Abstract
The long-term nesting ecology of birds was studied during 2002 and 2003 on the State Game Lands (SGL) 33 Research and Demonstration Area, which is located along a 230-kV transmission right-of-way (ROW) of FirstEnergy (Penelec) in the Allegheny Mountain Region, Centre County, Pennsylvania, U.S. The objectives of this study were to compare nest abundance, success, and placement (1) in handcut versus herbicide-treated study sites (units) and (2) in wire versus border zones. In addition, results from this study were compared to those obtained in a previous study conducted in 1991–1992 on the ROW to better understand the long-term effects of vegetation maintenance management on wildlife. Thirty-three and 26 nests of 10 bird species were noted in 2002 and 2003, respectively. The most frequently encountered nests in 1991–1992 and 2002–2003 were those of bird species adapted to early successional habitats, for example, eastern towhee (Pipilo erythrophthalmus), created by the wire–border zone method of vegetation maintenance on the ROW. Thirteen (39%) of 33 nests of all species combined fledged young in 2002 compared to 17 (65%) of 26 nests in 2003. Nesting success in 2003 on the SGL 33 ROW was typical of most studies of bird nesting success in a variety of habitats and was comparable to that recorded in 1991–1992. The low-volume basal unit was more important as nesting habitat than either handcut or mowing plus herbicide units, with nine species nesting in the low-volume basal unit versus only four species in each of the other two units. Thirty-five (59%) of the 59 nests on the ROW were in wire zones, whereas 24 (41%) nests were in border zones. In conclusion, mowing plus herbicide treatment on a ROW may be the best application of the wire–border zone method in terms of resistance to seedling invasion of undesirable trees, cover-type development in the wire zone, and its value as wildlife habitat. Because early successional habitat is becoming less common in the eastern United States and because species dependent on these habitats are showing populations declines, the maintenance of a ROW via the wire–border zone method is extremely valuable to the long-term conservation of early successional bird species.
The State Game Lands (SGL) 33 Research and Demonstration Project in Centre County, Pennsylvania, U.S., has been studied since 1953, making this 51-year-old project the longest continuous project documenting the effects of mechanical and herbicidal maintenance on flora and fauna along an electric transmission right-of-way (ROW) (Yahner et al. 2002a; Vistas 2003). Transmission ROW are linear corridors that often traverse contiguous forests, thereby making these ROW extremely valuable for bird species requiring early successional habitats (Bramble et al. 1992a, 1994; Yahner et al. 2002a; Yahner 2003a). For instance, most nests found on the SGL 33 ROW in 1991–1992 were those of early successional species, including field sparrow (Spizella pusilla), gray catbird (Dumetella carolinensis), eastern towhee (Pipilo erythrophthalmus), common yellowthroat (Geothlypis trichas), and indigo bunting (Passerina cyanea) (Bramble et al. 1994).
Early successional habitat has become less common in the forests of the eastern United States in recent decades (Trani et al. 2001), and the maintenance of a ROW via the wire–border zone method creates early successional habitat for bird populations (e.g., Yahner et al. 2002b). Thus, because bird species adapted to early successional habitat have experienced population declines over recent decades in the northeastern United States (Robbins et al. 1989; James et al. 1996; Yahner 2000, 2003a; Askins 2001; Brawn et al. 2001), a ROW, if properly maintained using the wire–border zone method, represents important nesting habitat for many bird species.
In the present study, we examined the long-term response of breeding birds to ROW vegetation maintenance on SGL 33 in the Allegheny Mountain Region of central Pennsylvania in 2002 and 2003. The objectives of this study were to compare nest abundance, success, and placement (1) in handcut versus herbicidal-treated study sites (units) and (2) in wire versus border zones. In addition, results from this study then were compared to those obtained in a previous study conducted on the ROW in 1991–1992 (Bramble et al. 1994) to better understand the long-term effects of ROW maintenance on bird nesting ecology.
METHODS
The wire–border zone method of vegetation maintenance was initiated on the SGL 33 ROW in 1987 (Bramble et al. 1992a) (Figure 1). This method of vegetation maintenance along a ROW is designed to produce a tree-resistant forb–grass–shrub cover type in wire zones while simultaneously maintaining a tall shrub cover type in border zones. As a result, the wire–border zone method creates a diverse wildlife habitat on the ROW, with low-lying vegetation in wire zones and taller vegetation in border zones to produce habitat diversity. The total area of wire zones was approximately equal to that of border zones.
Diagram of a 230-kV electric transmission line, showing wire and border zones. A combination of a low-growing forb–shrub–grass cover type develops in the wire zone, and a tall shrub cover type occurs in the border zone. Adjacent to the border zone is mature forest.
Three major principal study sites (units) were selected for study: handcut [1.2 ha (3 ac)], low-volume basal spray [1.1 ha (2.7 ac)], and mowing plus herbicide [0.8 ha (2 ac)]. These were the same units used in a previous study of bird nesting ecology in 1991 and 1992 (Bramble et al. 1994). As time permitted, nest searches were conducted in five other units to give additional data on bird nesting ecology: two low-volume basal spray units, a mowing plus herbicide unit, a stem-foliage unit, and a foliage-spray unit.
