Characteristics of northern hardwood trees used by cavity-nesting birds

Characteristics of nest trees used by 4 species of cavity-nesting birds were investigated in a northern hardwood forest in Vermont. A total of 110 nests was found for the yellow-bellied sapsucker (Syphrapicus varius), hairy woodpecker (Picoides villosus), downy woodpecker (P. pubescens), and black-capped chickadee (Parus atricapillus). The 110 nest trees were compared to 440 selected non-nest trees to identify tree characteristics associated with the presence of active cavity nests. Criteria to identify potential cavity-nest trees were developed. Woodpecker and sapsucker nests were associated with live, deciduous trees with fruiting bodies of heartwood decay fungi, branch stubs, broken tops, or previously excavated cavities. Chickadees used well decayed, deciduous snags with broken tops. J. WILDL. MANAGE. 51(1):217-223 Primary cavity-nesting birds excavate their own nest holes and are dependent upon trees suitable for excavation; hence populations may be limited by potential nest sites. Previous studies have confirmed that primary cavity-nesting birds have specific requirements for nest sites (Kilham 1971, Conner et al. 1976) and that territory selection and reproductive success depend on the presence of suitable nest sites (von Haartman 1957, Kilham 1966). Shortages of potential cavity-nest sites may exist in young forests (Flack 1976) and in all ages of intensively managed forests (McClelland 1977, Carey 1983). Suitable nest sites for primary cavity-nesting birds are generally considered to be old, dead or dying, partially decayed trees. Timber management practices uch as clearcutting, timber stand improvement, short harvest rotations, and removing snags to reduce fire and safety hazards may result in elimination of cavity-nest sites from the forest. It should be expected that unmanaged timber stands will have higher densities of cavity trees and snags than managed stands (Cline et al. 1980). Furthermore, as the use of wood for energy production continues to increase, trees formerly regarded as unmerchantable will be harvested extensively. Our study was designed to provide information needed to manage for trees suitable for 'Present address: Wyoming Cooperative Fish and Wildlife Research Unit, Department of Zoology, University of Wyoming, Box 3166, University Station, Laramie, WY 82071. This content downloaded from 157.55. 39.177 on Fri, 18 Nov 2016 04:13:55 UTC All use subject to http://about.jstor. org/terms 218 CAVITY-NEST TREES * Runde and Capen J. Wildl. Manage. 51(1):1987 nesting use by primary cavity-nesting birds in a northern hardwood forest. The objective was to identify characteristics of nest trees used by 4 species of primary cavity-nesting birds. This study was funded by the Windham Found. , Grafton, Vt. and Coop. For. Res. Funds administered through the School Nat. Resour. , Univ. Vermont. We thank W. P. Roberts, S. G. Parren, F. R. Thompson, and M. R. Scott for assistance in the field. J. D. Gill and M. G. Raphael provided helpful suggestions on the manuscript. STUDY AREA AND METHODS This study took place on the University of Vermont's Grafton Forest Resource Project Area, a 1,225-ha site in the eastern foothill region of the Green Mountains of southeastern Vermont. Elevations range from 260 to 550 m. Acidic, glacial till soils predominate. Most of the study area is an all-aged mix of northern hardwoods and conifers. Hardwood cover types occur on 56% of the area, softwood types on 26%, and mixed types on 8%. Miscellaneous or agricultural types make up 10% of the area. Early forest seral stages are composed primarily of white pine (Pinus strobus), paper birch (Betula papyrifera), red maple (Acer rubrum), and quaking aspen (Populus tremuloides). Older stages are primarily American beech (Fagus grandifolia), yellow birch (B. alleghaniensis), and sugar maple (A. saccharum), with lesser amounts of Canada hemlock (Tsuga canadensis), northern red oak (Quercus rubra), white ash (Fraxinus americana), and American hophornbeam (Ostrya virginiana). Red spruce (Picea rubens), balsam fir (Abies balsamea), paper birch, and northern red oak occur on shallow soils at higher elevations. We examined active nest sites of 4 species of primary cavity-nesting birds: yellow-bellied sapsuckers, hairy woodpeckers, downy woodpeckers, and black-capped chickadees. These common birds reflect a range of excavation ability in cavity nesters of the Northeast. Hairy woodpeckers are the strongest excavators, followed by yellow-bellied sapsuckers, downy woodpeckers, and black-capped chickadees. Nest searching began 1 May and ended 15 July in 1979 and 1980. Early in the breeding season, nests were found by observing birds exhibiting territorial, nest building, or courtship behaviors. Several territories of the yellow-bellied sapsucker were located by imitating their territorial tapping. Later in the season, nests were found by listening for nestlings calling from within a nest or by observing adults carying food into a cavity. We identified "cavity trees" as those with excavated or natural cavities and classified "potential cavity trees" as those exhibiting characteristics that indicated potential for excavation. In this paper only standing dead trees >5 m tall are referred to as "snags"; dead trees 5-cm penetration by a knife blade. We noted whethr the top of each tree was broken or intact, and whether the tree was living. Adjacent nonnest trees were measured as nest trees, except no increment cores were taken, and dnh was measured at the height of the adjacent nest hole. We characterized nest trees and compared them to non-nest trees by testing with the Chisquare statistic whether the proportion of nest trees with a given characteristic was equal to the proportion of non-nest trees with the same characteristic. Differences in sizes of nest and non-nest trees were identified using Student's This content downloaded from 157.55. 39.177 on Fri, 18 Nov 2016 04:13:55 UTC All use subject to http://about.jstor. org/terms J. Wildl. Manage. 51(1):1987 CAVITY-NEST TREES * Runde and Capen 219 t-test. To provide a more rigorous 1-sided t-test for differences between upper stem diameters and total height of nest and non-nest trees, we excluded non-nest trees known to be out of the size range of active nest trees. For woodpecker species, non-nest trees were included only if dbh was >20 cm (min. dbh of woodpecker nest trees in this study). For black-capped chickadees, nonnest trees were included if dbh was <45 cm (max. dbh of chickadee nest trees in this study).