Bird Population & Nesting Research

Bird populations have been extensively studied on the State Game Lands 33 (SGL33) right-of-way (ROW) and Green Lane Research and Demonstration area (GLR&D) since 1982. Previous study of bird communities at SGL 33 and GLR&D have shown from the early 1980’s through 2006 that anywhere between 31–44 species of birds have utilized the ROW sites per year. Researchers detected between 710 and 819 individual birds representing 35–49 species per year from 2015–17 within the wire and border management zones and adjacent forest at SGL33 with the most common being those that nest in brushy or grassy vegetation created by integrated vegetation management practices. We observed between 135 and 194 individuals representing 12–16 different bird species per year within the wire management zone and an additional 211–292 individuals ranging from 14–19 bird species per year within the border management zone of the ROW. The most abundant birds within the wire and border management zones of the ROW included early successional habitat obligates such as chestnut-sided warbler, field sparrow, eastern towhee, common yellowthroat, gray catbird, and indigo bunting. Since artificial disturbances not created solely for natural resource conservation now make up a majority (approximately 80%) of early successional habitats, it is important to make informed decisions about how these areas are created and managed. Therefore, ROW maintained using integrated vegetation management (IVM) such as those at SGL 33 and GLR&D will be vital to and can be used as examples of early successional habitat management for bird conservation. 

The effects of herbicide use often are equated to but should not be misconstrued or confused with the effects of pesticide use and the possible harm pesticides may demonstrate toward non-target plants and animals. Throughout the history of the research conducted at SGL 33 and GLR&D, numerous studies have demonstrated that proper use of herbicides via IVM has been compatible with and even beneficial to plant and animal communities. In particular, Bramble et al. (1984) and Yahner et al. (2002) emphasized the benefits of IVM and positive response of bird communities to sections of ROW maintained in an early successional state with the proper use of herbicides. Our current research findings from 2015–17 further indicate support for IVM that incorporates the proper use of herbicides.  Low volume stem foliar and high volume foliar application on sections of GLR&D contained the highest abundance and richness of breeding birds. Additionally, sections of ROW at SGL 33 managed using herbicides were comparable or more beneficial to bird communities in terms of abundance, species richness, indices of productivity, and nesting success than sections maintained via mechanical treatments both at the end of a four year IVM cycle (2016) and during the first breeding season post treatment (2017). On both SGL 33 and GLR&D, the most abundant bird species were either insectivores (barn swallow, chestnut-sided warbler, common yellowthroat, indigo bunting, and prairie warbler) or omnivores (eastern towhee, field sparrow, and gray catbird); further clarifying the differences in effects of insecticides versus herbicides and supporting IVM incorporating the proper use of herbicides along ROW. 

The wire zone – border zone IVM approach was applied at SGL33 and GLR&D in the mid-1980’s. The zone located directly under transmission lines (wire zone) is managed to maintain a plant community comprised of grass, forbs and low shrubs in order to minimize reinvasion of tall-statured trees and shrubs that could possibly interfere with electrical transmission lines. Either or both sides of the wire zone adjoin a narrow border zone dominated by of low- to medium-sized shrubby vegetation before the ROW transitions to natural forest. Past research on the two study locations indicated that within the ROW, nearly four times as many birds were observed in the shrubby border zones as in the wire zones.  During 2015–17, we detected more individuals and more bird species within the border compared to the wire zone for all three years of surveys at SGL 33. Additionally, avian productivity in the form of number of successful nests was comparable between border and wire zones (11 versus 12 nests and 10 versus 12 nests in 2016 and 2017, respectively) despite the wire zone being 25 feet wider than the border zone in 2016 and more than three times the total area of the border zone in 2017. In 2016 sections of ROW at SGL 33 that contained no borders had the lowest number of individuals, species richness, breeding bird productivity, and number of successful nests compared to the other five management types that contain border zones. The same was true in 2017 with the exception of mowing only management section which was the least beneficial to breeding birds following extensive treatment of the ROW at SGL 33 in the fall of 2016. Hence, the border zone is a very important component of IVM as it adds habitat for bird species that require a combination or mix of herbaceous vegetation, shrubs, and sapling tree species. A concerted effort needs to be made to retain borders and border vegetation especially with the new federal safety regulations requiring increased clearance between vegetation and the electrical transmission lines. 

