(also known as spot mapping, and by the name of the program that has used this technique, the Breeding Bird Census)

Author: Sam Droege, USGS Patuxent Wildlife Research Center, 12100 Beech Forest Rd., Laurel, MD 20708, frog@usgs.gov, 301-497-5840.

Species list
Description of technique
       Field time components
       Office time
       Equipment
Things that could bias your counts
       Observer skill
       Plot size
       Number of visits
       Interpretation of maps
Advantages and disadvantages
Approaches to analyzing your data
Existing protocols and programs using this technique
Estimates of variation of counts for this technique
Studies that have used this technique
Literature cited
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Any species of land bird that maintains discrete territories throughout the main portion of the breeding season is a possible candidate, but the technique is most often used for smaller land birds whose territory size fits well within the typical plot.

The objective of territory mapping is to estimate population densities for each bird species present within a surveyed plot of land (Robbins 1970, Williams 1936). Square or rectangular plots are most often used and are usually subdivided into 50m grids along which the observer travels during the census. The locations of all birds, and particularly singing males and nest sites, are mapped on paper replicas of the plot during a series of visits made during the breeding season (usually 8 or more). These individual visits are transcribed to species-specific sheets at the end of the season and territory boundaries are identified for individual males or pairs. Visits are spaced throughout the breeding season, including visits during spring for species breeding early in the year. Rules for interpreting the territory boundaries were first developed in North America by Williams (1936) and then modified by Hall (1964), Robbins (1970), and Van Velzen (1972). In Britain, the Common Birds Census uses Territory Mapping and a detailed summary of the protocol exists in Bibby et al. (2000).

Usually the observer spends the entire morning surveying one plot. If plots are small and close together it is possible to do 2 plots. Time to do the survey depends on the size of the plot; the complexity and density of the bird community, the care with which the observer estimates the singing locations of the birds, the amount of time spent looking for nests, and the speed in which the observer walks. Rough times for a 10 ha woodland plot would be 3-4 hours per visit.

Time to initially establish the plot can take from 1 to several days depending on the size of the plot, accuracy and precision of the surveying, and the skills of the surveyors. Two people are usually required to lay out the lines, run the measuring tapes, and efficiently use the surveying equipment.

At the start of each field season the plots need to be checked for missing grid markers and flagging added to reestablish the walked grid lines.

Transcribing the data from the individual visits to an individual species maps is tedious. Transcription of all the data and the resulting interpretation of the maps can take quite a few hours. Time to do one plot could range between 8-20 hours depending on the complexity and number of visits.

• Paper
• Pencil
• Clipboard
• Binoculars

Things that could bias your counts

Any observer involved in mapping territories must know the songs and calls of the birds on the plot. Most mapped detections are of birds not seen, but heard and their estimated location plotted on the paper replica of the plot. Compared to point counts and other techniques that have a strict time component, territory mapping does accommodate more deficits in an observer’s knowledge of bird identification in that they have the opportunity to track down birds whose calls they cannot identify. However, a great deal of time spent tracking down unknown birds can eat into survey time and often the calling or singing bird remains undiscovered. There is likely to be discernable and perhaps major differences between efforts by practitioners comfortable with their identification of birds and the novice.

It is intuitive that the smaller the plot the fewer the number of birds will be found in that plot. By extension… small plot sizes will more poorly estimate bird densities than large plots as the chance inclusion or exclusion of bird territories will have greater impacts on small plots than large ones. Territories of birds found on the edge of a plot present problems as the birds are both less likely to be detected and the proportion of the territory in the plot vs. outside the plot usually cannot be estimated as the territory outside the plot is unmeasured. Consequently, the smaller the plot the greater the amount of edge to interior of the plot there will be and the less accurately true density will be estimated. In fact, two studies (Verner 1985, Scherner (1981) independently demonstrated an inverse relationship between plot size and density, an indication that bird densities were being overestimated when plots sizes were small. While no consensus exists as to the appropriate plot size, recommendations are for 40-100 hectare plots in open country habitats and 10-30 ha in closed habitats (Robbins 1970).

In a similar vein to the problem of small plot sizes, the fewer the number of visits to a plot the less likely that clearly defined territory boundaries will appear from the data collected. Too many site visits can lead to the opposite problem with false territories being identified due to shifts in territory boundaries and the presence of floating non-territorial males. Verner (1985) indicates that based on an analysis of published and unpublished studies that 8-14 site visits is the best compromise between too few and too many visits.

Territories boundaries and the numbers of territories of birds can change during any breeding season. Males disappear, new males attempt to establish territories, 2 territories can merge into one, and boundaries shift, making it difficult to define and justify at times a single density figure. Added to those complexities of avian reproductive ecology is our inability to detect and measure those patterns of territory change. While our repeated visits to these plots give us more insights into bird populations than almost any other bird survey technique it is insufficient to detect all the changes that have taken place. Consequently, our final maps leave us enough ambiguities regarding territory placement that different map interpreters looking at the same set of maps come up with a very different estimated number of territories for some species (O’Connor 1990, O’Connor and Marchant, J.H. 1981) The folks at the Common Birds Census have done the most work to minimize these errors, going so far as using the same set of technicians to determine territory boundaries for all their plots. A summary of their techniques can be found in Bibby et al. (2000).

As the real story regarding the number of territories in a plot will almost never be known, standardization of how territories are determined and who determines them becomes important. On individual plots over a small number of years the answer would be to have a single individual do the map interpretation. While the actual bias will be unknown at least it will be consistent. A consequence of this is that any collection of plots will have different and unknown degrees and directions of bias affecting the density estimates, thus, increasing the variances of any tests or descriptors applied to that collection of data.

