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Towards Ecological Sustainability: Understanding Habitat Size Requirements for Declining Grassland Birds Alexander Keyel & J. Michael Reed Dept. of Biology, Tufts University, 163 Packard Ave, Medford, MA 02155, Summary: Habitat loss and fragmentation are the leading causes of species extinction today. Some bird species have declined more than expected by habitat loss alone, and it is important to understand the causes of these declines. In part, these declines are due to species being absent from small habitat patches that are apparently suitable. The mechanism(s) causing these absences is not understood, but predator avoidance may be especially important for grassland species. In particular, grassland birds may be choosing open habitat to increase predator (hawk) detection, and birds in less open patches (i.e. smaller patches) may need to spend more time being vigilant to spot predators coming from the edge. I tested the hypotheses that patch openness drives grassland bird density, and that vigilance is greater in less open grassland patches. We measured grassland bird density, openness, vigilance, flight initiation distance, and vegetation cover at up to 49 fields at 23 different sites between May and November 2009. Results from my analyses are being prepared for publication and will be available from TIE upon publication. Background: Habitat loss and fragmentation are the leading causes of species extinction today.1 The obvious effect is that less habitat means smaller populations. Habitat fragmentation exacerbates this problem by isolating the remaining individuals, thus reducing connectivity, which prevents colonization and population rescue.2 An important observation is that some species are absent from smaller habitat patches that appear otherwise suitable.3 This phenomenon is referred to as area sensitivity; a species is area sensitive if it is absent from, or occurs in reduced densities, in small habitat patches that are large enough to support territories (Fig. 1).4 The mechanisms underlying area sensitivity are poorly understood. Many studies have focused on edge effects on predator or food abundance for area sensitivity (e.g.5,6,7); however, behavioral mechanisms might cause area sensitivity as behavior can mediate population dynamics and extinction risk,8,9,10 predator avoidance and escape,11 and habitat selection.12,13,14,15,16 Predators influence bird behavior,17 and individuals in smaller patches may be more sensitive to edge-dwelling predators than are those in large patches. Consequently, the ability to detect predators may affect whether or not birds settle on a site. I predict that in grassland ecosystems the ability to detect predators will depend on how open a patch is and how much time is spent by birds looking for predators. How open a patch is depends on how large a patch is, as well as on the vegetation structure Site Size (ha)Population Density (#/ha)Small LargeArea SensitiveNot area sensitiveSite Size (ha)Population Density (#/ha)Small LargeArea SensitiveNot area sensitive Figure 1: An area sensitive species shows a positive relationship between density and site size; one possible relationship is shown above, contrasted with a species that is not. surrounding the patch and the patchs topography. Time spent looking for predators would represent a cost to the individual animal, as they are more exposed to predators themselves, and are not able to spend this time foraging. Consequently, less-open patches may be more costly to occupy due to an increased need to be vigilant (i.e. look for predators). Hypotheses: I tested the hypothesis that grassland birds are sensitive to how open a site is rather than to patch size per se. This may lead to the appearance of area sensitivity when openness and patch area are strongly correlated. In addition, I tested whether vigilance increases in less open patches. Study species: I recorded occupancy and abundance data on all grassland obligate bird species; however, data were collected to ensure that a meaningful sample size was collected for Bobolinks (Dolichonyx oryzivorus). Bobolinks were selected as a focal species for these questions because they are: 1. area sensitive; 2. abundant; 3. conspicuous; and 4. in need of conservation: Bobolinks have declined by 52% nationwide over the last 40 years. 2009 Accomplishments: Assessing area sensitivity and patch openness: To test for effects of patch size vs. openness, we surveyed grassland bird density across a spectrum of patch sizes (0.3-50 ha) and of varying degrees of openness, using multiple measures of openness. During the breeding season we used 100 m line transect surveys to assess bird density. During the non-breeding season entire fields were surveyed for birds either via a walk-through or by dragging a rope between two individuals. Bird density data were compared to field area, perimeter/area ratio, a GIS edge-based prediction model and 11 openness indexes, measured with either an inclinometer or a Solar Pathfinder. Each openness index is a composite of patch size, patch shape, surrounding vegetation type and height and slope of the patch, and changes to any of these variables changes the openness index. Note that small grassland patches can be more or less open, depending on edge habitat. Larger patches are less affected by edge, but more structurally complex edge habitat might be avoided by bobolinks. Assessing vigilance: Bobolink vigilance was measured by 3 complementary measures. 