These maps indicate the number of birds seen on BBS routes, grouped into
convenient categories of relative abundance. The maps predict the average
number of birds of the species that could be seen in about 2.5 hours of
birdwatching along roadsides (by very good birders). They are based on
mean counts on BBS routes over the interval 2011 - 2015.
The maps have some error associated with both the underlying data and
the mapping procedure. On rare occasions, the procedure inserts edges
that omit some marginal routes. Also, note that the lowest relative
abundance category occasionally indicates the presence of a species
outside its recognized range of the species, representing an accidental
observation. Finally, the mapping procedure causes the lowest relative
abundance category to extend slightly beyond the survey routes on
which the species occurs. We have left these edges and accidental
observations in the map to emphasize that unusual observations (and
misidentifications) occur in any survey that is based on data
collected by many observers.
Also, bird populations are changing, and species can be more (or less)
abundant than indicated by the 5-year mean of counts on nearby survey routes.
So, be warned! You will not always be able to see the species at all
locations where the maps indicate they occur at low abundances.
Many investigators have used bird survey data to develop
contour maps of bird abundance based on mean counts on survey
routes. Root (1988) provided a grid of smoothed relative
abundances for species observed on Christmas Bird Counts. Sauer and
Droege (1989) mapped relative abundances of Eastern Bluebirds
(Sialia sialis) just after severe winters in the mid
1970's and
after their populations returned to pre-winter levels. We have
also used relative abundance maps to document the ranges of several
species (e.g., Droege and Sauer 1990). See Sauer et al. (1995
a and b) for applications and discussions regarding mapping of
survey data.
These maps are based on exactly the same data that are used in the BBS trend analyses, and route summaries
are simple averages of counts on routes over time. Please note that these simple averages do not
account for observer differences in counting ability or for other factors that could be controlled
in more sophisticated analyses. We also note that the BBS data are edited to remove data that are of questionable
quality or represent birds that are thought to be migrating rather than breeding. Please refer to the metadata for the BBS
dataset for more information on editing and quality control of the BBS data.
Preparation of Maps from BBS Data
We developed a map of starting locations of BBS routes.
Latitude and longitude (degree-minute) of the starting locations
were taken from topographic maps of the route path. Of course, the
route is 24.5 miles in length, hence any point used to characterize
the route is arbitrary.
We estimated average counts from the interval 2011 - 2015 on
each route for each species, and copied them into database files.
We developed contour maps of bird relative abundances, using the
route relative abundances as input to smoothing procedures (Isaaks
and Srivastava 1989, Cressie 1992).
We used inverse distancing (Isaaks and Srivastava 1989) to
prepare a smooth of the data. This procedure estimates the
abundance at a location as a distance-weighted average of counts
from nearby survey routes. We used inverse distancing to estimate
abundances for a grid of points overlaid on the survey area, then
used Arc/Info to make a contour map from the estimated abundances
(Environmental Systems Research Institute
1991). See below for more datails of the analysis.
Arc/Info provided an arc coverage of
contours that connect points having the same value.
Depending on the maximum relative abundance of the species, we used
levels of 1, 3, 10, 30, and 100 for contours. The maps
end at a minimum level of 0.1, which was chosen as a possible
edge-of-range index after some comparisons of contours with known
edges of ranges (S. Droege and D. Bystrak, Personal Communication),
and the larger cutpoints were chosen as a series of powers of 3,
rounded up for ease of presentation.
The maps presented here are quite similar to the maps in the
those of earlier versions of the Home Page, but several differences exist
between the procedures used to prepare the earlier maps and these
maps. To make the 1966 - 1992 maps, we used Kriging, a procedure
in which a variogram is estimated for the species and is used to
define the distance-covariance relationship for the smooth (Cressie
1992). In theory, the Kriging should provide a more
accurate surface than a procedure such as inverse distancing, which
never uses information from the data to adjust the weighting.
However, in our experience the variograms were not particularly
informative, suggesting that at the scale of the BBS there is
little advantage in using Kriging.
We acknowledge, however, that the maps provided here are
designed to provide a large-scale summary of the data, and
if a species is of particular interest, a more intensive
analysis should be conducted using Kriging or some other smoothing
procedure. Kriging is a model-based
estimation procedure, and if the model is appropriate for the data
we can put confidence intervals on the resulting surface. By
developing a semivariogram model that more accurately portrays the
spatial covariance among routes, the resulting Kriged surface will
better reflect the patterns of change among the routes. Often
features such as directionality of the semivariogram and trend in
the data will require the use of more complex models.
See Isaaks and Srivastava (1989) for a useful discussion of the
technical details of fitting semivariograms to data, and Cressie
(1991) for a more technical discussion of all aspects of spatial
modelling.
Literature Cited
Cressie, N. 1992. Statistics for spatial data. Wiley, New York. 900pp.
Droege, S., and J. R. Sauer. 1990. Northern bobwhite, Gray
partridge, and ring-necked pheasant population trends
(1966-1988) from the North American Breeding Bird Survey.
Pages 2-20 in K. E. Church, R. E. Warner, and S. J. Brady,
eds. Perdix V: Gray partridge and ring-necked pheasant
workshop, Kans. Dept. Wildl. and Parks, Emporia.
Environmental Systems Research Institute. 1991. Surface Modeling
with TIN. Environmental Systems Research
Institute, Inc., Redlands, CA.
Isaaks, E. H., and R. M. Srivastava. 1989. An introduction to
applied geostatistics. Oxford University Press, New York.
561pp.
Root, T. 1988. Atlas of wintering North American birds.
University of Chicago Press, Chicago, Il.
Sauer, J. R., and S. Droege. 1990. Recent population trends
of the eastern bluebird. Wilson Bull 102:239-252.
Sauer, J. R., S. Orsillo, and B. G. Peterjohn. 1995a. Geographic
patterns in relative abundances and population trends
of breeding and wintering Loggerhead Shrikes in North
America. Proceedings Western Foundation of Vertebrate Zoology
6:128-141.
Sauer, J. R., G. W. Pendleton, and S. Orsillo. 1995b. Mapping of bird
distributions from point count surveys. USDA Forest Service,
GTR PSW-GTR-149:151-160.