In addition to being a vital component of ROW management for bird species requiring shrubby habitat, the border zone can help minimize the impacts of management conducted within the wire zone at the beginning of an IVM cycle. Bramble et al. (1992) and (1994) noted significant declines in bird populations following IVM at both SGL 33 and GLR&D. They also suggested that border zones were responsible for the retention of large and diverse bird populations on the ROW, as the wire zone – border zone method of IVM allowed for retention of shrub cover as the dominant vegetation component within the borders despite extensive changes to vegetation within the wire zone post management. We also detected the fewest birds in 2017 at SGL 33 following IVM in fall 2016 as compared to the pre-treatment breeding seasons of 2015 and 2016. Beside changes in avian abundance, breeding bird productivity can fluctuate quite dramatically from year to year and the presence of border zone vegetation may help to retain birds following extensive management within the wire zone. A nesting success rate of 68% was the highest recorded at SGL 33 in 1991–92 combined, whereas Yahner et al. (2004) detected differences in nesting success rates of 39% in 2002 compared to 65% in 2003. For comparison, nesting success was 42% at GLR&D within a similar time period (2003–04) and success rates average around 50% for different managed landscapes within Pennsylvania and Maryland. On the ROW at SGL 33, nearly half of the nests fledged young in 2016 (49% of 47 nests; with 52% wire zone nests and 46% border zone nests) compared to 36% of 61 nests in 2017 (34% wire zone nest success and 38.5% border zone nest success).  Fluctuations in breeding bird productivity and nest success have been attributed to many causes including ambient temperature differences between years that alter plant phenology (availability of nest cover) and nest chronology and varying population levels of different nest predators. However, differences in both wire zone and border zone nest success rates, plus overall nest success rates between 2016 and 2017 at SGL 33 likely were due to reduction of available nest cover with changing vegetation characteristics in both zones following the initiation and implementation of a new IVM cycle in the late summer and early fall of 2016. The wire zone – border zone method of ROW management appears beneficial to early successional birds as evidenced by the continued presence of a diverse avian community throughout the history of IVM at SGL 33 since 1982 and on the GLR&D since research began in 1987. It will be important to gain insight as to how breeding bird productivity responds to changes in vegetation throughout the course of an IVM cycle (mid-cycle compared to the end or beginning), as well as track the possible long-term changes in bird populations with the recent reduction of the border zones at SGL 33 and GLR&D, plus the increasing presence of invasive and exotic defoliating insects potentially eliminating vegetation cover on each of the ROW sites. 

Key Findings

  1. Integrated Vegetation Management provides valuable habitat for early successional bird populations during the spring and summer months. 8 9 10 11 15 16 17 21 30 31 37
  2. Bird abundance, richness, and breeding productivity was higher on herbicide treated units compared to those that were mechanically managed especially mow only treatments. 28 29 37
  3. Bird abundance, richness, and breeding productivity was higher in border zone than within the wire zone of each ROW. Hence, the retention of border zones with their mix of herbaceous, shrub, and tree species is critical to supplement the more herbaceous dominated wire zone for successful management of early successional bird communities when using IVM on ROW. 10 28 29 37


8Bramble WC, WR Byrnes and MD Schuler. 1984. The bird population of a transmission right-of-way maintained by herbicides. J. Arboric. 10(1):13-20.

9Bramble WC, WR Byrnes, and MD Schuler. 1986. Effects of special right-of-way maintenance on an avian population. J. Arboric. 12(9):219-226.

10Bramble WC, RH Yahner, and WR Byrnes.1992. Breeding-bird population changes following right-of-way maintenance treatments. J. Arboric. 18(1):23-32.

11Bramble WC, RH Yahner, and WR Byrnes. 1994. Nesting of breeding birds on an electric transmission line right-of-way. J. Arboric. 20(2): 124-129.

15Yahner RH. 1995a. Forest-dividing corridors and neotropical migrant birds. Conserv. Biol. 9:476–477.

16Yahner RH. 1995b. Forest fragmentation and avian populations in the Northeast: Some regional landscape considerations. Northeast Wildl. 52:93–102.

17Yahner RH. 2000. Long-term effects of even-aged management on bird communities in central Pennsylvania. Wildl. Soc. Bull. 28:1102–1110.

21Yahner RH. 2007. Searching for nests in early successional habitat. Birding 33:56-82.

28Yahner RH, RJ Hutnik, and SA Liscinsky. 2002. Bird populations associated with an electric transmission right-of-way. J. Arboric. 28(3): 123-130.

29Yahner RH, RJ Hutnik, and SA Liscinsky. 2003. Long-term trends in bird populations on an electric transmission right-of-way. J. Arboric. 29(3):156-164.

30Yahner RH, BD Ross, RT Yahner, RJ Hutnik, and SA Liscinsky. 2004. Long-term effects of rights-of-way maintenance via the wire-border zone method on bird nesting ecology. J. Arboric. 30(5):288-293.

31Yahner RH and HR Smith. 1990. Avian community structure and habitat relationships in central Pennsylvania forests. J. Pennsylvania Acad. Sci. 64:3–7.

37Mahan C, B Ross, H Stout, D Roberts, L Russo.2018. Floral and faunal research on utility rights-of-way at state game lands 33 and green lane research and demonstration areas 2015-2018. Report to Project Cooperators 2018. 126 pages.