Bird Populations Associated With an Electric Transmission Right-Of-Way

Vegetation on the ROW

Oak (Quercus spp.) and red maple (Acer rubrum) were common trees in border zones of the ROW and in the adjacent forest (e.g., Bramble et al. 1992; Yahner et al. 2001a). Common shrubs on the ROW were blackberry (Rubus allegheniensis), dewberry (Rubus spp.), blueberry (Vaccinium spp.), witchhazel (Hamamelis virginiana), and sweet fern (Comptonia peregrina). Abundant forbs included goldenrod (Solidago spp.), hayscented fern (Dennstaedtia punctilobula), and bracken fern (Pteridium aquilinum). Major grasses were poverty grass (Danthonia spicata), meadow fescue (Fescue elatior), and a mix of other grasses.

Treatments on the ROW

Six treatment units were selected for study: handcutting, low-volume basal spray, mowing plus herbicide, stem-foliage spray, foliage spray, and mowing. These units were the same study plots used in a previous study of bird populations on the ROW in 1987 and 1988 (Bramble et al. 1992). Units varied in size from 0.81 to 1.34 ha (2 to 3.3 ac). The total area surveyed on the ROW and the adjacent forest was each 6.87 ha (17 ac).

Maintenance treatments were applied in July 2000 (details of previous treatments can be found in Yahner et al. 2001a). Briefly, the 2000 treatments consisted of handcutting undesirable trees in wire and border zones, and the application of a low volume basal spray to undesirable tree and shrub species using Garlon 4 (25%) in basal oil (75%) in wire and border zones of other units. The handcutting unit was characterized by shrub–tree–forb cover type in the wire zone and shrub–tree cover type in the border zone (e.g., Yahner et al. 2001a, 2001b). The low volume basal unit was shrub–forb cover type in wire and border zones. The mowing plus herbicide unit was forb–shrub cover type in the wire zone and shrub–forb cover type in the border zone. The stem–foliage spray unit was forb–shrub cover type in the wire zone and shrub–grass–forb cover type in the border zone. The foliage spray mowing plus herbicide unit was forb–grass–shrub cover type in the wire zone and shrub–forb cover type in the border zone. Finally, the mowing unit was shrub–forb–grass cover type in both wire and border zones.

Bird Surveys on the ROW

Bird populations were surveyed during two time periods in 2000 (a pre-treatment survey in June and a post-treatment survey in August) and two periods in 2001 (June and August). During each time period, birds were counted on 6 consecutive days along the ROW. Counts were made between sunrise and 1000 hours. All birds seen or heard were noted; the location of each bird was recorded in relation the type of treatment unit and whether it was in the wire or border zone.

Differences Between Seasons and Between ROW and Adjacent Forest

Forty-four species were observed on the State Game Lands 33 Research and Demonstration Project during the 2 years on the ROW and adjacent forest combined (Table 1). In 1987 and 1988 combined, 39 species were noted on the ROW (Bramble et al. 1992); thus, in terms of numbers of species, the bird community on the ROW has changed relatively little over the past 13 to14 years.

In both 2000 and 2001, slightly more species occurred on the ROW in summer (n = 25–33) than in spring (n = 25–26) (Table 1). In contrast, considerably fewer species were noted in the adjacent forest in both spring (n = 8–13) and summer (n = 7). A greater number of species was also found on the ROW than in the adjacent forest during spring and summer in a previous study of bird populations on the State Game Lands 33 Research and Demonstration Project (Bramble et al. 1992).

The abundance of all species combined (number of individuals/100 ha/day) was nearly sevenfold higher on the ROW compared to the adjacent forest (Table 1). Moreover, abundance was much higher in 2001 than 2000; however, we have no biological explanation for this year-to-year difference. In other studies of bird populations in Pennsylvania, dramatic differences in bird populations have been noted between consecutive years (e.g., Yahner and Smith 1990), thereby attesting to the importance of more than one field season of data collection in ecological studies.

The most common species (≥50 individuals/100 ha/day) on the ROW in spring were chestnut-sided warbler (Dendroica pensylvanica), American redstart (Setaphaga ruticilla), common yellowthroat (Geothlypis trichas), eastern towhee (Pipilo erythropthalmus), field sparrow (Spizella pusilla), and indigo bunting (Passerina cyanea). Common bird species on the ROW in spring were mainly those adapted to early successional habitats in general, including those characteristic of habitat created by even-aged forest management in nearby (<20 km) forest stands (Yahner 1997, 2000b). These six species, plus red-eyed vireo (Vireo olivaceus) and black-capped chickadee (Poecile atricapillus), were the most abundant species on the ROW in summer. In contrast, the only common species in the adjacent forest was the red-eyed vireo. In 1987 and 1988 combined, the seven most abundant species noted on the State Game Lands 33 ROW were chestnutsided warbler, common yellowthroat, eastern towhee, field sparrow, song sparrow (Melospiza melodia), gray catbird (Dumetella carolinensis), and indigo bunting (Bramble et al. 1992). Perhaps the only notable change in abundance of individual bird species from 1987 to 1988 compared to 2000 to 2001 on the ROW was a decline in abundance of brown-headed cowbirds (Molothrus ater) in 2000–2001 (see Bramble et al. 1992). Brown-headed cowbirds are obligate brood parasites but probably have a negligible impact on nesting success of birds breeding on rights-of-way in central Pennsylvania (e.g., Yahner 1995a, 1995b).

