CERULEAN WARBLER (DENDROICA CERULEA) MICROHABITAT AND LANDSCAPE-LEVEL HABITAT CHARACTERISTICS IN SOUTHERN WEST VIRGINIA

Auk, The, Apr 2005 by Weakland, Cathy A, Wood, Petra Bohall

ABSTRACT

The Cerulean Warbler (Dendroica cerulea) is a species of conservation concern in eastern North America, where declines in its population have been documented over the past several decades. A high proportion of the population occurs in forested areas of southern West Virginia, where it may be threatened by loss and degradation of forested habitat from mountaintop mining and valley fill (MTMVF). We examined, from a landscape perspective, the effects of forest fragmentation (in particular, effects of fragment size and response to edges) on Cerulean Warblers, using territory mapping techniques and geographic information system (GIS) technology in portions of four counties in southwestern West Virginia. We quantified landscape characteristics from digitized aerial photographs and measured microhabitat characteristics on spot-mapping plots. Territory density of Cerulean Warblers was 4.6 territories per 10 ha in intact forest and 0.7 territories per 10 ha in fragmented forest. The best habitat model included both landscape and microhabitat variables and indicated that territory density increased with increasing snag density, percentage of canopy cover >6-12 m and >24 m in height, and distance from mine edge. Models for predicting microhabitat use at the territory level were weak, indicating that microhabitat characteristics of territories were similar to habitat available on spot-mapping plots. The species did not appear to avoid internal edges, such as natural canopy gaps and open-canopy or partially open-canopy roads. Territory placement on ridges was greater than expected, and in bottomlands (ravines) and midslopes less than expected, given availability. Fifty percent of all territories were on ridges. Preference for ridges suggests that MTMVF may have a greater effect on Cerulean Warbler populations than other sources of forest fragmentation, given that ridges are removed in MTMVF. Our data indicate that Cerulean Warblers are negatively affected by mountaintop mining from loss of forested habitat, particularly ridgetops, and from degradation of remaining forests, as evidenced by lower territory density in fragmented forests and lower territory density closer to mine edges. Received 20 February 2003, accepted 1 November 2004.

Key words: Cerulean Warbler, Dendroica cerulea, habitat, mining, territory density.

Características del Microhábitat y del Hábitat a Nivel de Paisaje de Dendroica cerulea en el Suroeste de Virginia

RESUMEN.-Dendroica cerulea es una especie con un estado de conservación preocupante en el este de Norteamérica, donde se ha documentado una disminución de sus poblaciones durante las últimas décadas. Una alta proporción de la población se encuentra en las áreas boscosas del suroeste de Virginia, donde las causas de la amenaza pueden estar relacionadas con la pérdida y degradación del hábitat boscoso debido a actividades mineras en las amas de los cerros y el relleno de los valles. Desde una perspectiva a nivel del paisaje, examinamos los efectos de la fragmentation del bosque (en particular los efectos del tamaño de los fragmentos y las respuestas a los bordes) sobre D. cerulea utilizando técnicas de mapeo de territories y tecnología de sistemas de información geográfica (SIG) en partes de cuatro condados del suroeste de Virginia. Cuantificamos las características del paisaje a partir de fotografías aéreas digitalizadas y medimos las características del microhábitat en los sitios en que se realizó el mapeo de los territorios. La densidad de territorios de D. cerulea fue de 4.6 territorios por 10 ha en bosques intactes y de 0.7 territorios por 10 ha en bosques fragmentados. El mejor modelo de hábitat incluyó tanto variables del microhábitat como del paisaje, e indicó que la densidad de territorios aumentó con la densidad de árboles vivos en pie, el porcentaje de cobertura del dosel entre 6-12 m y >24 m de altura y la distancia al borde de la mina. Los modelos para predecir el uso de hábitat a nivel de territorio fueron débiles, Io que indica que las variables de microhábitat de los territorios fueron similares al hábitat disponible en los sitios en que se realizó el mapeo de los territorios de las aves. Esta especie no pareció evitar los bordes internos, taies como aberturas naturales en el bosque, aberturas en el dosel o caminos con dosel semi-abierto. El establecimiento de territorios en las cimas fue mayor de lo esperado y en el fondo de las quebradas y laderas fue menor de lo esperado con relación a la disponibilidad. El 50% de todos los territorios estuvieron en las cimas. La preferencia por las cimas sugiere que las actividades mineras en las rimas de los cerros y el relleno de los valles pueden tener un mayor efecto sobre las poblaciones de D. cerulea que otras fuentes de fragmentación del bosque, dado que las cimas son removidas por las actividades mineras. Nuestros datos indican que las poblaciones de D. cerulea están siendo afectadas negativamente por las actividades mineras en la rima de los cerros debido a la pérdida de habitat boscoso, particularmente en las cimas de las montañas, y por la degradación del bosque remanente. Esto se evidencia en una menor densidad de territorios en los bosques fragmentados y cerca de los bordes con las minas.

CERULEAN WARBLERS (Dendroica cerulea) have been declining in many parts of their range (Sauer et al. 2004); a status assessment (Hamel 2000) suggested that the population is declining at "precipitous rates." The species has been identified by Partners in Flight as a priority for conservation in the upland forest community of the Ohio Hills and Northern Cumberland Plateau physiographic areas, which include southern West Virginia. Those two physiographic areas contain a large proportion of the total Cerulean Warbler population (Rosenberg et al. 2000). Southwestern West Virginia may represent a significant source population for this species in the eastern United States (Rosenberg and Wells 2000).

The primary threat to the species is loss and degradation of habitat (Hamel 2000, Hamel et al. 2004). In both the Ohio Hills and Northern Cumberland Plateau physiographic areas, a current potential risk to Cerulean Warbler populations is the coal-mining technique of mountaintop mining and valley fill (MTMVF). With this mining technique, rock and soil overburden is removed from an entire ridgetop to expose coal seams and is placed in head-ofhollow streams, creating valley fills. Once the coal is removed, reclamation includes returning some of the overburden to ridgelines, contouring valley fills, and seeding the site primarily to grasses and herbaceous vegetation. These extensive surface mines can affect areas on the order of 2,000 ha, converting a landscape that is predominantly forested to a landscape of predominantly early-successional habitats with remnant forest fragments (P. B. Wood et al. unpubl. data). It is imperative to understand how these landscape-level changes affect Cerulean Warblers, a species that inhabits large tracts of mature deciduous forest with large, tall trees. The species appears to use edges of small canopy gaps within large tracts; however, its use of openings and edges needs further study. High-priority research needs identified for Cerulean Warblers include occurrence and density in relation to landscape characteristics, especially in relation to forest fragmentation, habitat preferences in relation to vegetation structure, and response of populations to land management activities (Hamel 2000).

The major effects of MTMVF on Cerulean Warblers potentially include both loss and degradation of forested habitat. Large areas of mature hardwood forest are converted to earlysuccessional habitat, resulting in outright loss of forested habitat. Remaining forest patches may be degraded because of fragmentation, area, and edge effects. Currently, there are no published studies documenting the effects of MTMVF on forest-dwelling songbirds as forests are lost and fragmented by mining activities. However, because of the large size of most MTMVF areas, it is possible that they have severe negative effects on populations of forest interior species, such as the Cerulean Warbler, that require large blocks of unfragmented forest for breeding. Duration of the habitat loss or fragmentation will depend on which post-mining land use is selected for an area. Nontimber post-mining land uses, such as grazing or development, will result in permanent fragmentation of forest habitats. Even with timber as the post-mining land use, it is estimated that many hundreds of years may be required for a functioning forest to regenerate on such sites.