Date of Award
Doctor of Philosophy
Louis Gross, Craig Harper, Frank van Manen
Grassland bird populations have decline significantly over the past 30 years because of the dramatic decrease of native grasslands through clearing of non-forested land for agriculture, and discontinued use of fire. It is imperative to understand the distribution and productivity of these birds and the potential for grassland management to enhance these declining populations, especially in land areas where the landuses may be compatible with grassland bird conservation.
This study was conducted to provide needed basic life history and nest site habitat use information as well as information about populations and potential region-wide habitat availability to enhance current and future land management planning. This stydy focused on Grasshopper Sparrows (Ammodramus savannarum), Henslow’s Sparrows (Ammodramus henslowii), Field Sparrows (Spizella pusillia), Dickcissels (Spiza Americana), and Eastern Meadowlarks (Sturnella magna) The specific objectives of this project were: 1) to provide basic life history parameters for five species of conservation concern at Fort Campbell Army Base, Kentucky, over a five-year period (1999-2003; Chapter 2); 2) to use the basic life history parameters to examine population viability grassland bird populations at Fort Campbell, and examine the implications of management activities within the breeding season on these population viabilities (Chapter 3); 3) to examine nest site habitat selection of the five focal species (Chapter 4); and 4) to examine the potential for US Department of Defense installations in the eastern US to provide grassland habitat for breeding and wintering grassland bird populations (Chapter 5). Finally, in Chapter 6, I discuss the management implications developed from the results of this project.
A total of 811 nests of target species were monitored between 1999-2003, and nest success ranged between 14.7% and 33.8% for each species. Most nest failures were attributed to predation. Brown-Headed Cowbird (Molothrus ater) nest parasitism rates were very low for all species. Clutch size decreased during the nesting season for Dickcissels, Grasshopper Sparrows, and Field Sparrows. Nesting phenology suggests the possibility of at least double-brooding for all five species in this study. Eastern Meadowlarks initiated nests earliest, mid-April. Field Sparrow nest initiation started the next week, followed by Henslow’s Sparrows the next week, and then Grasshopper Sparrows. Dickcissels were consistently the last species to arrive and began nesting during the second week of May. For all species, nest initiation continued through mid-July, and nesting activity continued through August. This study provided the biological parameters necessary to create population models to evaluate population trajectories and alternative management plans.
I constructed a simple population model incorporating typical demographic parameters collected in the field supplemented by values found in the literature for Henslow’s Sparrows, Grasshopper Sparrows, Dickcissels, Eastern Meadowlarks and Field Sparrows. Species specific parameters collected in the field included clutch size, nesting phenology, Mayfield (1975) nesting success, and number of fledglings per successful nest. This analysis produced population viability plots with curves representing the threshold between source and sink populations. I also modeled the effects of breeding season length and hay management within the nesting season on the number of possible nesting attempts to examine the population trajectories of Grasshopper Sparrows and Henslow’s Sparrows.
For Henslow’s Sparrows (triple-brooded, 4 attempts), the basic model using the mean estimates of nest success and young per successful nest for all years combined indicated the population could not sustain itself without immigration. The estimates of nesting success and young produced per successful nest for 2 of the 5 years (2001 and 2003) indicated source populations with 4 nesting attempts (A) and 3 years (1999, 2000 and 2002) indicated sink populations. For Grasshopper Sparrows (triple-brooded) population viability for 3 of the 5 years (2000, 2002 and 2003) indicated source populations with 4 nesting attempts (A) and 2 years (1999 and 2001) indicated sink populations.
For Dickcissels (single-brooded), Field Sparrows (double-brooded), and Eastern Meadowlarks (double-brooded), the mean estimates of nest success and young per successful nest for all years combined were too low to indicate any source populations under the conditions of this model. For Dickcissels, the estimates of nest success and young per successful nest for any single year were also too low to indicate any source populations under the conditions of this model.