1. Focal samples, or continuous observations of a single individual were used to quantify individual vigilance; 2. Scan samples were used to determine the number and percent of vigilant individuals and provides an estimate of site-level vigilance. Scan samples were taken by scanning the field at a point in time and recording the number of individuals that are up. 3. We measured flight initiation distance, or the distance at which the animal moves in response to an approaching observer. Taking vigilance further: Due to the unusually high amount of rain in June, we replaced some of our field data collection with lab experiments designed to support our field studies. Because we did not have Bobolinks in captivity, we tested European Starling (Sturnus vulgaris) response to an avian predator mount (Sharp-shinned hawk, Accipiter striatus) in the lab in June, and then used this predator mount to assess Bobolink responses in the field in July. These results were presented at the joint meeting of the Cooper Ornithological Society, American Ornithologists Union and the Society of Canadian Ornithologists in February, 2010, in San Diego. See attached poster for project methods and results. Results: The results from these studies are being prepared for publication. In order to assure the quality of the analyses, the results will be available through the TIE office following publication. Acknowledgements: A huge thank you to all who made this possible: Field Assistance: David Peck, Chuong Le, Gautam Surya, Emily Wier. Feedback: Reed/Romero lab group esp. Dave DesRochers and Brian Tavernia. Thesis Committee: Michael Romero, Colin Orians. GIS/GPS Support: Barbara Parmenter and Patrick Florance. Office Assistance: Eileen Magnant, Liz Palmer and Karen Murphy, Tony Keevan. Landowners: Towns of Bedford, Lincoln, and Amesbury, Moore, Walden Pond, and Brookwood Farm State Parks, DCR Division of Water Supply, MassWildlife, Massachusetts Audubon Society, Trustees of Reservations, US Fish & Wildlife Service Eastern MA Wildlife Refuges Complex, Tufts Veterinary School. Farmers: Bill Foster, Heidi Tafel, Jim Philips Funding for this research provided by: Tufts Institute for the Environment Fellowship National Science Foundation Graduate Research Fellowship Nuttall Ornithological Club Tufts REU BUDGET Expenditures charged to TIE Travel $1,847.00 CO2 offset for travel $56.00 Field Assistant $2,097.00 TOTAL: $4,000.00 NOTE: Field assistant stipend was less than anticipated due to volunteer assistance & partial assistance from separately funded REU student. Literature Cited 1 Wilcove, D.S., D. Rothstein, J. Dubow, P. Ali, and E. Losos. 1998. Quantifying threats to imperiled species in the United States. Bioscience 48:607-615. 2 Gotelli, N. J. 1991. Metapopulation models - the rescue effect, the propagule rain, and the core-satellite hypothesis. American Naturalist 138:768-776. 3 Robbins, C. S., D. K. Dawson, and B. A. Dowell. 1989. Habitat area requirements of breeding forest birds of the middle Atlantic states. Wildlife Monographs 103:3-34. 4Horn, D. J., and R. R. Koford. 2006. Could the Area-sensitivity of some grassland birds be affected by landscape composition. Pages 109-116 in D. Egan and J. A. Harrington, editors. Proceedings of the 19th North American Prairie Conference. 5 Renfrew, R. B., C. A. Ribic, and J. L. Nack. 2005. Edge avoidance by nesting grassland birds: A futile strategy in a fragmented landscape. Auk 122:618-636. 6 Johnson, R. G., and S. A. Temple. 1990. Nest predation and brood parasitism of tallgrass prairie birds. The Journal of Wildlife Management 54:106-111. 7 Burke, D. M., and E. Nol. 1998. Influence of food abundance, nest-site habitat, and forest fragmentation on breeding ovenbirds. Auk 115:96-104. 8 Smith, A. T., and M. M. Peacock. 1990. Conspecific attraction and the determination of metapopulation colonization rates. Conservation Biology 4:320-323. 9 Reed, J. M. 1999. The role of behavior in recent avian extinctions and endangerments. Conservation Biology 13:232-241. 10 Reed, J. M., and S. H. Levine. 2005. A model for behavioral regulation of metapopulation dynamics. Ecological Modelling 183:411-423. 11 Lima, S. J., P. A. Zollner, and P. A. Bednekoff. 1999. Predation, scramble competition, and the vigilance group size effect in dark-eyed juncos (Junco hyemalis). Behav Ecol Sociobiol 46:110-116. 12 Hilden, O. 1965. Habitat selection in birds. Ann Zool Fenn 2:53-75. 13 Danchin, E., T. Boulinier, and M. Massot. 1998. Conspecific reproductive success and breeding habitat selection: Implications for the study of coloniality. Ecology 79:2415-2428. 14 Reed, J. M., T. Boulinier, E. Danchin, and L. W. Oring. 1999. Informed dispersal: prospecting by birds for breeding sites. Pages 189-259 Current Ornithology 15 Doligez, B., E. Danchin, and J. Clobert. 2002. Public information and breeding habitat selection in a wild bird population. Science 297:1168-1170. 16 Doligez, B., T. Part, E. Danchin, J. Clobert, and L. Gustafsson. 2004. Availability and use of public information and conspecific density for settlement decisions in the collared flycatcher. Journal of Animal Ecology 73:75-87. 17 Fontaine, J. J., and T. E. Martin. 2006. Parent birds assess nest predation risk and adjust their reproductive strategies. Ecology Letters 9:428-434.


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