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Habitat selection by Cerulean Warblers in eastern Kentucky.

Article Excerpt
The Cerulean Warbler (Dendroica cerulea) is a neotropical migratory songbird that has gained widespread attention as a species of conservation and management concern. It breeds in mature, deciduous forests of eastern North America and has experienced range-wide declines over the last 40 years (Sauer et al. 2007), especially in historically high-density areas including the Cumberland Plateau (Hamel 2000b). It is currently a high conservation priority and a species of concern in 13 states and Canada (Rosenberg et al. 2000). Its management is particularly challenging in Kentucky where <8% of the land is in public ownership (Kentucky Department of Fish and Wildlife Resources 2005).

Successful management of Cerulean Warblers depends on a thorough understanding of its habitat requirements. Vegetative characteristics associated with nesting Cerulean Warblers have been examined in many parts of its breeding range (Robbins et al. 1992, Oliarnyk and Robertson 1996, Hamel 2000a, Jones and Robertson 2001) where tract size, stand age, and extent of fragmentation are thought to be important (Hamel 2000b, Weakland and Wood 2005). However, habitat use by this species remains poorly understood and sufficient regional ecological variation exists that site-specific research is needed to develop appropriate conservation plans (Hamel et al. 2004).

Our objectives were to: (1) describe Cerulean Warbler breeding habitat in high-density breeding areas on the Cumberland Plateau (~7 territories/10 ha), and (2) identify which habitat features were predictive of its distribution within the environment. We examined Cerulean Warbler habitat at two spatial scales. First, we examined its distribution within a large forest tract. Second, we examined habitat features within territories, specifically song perches.

METHODS

Study Area.--The Morehead District of the Daniel Boone National Forest (DBNF) in eastcentral Kentucky includes portions of Rowan, Bath, and Menifee counties. It is on the Pottsville escarpment at the northwest boundary of the Cumberland Plateau and the southwest boundary of the Allegheny Plateau (Smalley 1986). The Cumberland Plateau is within the core of the breeding range of the Cerulean Warbler and Kentucky has been identified as one of the largest gaps in knowledge of the breeding distribution (Rosenberg et al. 2000:10). The region is characterized by rough topography with narrow, winding ridge tops, steep slopes, and deep, narrow ravines. Ridges are oriented in a north-south direction, and elevation ranges from 212 to 394 m above sea level.

Oak-hickory (Quercus-Carya) forest typically dominates drier ridge tops while mixed mesophytic forests are on east and north-facing slopes (Overstreet 1989, Martin 2003). Canopy co-dominants include tulip poplar (Liriodendron tulipfera), sugar maple (Acer saccharum), northern red oak (Quercus rubra), white oak (Q. alba), black oak (Q. velutina), chestnut oak (Q. prinus), white ash (Fraxinus americana), shagbark hickory (Carya ovata), mockernut hickory (C. tomentosa), and pignut hickory (C. glabra). Cucumbertree (Magnolia acuminata), American basswood (Tilia americana), black walnut (Juglans nigra), and chinquapin oak (Q. muhlenbergii) can be canopy components. Flowering dogwood (Comus florida), sourwood (Oydendrum arboreum), eastern redbud (Cercis canadensis), eastern hop-horn-beam (Ostrya virginiana), and downy serviceberry (Amelanchier arborea) are frequent in the understory. Common shrubs include spicebush (Lindera benzoin), pawpaw (Asimina triloba), greenbriar (Smilax spp.), and a variety of tree saplings.

Point Counts.--We established five 1-[km.sup.2] survey grids in spring 2004 in Buffalo Branch (38[degrees] 12' N, 83[degrees] 21' W), a 1,600-ha stand of 50-100 year-old mixed hardwoods in DBNF near Morehead, Kentucky. Grids were placed randomly and point count stations within grids were established systematically at 250 m intervals. Each grid contained 20 point count locations for a total of 100 points. Grids were spaced at least 250 m apart.

We conducted point counts in 2004 to detect Cerulean Warbler presence within the Buffalo Branch study area. All bird species detected during point counts were recorded with special attention given to avian nest predator (American crow [Corvus brachyrhynchos], Blue Jay [Cyanocitta cristata], and Common Grackle [Quiscalus quiscula]) and brood parasite (Brown-headed Cowbird [Molothrusater]) abundance. Ten minute, 50-m fixed radius point counts (Ralph et al. 1993, Hamel et al. 1996) were conducted on the five grids (n = 100 point count stations). Surveys were conducted between sunrise and 1000 hrs EDT on days without rain or high wind (>20 km/hr). Point counts were conducted by three observers skilled in bird identification and trained...