteristics, the following data were collectedfrom the literature: diameter at breast height (DBH) and its standard deviation (SD),sample size of trees with and without active nest, amount of nest and available treesdescribed as dead or with a broken crown, and amount of nest and available treesthat were lacking these characteristics. Further collected data included bird speciesnesting in the cavities and nest-building type (nonexcavator/excavator), forest type(coniferous/deciduous/mixed), biome (temperate/boreal), and naturalness (managed/natural). From these data, three effect sizes were calculated that describe potential nest trees in terms of DBH, vital status (dead/alive), and crown status (broken/intact). These tree characteristics can be easily recognized by foresters. The resultsshow that on average large-diameter trees, dead trees, and trees with broken crownswere selected for nesting. The magnitude of this effect varied depending primarilyon bird species and the explanatory variables forest type and naturalness. Biome hadlowest influence (indicated by AIC). We conclude that diameter at breast height,vitality, and crown status can be used as tree characteristics for the selection of treesthat should be retained in selectively harvested forests.