Bird species distributions are closely associated with physiographic and vegetation features. In recognition of this, physiographic strata for the BBS were developed by modifying existing vegetation and soil distribution maps (Robbins et al. 1986). Maps of the physiographic strata have appeared in recent Annual Summaries and elsewhere (Droege and Sauer 1989, Butcher 1990). Because bird abundances differ among strata, most of our analyses of BBS data use physiographic strata within states or provinces (state-stratum units) as the smallest unit for estimating population trends or average relative abundance. Regional averages are constructed from state-stratum averages, weighting by land areas. This approach minimizes the influence of sampling bias associated with regional differences in sampling intensity.
Strata names are: 1 Subtropical 2 Floridian 3 Coastal Flatwoods 4 Upper Coastal Plain 5 Mississippi Alluvial Plain 6 Coastal Prairies 7 South Texas Brushlands 8 East Texas Prairies 9 Glaciated Coastal Plain 10 Northern Piedmont 11 Southern Piedmont 12 Southern New England 13 Ridge and Valley 14 Highland Rim 15 Lexington Plain 16 Great Lakes Plain 17 Driftless Area 18 St. Lawrence River Plain 19 Ozark-Ouachita Plateau 20 Great Lakes Transition 21 Cumberland Plateau 22 Ohio Hills 23 Blue Ridge Mountains 24 Allegheny Plateau 25 Open Boreal Forest 26 Adirondack Mountains 27 Northern New England 28 N. Spruce-Hardwoods 29 Closed Boreal Forest 30 Aspen Parklands 31 Till Plains 32 Dissected Till Plains 33 Osage Plain-Cross Timbers 34 High Plains Border 35 Rolling Red Prairies 36 High Plains 37 Drift Prairie 38 Glaciated Missouri Plateau 39 Great Plains Roughlands 40 Black Prairie 53 Edward's Plateau 54 Rolling Red Plains 55 Staked Plains 56 Chihuahuan Desert 61 Black Hills 62 Southern Rockies 63 Fraser Plateau 64 Central Rockies 65 Dissected Rockies 66 Sierra Nevada 67 Cascade Mountains 68 Northern Rockies 80 Great Basin Deserts 81 Mexican Highlands 82 Sonoran Desert 83 Mojave Desert 84 Pinyon-Juniper Woodlands 85 Pitt-Klamath Plateau 86 Wyoming Basin 87 Intermountain Grasslands 88 Basin and Range 89 Columbia Plateau 90 S. California Grasslands 91 Central Valley 92 California Foothills 93 S. Pacific Rainforests 94 N. Pacific Rainforests 95 Los Angeles Ranges 96 S. Alaska Coast 98 Willamette Lowlands 99 Tundra.
Butcher, G. S. 1990. Audubon Christmas Bird Counts. Pp. 5-13 in J. R. Sauer and S. Droege, eds. Survey designs and statistical methods for the estimation of avian population trends. U.S. Fish Wildl. Serv. Biol. Rep. 90(1). Droege, S., and J. R. Sauer 1989. North American Breeding Bird Survey annual summary 1988. U. S. Fish. Wildl. Serv., Biol. Rep. 89(13):1-16. Robbins, C. S., D. Bystrak, and P. H. Geissler. 1986. The breeding bird survey: its first fifteen years, 1965-1979. U.S. Fish Wildl. Serv. Resour. Publ. 157. 196pp.