White oak (Quercus alba) was the most abundant undesirable tree species in the handcut unit (both wire and border zones), and northern red oak (Q. rubra) was the most common tree in border zones of other units (Yahner et al. 2002a). Red maple (Acer rubrum) was a minor component in handcut units, but it was an important tree species in the other units. The most important cover types were forb-grass-shrub in wire zones of all units. Shrubs were the most important cover type in border zones of all units, although forbs also were important in border zones.
In the handcut unit, woody vegetation in the wire zone was cut to a 10 cm (4 in.) height, and trees only were cut selectively in border zones in 2000 (Yahner et al. 2002a, 2002b). Also, in this same year, a low-volume basal spray using Garlon 4 (25%) in basal oil (75%) was applied in wire zones in all herbicidal-treated units to trees and tall shrubs [(including witch-hazel (Hammamelis virginiana) and bear oak (Quercus ilicifolia)] visible above the low herbaceous vegetation. The same low-volume basal spray was applied to undesirable trees in border zones, except to low-growing trees and shrubs important to wildlife (e.g., witch-hazel and bear oak).
Nest searches were conducted systematically from late May through early August 2002-2003 at 3- to 4-day intervals in the three principal units and less frequently in the other units. Two to three researchers slowly searched for nests, while watching for parental birds flushing from nests, carrying nest material, or feeding young. When a nest was found, the bird species that constructed the nest, date in which the nest was found, nest location [unit, wire versus border zone, height (cm) above ground], substrate used for the nest, and status of the nest (number of eggs, etc.) were noted. The status of each nest was monitored at 3- to 4-day intervals to determine nesting success and was considered successful if at least one young was fledged (Yahner 1991; Bramble et al. 1994).
Nest predation is the major factor affecting nesting success in most songbird populations (Heske et al. 2001). Potential predators of nests on the ROW included snakes (Yahner et al. 2001a), white-footed mice (Peromyscus leucopus) and eastern chipmunks (Tamias striatus) (Bramble et al. 1992b; Yahner 2003b), and a variety of larger mammals (Yahner 1991; Yahner et al. 2001b). When possible, the type of predator on a nest was determined based on nest appearance and mode of disturbance (e.g., peck holes) (Rearden 1951; Yahner and Scott 1988; Hernandez et al. 1997).
RESULTS AND DISCUSSION
Nest Abundance and Success
Thirty-three and 26 nests of 10 bird species were noted during 2002 and 2003, respectively (Table 1). The most frequently encountered nests were those of indigo bunting (n = 17), field sparrow (n = 12), gray catbird (n = 7), eastern towhee (n = 6), and common yellowthroat (n = 4). These are birds adapted to early successional habitats created by the wire–border zone method of vegetation maintenance on the ROW (Yahner et al. 2002b). These same bird species were the most common nesting species on the SGL 33 ROW in 1991–1992 (Bramble et al. 1994).
Nesting bird species and number of nests established (number of nests successfully fledging young in parentheses) in 2002, 2003, and both years combined on the right-of-way on the State Game Lands 33 Research and Demonstration Area, Centre County, Pennsylvania.
We found more nests on the ROW in 2002 than in 2003, in part, perhaps because of relatively colder ambient temperatures in 2003, which delayed plant phenology and, hence, reduced nest-site availability and concealment for breeding birds (R. Yahner, personal observation). Low ambient temperatures and resultant delays in vegetation growth can retard the onset of nest-building behavior in birds (Pettingill 1985). Similarly, fewer nests and apparent delays in nest building were observed on another ROW in southeast Pennsylvania in 2003 (Green Lane Research and Demonstration Area, Montgomery County; Yahner et al. 2003).
Only 13 (39%) of 33 nests of all species combined fledged young during 2002 compared to 17 (65%) of 26 nests during 2003 on the ROW (Table 1). No nests were parasitized by the brood parasite, the brown-headed cowbird (Molothrus ater) (Yahner 1995a). In 2002, a major nest predator, the eastern chipmunk, was extremely abundant regionally and was believed to have a major impact on avian nesting success on the SGL 33 ROW and elsewhere in central Pennsylvania (Yahner 2003b). However, nesting success in 2003 on the SGL 33 ROW was quite good, because nesting success of approximately 50% is typical of most studies of bird nesting success in a variety of habitats (Yahner 1991).
Forty-two (71% of total) nests were found in the three principal treatment units (Table 2). Clearly, the low-volume basal spray unit was more important as nesting habitat than either handcut or mowing plus herbicide units, with nine species nesting in the low-volume basal spray unit versus only five species or less in each of the other two units (Figure 2). Moreover, more than twice the number of nests per area were noted in the low-volume basal spray unit [11.4 nest/ha/yr (4.6 nests/ac/yr)] than in the mowing plus herbicide unit [5.0 nests/ha/yr (2 nests/ac/yr)]; in turn, the number of nests per area in the mowing plus herbicide unit was 25% greater than that of the handcut unit [3.8 nests/ha/yr (1.5 nests/ac/yr)]. Thus, based on 2 years of data and results from a previous study (Bramble et al. 1994), we view the low-volume basal spray unit as the best nesting habitat and the handcut unit as the least suitable of the three units studied intensively. The low-volume basal spray unit was extremely heterogeneous in plant species composition and structure compared to handcut and mowing plus herbicide units (Yahner et al. 2002a, 2002b). Furthermore, in both years, oak sprouts, which were common in the handcut unit, leafed-out quite late compared to other species, making this plant suitable as a nest site only after mid-June.
