Long-Term Trends in Bird Populations on an Electric Transmission Right-Of-Way

Vegetation on the ROW

White ash (Fraxinus americana), sassafras (Sassafras albidum), black cherry (Prunus serotina), and eastern redcedar (Juniperus virginiana) were common trees in border zones, whereas white ash and sassafras were abundant trees in wire zones (Yahner et al. 2002a). Common shrubs on the ROW were blackberry (Rubus allegheniensis) and Japanese honeysuckle (Lonicera japonica); the dominant forb was goldenrod (Solidago spp.).

Treatments on the ROW

We selected five treatment units for study: mowing (0.58 ha), foliage spray (0.50 ha), mowing plus herbicide (0.79 ha), stem–foliage spray (0.60 ha) (Figure 2), and handcutting (0.27 ha). The total acreage sampled on the ROW was 2.74 ha, with approximately 50% each in wire zones and border zones. These were the same treatment units used in previous sampling of bird populations in 1987 and 1988 (Bramble et al. 1992).

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Figure 2.

A stem–foliage spray unit on the Green Lane Research and Demonstration Area. The border zone is on the left, and the wire zone is on the right (photo by R. Yahner, May 2002).

Maintenance treatments were applied in July 1999 (details of previous treatments can be found in Yahner et al. 2002a). Briefly, all undesirable trees were cut in wire and border zones of the handcutting unit and in border zones of other units. Wire zones in each unit (excluding handcutting) were mowed and/or treated with herbicides. The mowing unit was shrub–forb cover type in both wire and border zones. The stem– foliage spray unit was forb–grass–shrub cover type in the wire zone and shrub– forb cover type in the border zone. The mowing plus herbicide unit was grass– forb–shrub cover type in the wire zone and shrub–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. Lastly, the handcutting unit was characterized by shrub–tree–forb cover type in both wire and border zones.

Bird Surveys on the ROW

Bird surveys were conducted for 6 days each in spring (late May to early June) and summer (late July to early August). The spring corresponds to the breeding season, and the summer is the period when young are fledged from the nest and are typically associated in family groups with their parents. Approximately the same schedule was followed in both years. A minimum of 2 years of data is important because of year-to-year differences that may occur in population studies resulting from weather or other biological phenomenon.

Surveys were conducted from sunrise to about 10:00 A.M. (1000 hours). All birds seen or heard were noted to species; if possible, sex and age of each bird were also recorded. The location of each bird (i.e., treatment unit used and whether it was in the wire or the border zone) was determined. The average total number of birds of all species combined per day and the average number of individual birds per species per day were calculated on a per-hectare basis for each treatment (n = 5 treatments) and for each zone (wire versus border) in each season.

Diversity and Relative Abundance in Spring Versus Summer

Thirty-nine and 30 bird species were observed in spring and summer 2001 and 2002, respectively, on the Green Lane Research and Demonstration Area (Table 1*). In both seasons, the three most common species were field sparrow (scientific names are given in Table 1), common yellowthroat, and indigo bunting. In spring, prairie warbler, northern cardinal, American goldfinch, gray catbird, and eastern towhee were also abundant. In summer, additional common species included gray catbird, house wren, eastern towhee, northern cardinal, and American goldfinch. Each of these species is adapted to early successional habitats or edge habitats (Fergus 2000), which is characteristic of habitat created by the wire zone/border zone method of vegetative management. Similarly, early successional bird species (e.g., field sparrow, common yellowthroat) were common on the State Game Lands 33 right-of-way in Centre County, Pennsylvania (Yahner et al. 2002b). Early successional habitats are critical to populations of these bird species (Yahner 2000a, 2000b; Hunter et al. 2001), particularly because habitat in Pennsylvania and in the northeastern United States is beginning to revert from abandoned farmland to mature forest (Litvaitis 1993; Yahner 2000a; Hunter et al. 2001).

Bird populations in 2001 and 2002 were quite similar to bird observed in 1987 and 1988, except for subtle differences. For example, 44 bird species (counting the “Brewster’s” warbler as a separate “species”) were observed in the 2001– 2002 era compared to 42 species in the 1987–1988 era (Bramble et al. 1992). Two common species on the ROW in spring during both eras, blue-winged warbler (and the hybrid, “Brewster’s” warbler) and prairie warbler, fledged young and disbanded family groups before the summer counts. These warblers produce one brood each year and then migrate south for winter relatively early in the summer (e.g., Fergus 2000). However, the “Brewster’s” warbler [hybrid between blue-winged and golden-winged warbler (Vermivora chrysoptera)] was not seen on the ROW in the 1987–1988 era, but the golden-winged warbler was uncommon.

Infrequently occurring species noted on the ROW in the 1987–1988 era [e.g., mockingbird (Mimus polyglottus)], were seen near the ROW in the 2001–2002 era; conversely, wild turkey were seen on the ROW in 2001 and 2002 but only in the vicinity of the ROW in 1987 and 1988 (Table 1) (Bramble et al. 1992). Brown-headed cowbirds, which are brood parasites, remained low in numbers in both 1987–1988 and 2001–2002 eras (Bramble et al. 1992); that is, the presence of the ROW did not increase cowbird abundance in the area (see Yahner 1995). One notable change, however, was the presence of fish crows on the ROW (Table 1). This species has steadily increased its numbers and geographic range throughout Pennsylvania and the U.S. northeast in recent years (Fergus 2002).

