BIRD POPULATION RESPONSES TO CONSERVATION GRASSLANDS IN PENNSYLVANIA

Open Access
Author:
Wilson, Andrew Mark
Graduate Program:
Ecology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 17, 2009
Committee Members:
  • Margaret Brittingham, Dissertation Advisor
  • Margaret Brittingham, Committee Chair
  • Duane R Diefenbach, Committee Member
  • Murali Haran, Committee Member
  • Walter Matthew Tzilkowski, Committee Member
Keywords:
  • grassland bird
  • conservation
  • CRP
  • Pennsylvania
  • CREP
Abstract:
Grassland obligate birds have been in precipitous decline across North America for several decades. One of the principal mechanisms for stemming these declines is farmland conservation programs, among which the Conservation Reserve Program has been particularly successful in delivering millions of acres of new grasslands. Evidence of the efficacy of such programs for grassland bird conservation in the eastern United States is lacking. In 2000 the Conservation Reserve Enhancement Program (CREP) was established in Pennsylvania with the provision of grassland wildlife habitat one of its key aims. I show that within Pennsylvania, grassland obligates species showed sustained declines from the 1960s onwards. I use several bird survey data sources to assess the population-scale effects of CREP on a range of species within the state. A bird monitoring program was established in 2001 to assess farmland and grassland bird population trends in southern Pennsylvania. Trends for the period 2001 to 2005 provide scant evidence of population-scale changes in bird numbers; most grassland obligates continued to decline, but declines for some species were slower in areas with high rates of CREP enrollment. Eastern meadowlark was the only songbird for which I could demonstrate a strong response to higher rates of CREP enrollment, results for grasshopper sparrow and bobolink suggest that these species may have benefited, but the results are more equivocal. My analysis was hampered by the short time-series – fields were still being planted when the monitoring was curtailed. Further, the study coincided with the emergence of West Nile virus (WNV) in Pennsylvania, which further complicated the analysis. I show that WNV caused significant decreases in seven common bird species, notably corvids and cavity nesting songbirds. A survey of winter raptor numbers initiated in 2001 provided an ideal opportunity to test the hypothesis that CREP also provides habitat for wintering birds of prey. I show that at broad spatial scales there were increases in raptor numbers in areas where there was the most CREP, the results being particularly pronounced for northern harrier. Although these relationships are merely correlational, they do suggest that the new habitat provided by CREP has been sufficient to cause a significant shift in the wintering populations of this species. I use point count data from the 2nd Pennsylvania Breeding Bird Atlas to estimate grassland songbird populations and assess the relative importance of farmland for these species. I show that although densities are often low, farmland supports the majority of the populations of most grassland songbirds in Pennsylvania. I suggest that management, enrollment and re-enrollment of CREP fields could be better targeted. Large CREP fields in south central Pennsylvania should be managed specifically for eastern meadowlarks and grasshopper sparrows, while in the west of the state; further research could allow a targeting of CREP toward populations of the threatened Henslow’s sparrow. Small fields, which may be less suitable for grassland obligates, should provide habitat for species characteristic of early successional habitats, including the declining Field Sparrow. Population estimates show that CREP fields support only a small proportion of the population of most grassland songbirds. This highlights the importance of conservation efforts in known areas of high population density, such as reclaimed surface mines. Better targeting and careful management of CREP fields could potentially increase their value for grassland birds, but more research is needed if this is to be realized.