Advantages:

While there is bias associated with the density estimates produced by territory mapping and its magnitude remains unmeasured and uninvestigated, it is likely to be the least biased and most precise technique of those all those techniques available for estimating bird density; short of intensive studies of color marked individuals. Furthermore, territory mapping provides information on where each species is nesting within a plot and their relationships to neighbors, vegetation and other plot features. These is also a large array of published data from North America (printed versions appear sequencially in: Audubon Field Notes, American Birds, and as a supplement to the Journal of Field Ornithology but are no longer being published, and accessible databases for Canadian data and a subset of large long-term North American plots) that use the same technique that can be used for comparisons or, in many instances, resurveys of old plots. Of further benefit is the intimacy that develops between the observer and the birds of the plot. Insights into behavior, nesting ecology, and the immeasurable benefits of watching and closely observing birds are afforded during territory mapping sessions. Other techniques require great focus and presence of mind on the task of recording data and leave little time to develop the human-bird relationship.

Disadvantages:

• Territory mapping is time-consuming and sample sizes accumulate only very, very slowly. Additionally, because the mapping process is assumed to be complete (likely to be often violated as mentioned above) there are no standard errors associated with a year’s map for a site, thus some standard statistical tests cannot be used to investigate changes.
• There is no way to estimate a within season standard error associated with the number of territories for a given species.

Traditionally those analyzing territory mapping data make the presumption that there is no error in estimating density. This is untrue, but in reality these is no way to estimate the error, so the presumption must stand but needs to be explicitly stated when results are presented. No special tests or analytical approaches to the analysis of BBC data have been created, but see our section on analyzing trend data for examples and ideas regarding estimating trend over time.

Examples of analysis approaches to collections of several to many plots for can be found in the following publications:

Johnston, D.W. and J.M. Hagan, III. 1992. An analysis of long-term Breeding Bird Censuses from eastern deciduous forests. Pages 75-84 In J.M. Hagan, III, and D.W. Johnston, eds. Ecology and conservation of neotropical migrant landbirds. Smithsonian Inst. Press, Washington, D.C.

Taub, S.R. 1990. Smoothed scatterplot analysis of the long-term Breeding Bird Census data. Pages 80-83 In J.R. Sauer and S. Droege, eds. Survey designs and statistical methods for the estimation of avian population trends. U.S. Fish and Wildl. Serv. Biol. Rept. 90(1).

Common Birds Census

Breeding Bird Density Database

Briggs, S.A. and J.H. Criswell. 1978. Gradual silencing of spring in Washington. Atlantic Nat. 32:19-26.

Hall, G.A. 1984. A long-term bird population study in an Appalachian spruce forest. Wilson Bull. 96:228-240.

Hall, G.A. 1984. Population decline of neotropical migrants in an Appalachian forest. Am. Birds 37:14-18.

Holmes, R.T., T.W. Sherry and F.W. Sturges. 1986. Bird community dynamics in a temperate deciduous forest: long-term trends at Hubbard Brook. Ecol. Monogr. 56:201-220.

Holmes, R.T. and T.W. Sherry. 2001. Thirty-year bird population trends in an unfragmented temperate deciduous forest: importance of habitat change. Auk 118:589–609.

Kennedy, J.A., P. Dilworth-Christie and A.J. Erskine. 1999. The Canadian Breeding Bird (Mapping) Census Database. Technical Report Series No. 342, Canadian Wildlife Service, Ottawa, Ontario Cat.No.CW69-5/342E-IN.

Noon, B.R., D.K. Dawson and J.P. Kelly. 1985. A search for stability gradients in North American breeding bird communities. Auk 102:64-81.

Wilcove, D.S. 1988. Changes in the avifauna of the Great Smoky Mountains: 1947-1983. Wilson Bull. 100:256-271.

Bibby, C. J., N.D. Burgess, D.A. Hill, and S.H. Mustoe. 2000. Bird census techniques. 2nd ed. Academic Press, London and San Diego, Calif. 302 p.

Cooke, M.T. 1923. Report on bird censuses in the United States 1916 to 1920. U.S. Dep. Agric., Dep. Bull. 1165. 36 pp.

Hall, G. 1964. Breeding-Bird Census Instructions. Audubon Field Notes 18: 413-416.

Johnston, D.W. 1990. Descriptions of surveys: Breeding Bird Census. Pages 33-36 In J.R. Sauer, and S. Droege, eds. Survey designs and statistical methods for the estimation of avian population trends. U.S. Fish and Wildl. Serv. Biol. Rept. 90(1).

Kendeigh, S.C. 1944. Measurement of bird populations. Ecol. Monogr. 14:67-106.

Robbins, C.S. 1970. Recommendations for an international standard for a mapping method in bird census work. Aud. Field Notes 24:723-726.

O’Connor, R.J. 1990. The Common Birds Census in the United Kingdom. p.47-53. In Sauer, J.R. 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). 166p.

O’Connor, R.J. and J.H. Marchant. 1981. A field evaluation of some common birds census techniques. Report from British Trust for Ornithology to Nature Conservancy Council, Huntingdon, England.

Scherner, E.R. 1981. Die Flächengrösse als Fehlerquelle bei Brutvogel-Bestandsaufnahmen, Ökol. Vögel 3:145-175.

Stewart, R.E. and J.W. Aldrich. 1949. Breeding bird populations in the spruce region of the central highlands. Ecology 30:75-82.

Van Velzen, W.T. 1972. Breeding-Bird Census Instructions. American Birds 26: 1007-1010.

Verner, J. 1985. Assessment of counting techniques. Pages 247-302 in R.F. Johnson, ed. Current Ornithology, Vol. 2. Plenum Pres, New York.

Williams, A.B. 1936. The composition and dynamics of a beech-maple climax community. Ecol. Monogr. 6:317-408.