For Grasshopper Sparrows, the mowing model indicated “No mowing” and mowing after 1 August allowed for the possibility of a source population with the overall estimates of nest success and young produced per successful nest. Point-estimates for 1999, 2001, and 2002 indicated source populations only with no mowing. Mowing on 15 June or after 15 July allowed the Grasshopper Sparrow population in 2000 to be a source, whereas mowing 15 May, 1 June, and 1 July caused sink populations under the model assumptions. In 2003, the nesting success rate and the number of young produced per successful nest were great enough to compensate for mowing after 15 Jul with maximum breeding effort. For Henslow’s Sparrows, mowing before 15 July indicated sink populations in all years (Figure 3-15). The “no mowing” threshold did not allow a possible source population for the nesting success and young per successful nest estimates for all years combined with maximum breeding effort, but the variation indicated “no mowing” may allow for a source population. In 1999, 2000, and 2002, under conditions for maximum breeding output the estimates indicated a sink population even with no mowing. Year 2003 was a source population only with “no mowing.” Mowing after 1 August allowed the 2001 population to be a source.
I examined habitat differences between selected nest sites and available habitats (univariate analysis), and examined microhabitat selection (niche) relationships among the five target species (multivariage analysis). Based on the univariage analysis, litter depth was significantly greater at the nest sites for all species than at the random sites. The random sites also had greater bare ground cover and lower grass height than all species except Grasshopper Sparrows. Henslow’s Sparrow nest sites had the greatest warm-season grass cover and Eastern Meadowlark nest sites had the greatest cool-season grass cover. Field Sparrow nest sites had the greatest cover in woody vegetation. Based on the multivariate analysis, Field Sparrows and Dickcissels were using similar habitats; the discriminant function analysis had difficulty separating the nest sites of these species. The random vegetation plots, representing available microhabitat at Fort Campbell, were centrally located when plotted using the discriminant function coefficients calculated with the vegetation measurements at the nest sites of the five grassland species. Thus available habitat, on average, had intermediate litter depth and vertical cover, and relatively high forb cover and low warm-season grass cover. The random locations also occupied a relatively large area in the multivariate space, and extended well beyond the area of overlap of the five species along the second discriminant function axis. This result demonstrated that available habitat included unsuitable areas of grassland habitat for these five species. In a large potion of the random areas, burning occurred annually, which was too frequent to create suitable habitat for these grassland species of high conservation concern.
Finally, I used a course-filter analysis to determine which military installations in the eastern United States have the potential to provide significant grassland habitat by identifying military installations that contain large (>40 ha) grassland patches in the eastern US, identifying areas where open habitats (e.g., grassland, hayfields, agriculture) occupy a significant portion of the landscape, and overlaying the areas of high diversity for obligate grassland birds during the breeding and wintering seasons in the eastern US. I also conducted a buffer analysis to determine if the extent of grassland within the military installation was representative of grassland habitat within the surrounding landscape, and determine how much potential the surrounding landscape (within 30 km) had for grassland restoration.
Of the 186 land areas in the eastern US managed by the DOD, 45 contained at least one large (>40 ha) patch of grassland, including 1 port managed by the Army Corps of Engineers, 23 Army, 3 Air Force, 3 marine, 11 Navy, and 4 National Guard installations. Military installations with significant grassland habitat were found throughout the eastern US providing at least 65,000 ha of grassland in patches greater than 40 ha. Most of the selected military installations were located in the southern US within 300 km of the coast, and could be especially important for wintering habitat. Military installations could have major positive impacts on the declining populations of bird and other wildlife species, which depend on frequent habitat disturbance to maintain early-successional habitats like grasslands. Because many military activities require or cause the maintenance of large areas of open, grassy or shrubby habitats, tailoring habitat management to enhance grassland populations would not require major changes in existing management plans. The location of some of the larger eastern US military installations in landscapes with relatively large amounts of open habitats may also serve as a refuge for many grassland species displaced by modern, “clean” farming practices. With a few considerations to the type and timing of disturbances, military installations could serve as a model for other federal and private land management for the conservation of grassland habitats, and may even serve as a control sites for comparison with grassland restoration efforts.
Military lands comprise over 10-million ha of land in the US and could provide unique management opportunities to provide breeding and wintering habitat for birds. Conservations strategies for grassland species could be developed to take advantage of the unique need for open habitats for military training, especially in the eastern US. Land managers need to understand although grassland habitat used by different species superficially may seem very similar, different management actions will benefit different sets of species and may negatively impact others. Local habitat conditions can influence not only the presence of grassland birds but also other life history parameters like the number of successful broods and the number of nesting attempts. Planning across a temporal gradient is needed to provide suitable habitat for all species of concern.
Giocomo, James J., "Conservation of Grassland Bird Populations on Military Installations in the Eastern United States with Special Emphasis on Fort Campbell Army Base, Kentucky. " PhD diss., University of Tennessee, 2005.