For the 26 most abundant species of warblers in eastern and central North America (Table 2), as analyzed with NSRR, the largest estimated total populations in recent years (1986-1988) have been in four regions: the Spruce-Hardwood and Boreal Forest, the Appalachian Mountains, the Eastern Foothills, and the Eastern Coastal Plain (Table 1).
At the other end of the list are at least eight species that are estimated with both NNRR and NSRR to have increased substantially, although not all estimates are statistically significant by both methods.
- On the basis of data for 208 routes in 12 strata (Table 2), the Blue-winged Warbler is estimated to have increased more than any of the other 26 species in our sample (61% and 38% between 1970-1972 and 1986-1988 with NNRR and NSRR, respectively).
- On the basis of BBS data from 422 routes in 20 strata, the Ovenbird population is estimated to have increased substantially during the period analyzed (29% and 18% estimated increases between 1970-172 and 1986-1988 with NNRR and NSRR, respectively; both figures are statistically significant by a z test).
Application of the probit-normal model to a matrix of increases and decreases by species and strata provides a formal overall test of this possibility and indicates that, indeed, there is statistically significant overall geographic variation in population trends (P [less than] 0.01 with both NNRR and NSRR).
With NSRR, for example, the strata with the largest percent declines for particular species were the Adirondack Mountains for Canada Warblers, the Cumberland Plateau for Cerulean and Prairie Warblers, the Blue Ridge Mountains for Prairie Warblers, and the Ouachita Mountains for Cerulean Warblers.
1990), NSRR has indicated that unusual declines have occurred in warblers in the Adirondack Mountains.
Physiographic strata where the estimated probability of a decline in a hypothetical species between 1970-1972 and 1986-1988 was [greater than]0.7 (a) or [less than]0.3 (b) on the basis of a probit-normal model applied to a matrix of increases and decreases for each species estimated by either NNRR or NSRR. Strata with high values have unusual estimated proportions of species that decreased; those with low values have unusual proportions of species that increased (see Figs.
We found that the median trend for all 26 species of warblers is lower with NSRR (2% increase) than with NNRR (16% increase, see end of Table 2).
Nonparametric regression methods (such as NNRR and NSRR) estimate a regression curve without making strong assumptions about the shape of the true regression function (Altman 1992).