Total abundance of all species combined was relatively similar between spring (1,572 birds/100 ha/day) and summer (1,630 birds/100 ha/day) (Table 1). A factor contributing to a somewhat higher total abundance in summer than in spring was the presence of many family groups on the ROW in summer (Table 2). Family groups of 15 species were observed on the ROW, totaling 252 family groups/100 ha/day. The most frequently encountered family groups were those of common yellowthroat, gray catbird, field sparrow, house wren, indigo bunting, eastern towhee, and northern cardinal. In addition, family groups of woodland species (e.g., black-capped chickadee and tufted titmouse) were observed on the ROW. Similar results were found on the State Game Lands 33 Research and Demonstration Area in Centre County, Pennsylvania (Yahner et al. 2002b). Other recent studies have shown that early successional habitats are important to family groups or fledgling forest birds as sources of food and cover (e.g., Pagen et al. 2000). Presumably, many family groups of these 15 species were present on the ROW in summer because of abundant arthropods (insects, etc.) as food for young (e.g., Yahner et al. 2002b). In several cases during summer, adult birds were noted with food in their beaks, which probably was intended for young fledgling birds in the immediate vicinity (R. Yahner, personal observation). Thus, the Green Lane Research and Demonstration Area, which is managed via the wire zone/border zone method, provided food and cover for numerous family groups of early successional, edge, and woodland bird species.

Several bird species were not seen on the ROW but were noted within the immediate vicinity (<200 m) or flying over the ROW in 2001 or 2002. These were great blue heron (Ardea herodias), turkey vulture (Cathartes aura), Cooper’s hawk (Accipiter cooperii), black-billed cuckoo (Coccyzus erythropthalmus), hairy woodpecker (Picoides villosus), pileated woodpecker (Dryocopus pileatus), tree swallow (Tachycineta bicolor), mockingbird, white-crowned sparrow [Zonotrichia leucophyrs (migrant)], and rose-breasted grosbeak (Pheucticus ludovicianus).

Diversity and Relative Abundance Among Treatments

The number of species per unit varied from 23 in the foliage–spray unit to 33 in the mowing plus herbicide unit (Table 3). The number of species per unit may be partially a function of unit area, with mowing plus herbicide being the largest unit. Total abundance of all species combined was highest in the handcutting unit (2,571 birds/100 ha/day) versus other units (1,246 to 1,764 birds/100 ha/day). Also, high abundance in the handcutting unit was attributed in part to the use of the transmission tower in the unit by fish crows (Table 4) and to the patchy nature of the handcutting unit. This unit was characterized by shrubs and open herbaceous areas, unlike the more homogeneous, shrubby handcutting units found on the State Game Lands 33 Research and Demonstration Area (Yahner et al. 2002b). Interesting, the handcutting unit was the least suitable unit to birds in a previous study (Bramble et al. 1992).

Diversity and Relative Abundance in Wire Versus Border Zones

Nearly four-fifths (79%) of the bird observations on the Green Lane ROW in 2001 and 2002 were in border zones, despite border zones being approximately equal in area to that of wire zones (Table 5). This finding attests to the importance of this zone to bird populations and to the relevance of the wire zone/border zone method as a sound wildlife management practice along rights-of-way. Border zones tended to be more often used by birds in the two mowed units (mowing and mowing plus herbicides) than in the other units (88% versus 66% to 75%, respectively).

Nonmowed units were characterized by some shrubs in the wire zone, which were used as perch sites by birds (Figure 3). Hence, a combination of wire zones and border zones, regardless of treatment type, provides a variety of habitats for numerous bird species adapted to various vegetative cover types along a right-of-way.

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Figure 3.

A common yellowthroat using a perch site on the Green Lane Research and Demonstration Area (photo by R. Yahner, June 2002).

Of the total number of bird observations (n = 1,059) on the Green Lane ROW in both years, 53 (5%) were birds perched on transmission wires or on transmissionline towers (Table 4). Thus, 11 bird species used wire zones to some extent because of the presence of artificial substrates (e.g., either wires or towers, rather than vegetation created by the right-of-way management). Only the mourning dove used the wires as perch sites, whereas the other ten species used the towers as perch sites.

In summary, based on this 2-year study, the bird community on the Green Lane Research and Demonstration Area is quite diverse on all treatment units. In addition, despite continued mechanical and herbicidal maintenance of the right-of-way, the bird community has changed very little over the past 15 years. Bird species on the ROW are adapted to early successional or edge habitats created by the wire zone/border zone method; species in the summer also include those that breed in woodlands but use the ROW as foraging sites. This study also confirms the importance of the border zone as habitat to birds in both spring and summer. The presence of many family groups of birds supports the fact that food resources are common on the ROW. We strongly recommend the use of the wire-zone border-zone method for rights-of-way management.