Nesting bird species, number of nests established in both years combined for each of the three principal treatment units, and number of nests per unit area for each unit on the right-of-way on the State Game Lands 33 Research and Demonstration Area, Centre County, Pennsylvania.
An eastern towhee nest in a low-volume basal spray unit (photo taken by R. Yahner, May 2002).
Plants Used As Nest Sites
Twelve plant species were used as nest sites (Table 3). The major plants used were blackberry (Rubus allegheniensis), witch-hazel, blueberry (Vaccinium spp.), and sweet fern (Comptonia peregrina). Blackberry was common in the wire zone of the low-volume basal spray unit, blueberry and sweet fern were common in wire zones of both low-volume basal spray and mowing plus herbicide units, and witch-hazel was common in border zones of all units (Yahner et al. 2002a).
Number of nests established per bird species in various plant species on the right-of-way on the State Game Lands 33 Research and Demonstration Area, Centre County, Pennsylvania.
Indigo bunting and chestnut-sided warbler (Dendroica pensylvanica) preferred blackberry as nest sites (Table 3). Similar findings were noted in 1991–1992 on the SGL 33 ROW (Bramble et al. 1994). Indigo buntings also used relatively small [< 1 m (3 ft) in height)] witch-hazel for nest placement. On the other hand, nests of field sparrow were found most often in low-growing blueberry and sweet fern. Thus, a diversity of plant species provided suitable nest sites for a diverse breeding bird community on the ROW.
Use of Wire Zones Versus Border Zones
Thirty-five (59%) of the 59 nests on the SGL 33 ROW were in wire zones, whereas 24 (41%) nests were in border zones (Table 4). A primary reason for most nests occurring in wire zones was the presence of common nests sites (e.g., blackberry, blueberry, and sweet fern). The second most common nesting species, the field sparrow (Table 4), placed nests exclusively in wire zones; thus, if field sparrow nests were eliminated from the data set, approximately the same number of nests occurred in wire zones as in border zones (23 vs. 24 nests, respectively) (Table 4). As nesting habitat, therefore, we strongly recommend that border zones be established on a ROW, which, in combination with wire zones (Figure 1), provide abundant and diverse nest sites for the nesting bird community (Bramble et al. 1994; Yahner et al. 2002b).
Number of nests established per bird species in wire versus border zones in both years combined on the right-of-way on the State Game Lands 33 Research and Demonstration Area, Centre County, Pennsylvania.
CONCLUSIONS
The State Game Lands 33 Research and Demonstration Area continues to provide nesting habitat for a variety of bird species. Native vegetation (shrubs and forbs) in both wire and border zones serves as excellent nest sites. Compared to other units, low-volume basal areas appear to be very good locations for placement of bird nests. However, given the probability of a “blackout” of electrical power if wires come in contact with tall trees, mowing plus herbicide treatment on a ROW may be the best application in terms of cover-type development in wire zones, resistance to seedling invasion of undesirable trees, and its value as wildlife habitat (e.g., Bramble et al. 1990). Cover type in mowing plus herbicide units consists of a grass–forb–shrub combination, which has restricted tree invasion to 185 trees/ha (75 trees/ac) compared to 495 trees/ha (200 trees/ac) in other units on the SGL 33 ROW (Yahner et al. 2002a). This study and others (e.g., Bramble et al. 1992a; Yahner et al. 2002b) have shown that mowing plus herbicide is simultaneously valuable to many wildlife species.
High rates of nesting success on the ROW, particularly in 2003, attest to the fact that this linear corridor of early successional habitat does not result in a dramatic increase in rates of nest predation (Gates and Gysel 1978; Yahner 1995a, 1995b, 2000). Instead, because early successional habitat is becoming less common in the eastern United States (Trani et al. 2001) and because species dependent on these habitats are showing populations declines in recent decades (Yahner 2000, 2003a; Askins 2001; Brawn et al. 2001), the maintenance of a ROW via the wire–border zone method is extremely valuable to the long-term, regional conservation of early successional species. For instance, indigo bunting and field sparrow, which are the two most abundant species nesting on the SGL 33 ROW, are among the bird species showing significant statewide population declines according the breeding bird surveys conducted by the U.S. Fish and Wildlife Service (www.mbr-pwrc.usgs.gov/bbs/bbs/html).
Acknowledgments
Cooperators were Asplundh Tree Expert Tree Company, Dow AgroSciences, FirstEnergy (Penelec), and Pennsylvania Game Commission. A sad note—Mr. Steve Liscinsky passed away in 2003.
- © 2004, International Society of Arboriculture. All rights reserved.
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