One reason for an increase in the abundance of birds in summer versus spring on the ROW was the presence of family groups on the ROW (Table 2). Family groups of 18 species were observed, with 12 species noted in August 2000 and 17 in August 2001. The most prominent family groups (≥0.75 groups/day) were those of common yellowthroat, field sparrow, black-capped chickadee, eastern towhee, black-and-white warbler (Mniotilta varia), and red-eyed vireo. Chickadees and vireos are forest-nesting birds; family groups of these two species and other species adapted to early successional habitats were presumably on the ROW because of the abundance of arthropods (insects, etc.) as food for young throughout the growing season (e.g., Bramble et al. 1994). On several occasions, adult birds were seen in summer with food in their beaks, which probably was intended for young birds in the immediate vicinity that were recently fledged from nests (R. Yahner, personal observation). Thus, the State Game Lands 33 Research and Demonstration Project, which is managed using the wire zone–border zone method, not only provided habitat and food resources for birds nesting in early successional habitat (e.g., towhee and yellowthroat), but it also provided habitat and food resources for family groups of forest species [e.g., vireo, black-throated green warbler (Dendroica virens), ovenbird (Seiurus aurocapillus)] that shifted home ranges to include the ROW. Other recent studies also have shown that other types of early successional habitats are important to fledgling forest birds as sources of food and cover (e.g., Pagen et al. 2000).

Diversity and Relative Abundance per Treatment Unit

Forty-three of the 44 bird species recorded in this study were observed on the ROW in both years combined (Table 3). Eight species were found in each of the six treatment units, including black-capped chickadee, redeyed vireo, chestnut-sided warbler, black-and-white warbler, American redstart, common yellowthroat, eastern towhee, and indigo bunting. Most species occurred in the low-volume basal spray and foliage spray units (n = 29 and 28 species, respectively), and fewest species were noted in the handcutting unit (n = 19 species). With the exception of the abundance of birds of all species combined in the handcutting unit (552 birds/100 ha/day), abundance was at least 713 birds/100 ha/day in each of the remaining units; the greatest abundance of birds of all species combined was observed in the low-volume basal spray unit (1,361 birds/100 ha/day).

The common yellowthroat was ubiquitous as a common species (≥50 individual birds/100 ha/day) in each of the six treatment units (Table 3). In addition, other common species in decreasing order of abundance were eastern towhee and American redstart in the handcutting unit; field sparrow, indigo bunting, gray catbird, chestnut-sided warbler, eastern towhee, American redstart, song sparrow, and red-eyed vireo in the low-volume basal unit; black-capped chickadee, indigo bunting, eastern towhee, black-and-white warbler, and American redstart in the mowing plus herbicide unit; field sparrow and indigo bunting in the stem–foliage spray unit; field sparrow, eastern to-whee, red-eyed vireo, indigo bunting, chestnut-sided warbler, black-capped chickadee, and song sparrow in the foliage spray unit; and field sparrow, eastern to-whee, American redstart, chestnut-sided warbler, indigo bunting, cedar waxwing (Bombycilla cedrorum), and black-capped chickadee in the mowing unit. Thus, regardless of unit, common species were typically those adapted to early successional habitats.

Diversity and Relative Abundance in Wire Versus Border Zones

Considerably more bird species were observed in border zones than in wire zones of the ROW in 2000 and 2001 combined (n = 39 versus 17 species, respectively) (Table 4). Moreover, abundance of all bird species combined was nearly fourfold higher in border zones (1,530 individual birds/100 ha/day) than in wire zones (393 birds/100 ha/day).

Seven relatively common bird species (i.e., ≥50 birds/100 ha/day) were noted in border zones: common yellowthroat, field sparrow, eastern towhee, American redstart, indigo bunting, chestnut-sided warbler, redeyed vireo, and black-capped chickadee (Table 4). These species represent a combination of those adapted to early successional habitats and forested habitats (latter two species). Thus, the border zone is a very important habitat for birds along a ROW, with its combination of shrub cover types mixed in with forb, grass, or tree cover type, depending on the treatment unit. In contrast, only field sparrow and common yellowthroat were abundant in wire zones of the ROW.

Cooper’s hawk (Accipiter cooperii), American crow (Corvus brachyrhynchos), common raven (Corvus corax), and mourning dove (Zenaida macroura) were found exclusively in the wire zone (Table 4). These four species, plus an additional three species, were observed on towers or lines of the ROW (Table 5). Ravens, in particular, were typically observed only on towers on the ROW.

In summary, based on this 2-year study, the bird community on the State Game Lands 33 Research and Demonstration ROW is quite diverse on all treatment units. Bird species in spring are especially adapted to early successional habitat created by the wire-zone–border-zone method; species in the summer also include early successional species as well as those that breed in more mature forests but use the ROW as foraging sites. The presence of many family groups of birds supports the fact that food resources are common on the ROW.