Biology 363/563 Ornithology

Dr. David Swanson, Office: CL 180



The knowledge of the size or density of a population of birds is potentially a very valuable piece of information. This information is essential for studies of avian population biology, community ecology, and conservation. Many techniques exist for acquiring estimates or relative indices of bird population sizes (see Ralph and Scott 1981; Verner 1985). In this laboratory, you will conduct two distinct census techniques to arrive at estimates of bird population sizes for a riparian deciduous woodland community. The same techniques, with slight modifications, can be used for bird censuses in other community types. Separate portions of the class will perform each method on the same day in the same areas so that population estimates by the two methods will be directly comparable. A period of at least 15 minutes will be allowed between separate sampling techniques to allow the birds in the area to recover from disturbance associated with censusing techniques. If censuses are conducted during the breeding or wintering season, when populations are fairly stable, the requirement for sampling identical areas on the same day may be relaxed. The two census techniques to be used in this laboratory are the Variable Circular-plot Method (Reynolds et al. 1980; Skirvin 1981) and the Variable-strip Transect Method (Emlen 1971,1977; O'Meara 1981; Franzeb 1981; Conant et al 1981). Both methods, as adapted for riparian deciduous woodlands, are described below in some detail. Censuses should be conducted early in the morning if possible (Grue et al. 1981; Skirvin 1981), but should provide acceptable data at any time of day if conducted slowly and methodically (Robbins 1981).


  1. Description of Method

    For this method, several stations are established within a riparian deciduous plant community (or other plant community) at intervals (200 meters for this laboratory) along a transect through the community. A fixed time period for observation (by a stationary observer) at the center of each station is established depending on the complexity of the plant community (increasing complexity requires increasing observation times; we will use 10 min sampling periods in this laboratory). Each bird seen or heard is counted and the straight-line distance from the station center is estimated. No maximum distance restrictions apply for this method. Count only those birds which are actively using the study area; do not count birds flying overhead unless they may actively use the area (e.g., do not count a Great Blue Heron, do count an American Kestrel). Birds that are flushed while approaching the station are counted and their distance from the station center is estimated. Raw data will consist of a list of individual birds observed and their distance from the station center.

  2. Analysis of Data - also see Appendix 1
  3. The first step in the analysis is to plot (for each species) the density (birds/m2) within concentric bands (10 m width for this laboratory) vs. the distance from the station center. This will allow determination of the distance at which the number of birds detected begins to decline (= inflection point). For this plot, the area in concentric bands must be calculated so that the number of birds per area (density) can be used for the plot. This conversion is necessary because all the concentric bands do not possess the same area so comparison of absolute bird numbers may lead to incorrect inflection point determination. Do NOT simply plot numbers of birds vs. distance for this method! Areas for the concentric 10 m bands are as follows: 0-10m = 314m2, 10-20m = 943m2, 20-30m = 1570m2, 30-40m = 2200m2, 40-50m = 2827m2, 50-60m = 3456m2, 60-75m = 6361m2. The population density for each species in the habitat (birds/m2 or birds/km2) is then estimated by summing the densities inside of the inflection point (both the number of individuals and the areas must be summed; be sure to include areas of all bands inside the inflection point, even if there were no birds observed in that particular band). If greater than one station is used, the density estimate for all stations must be divided by the number of stations censused to provide a density estimate for the habitat as a whole. See Appendix 1 for a sample calculation for this method.


  1. Description of Method

    For this method, a linear transect of a predetermined distance (1 km for this laboratory) is established within a plant community. The observers walk this transect at a slow speed (approx. 1 km/h, 0.75 mph; Emlen 1977). Each bird seen or heard is counted and its distance from the transect line estimated. Again, no maximum distance restrictions apply for this method. Birds flying overhead should be counted according to identical restrictions as for the Variable Circular-plot Method (see above). Raw data will consist of a list of individual birds observed and their distance from the transect line.

  2. Analysis of Data

    The first step in the analysis is to plot (for each species) the number of individuals observed in successive strips (5 m divisions on each side of the transect line, 10 m total width) extending outward from either side of the transect line. The point on this plot where the number of birds detected begins to decline is the inflection point. The population density for each species in the area is then calculated by summing the densities for all strips inside the inflection point. Strip area is determined by multiplying the total strip width by the transect length (10 m X 600 m = 6000 m2 = 0.006 km2 for each strip in this example). Remember that each 5m strip extends to both sides of the transect line, so each strip is 10 m in width, not 5m.


Each student will be responsible for collecting data on the attached sheets and turning in the completed data sheets. Each student will also be required to calculate density estimates for three species of birds (or two species plus all birds combined) for each census method. These may or may not be the same three species for both methods (your choice).


Conant, S., M.S. Collins, and C.J. Ralph. 1981. Effects of observers using different methods on the total population estimates of two resident island birds. Stud. Avian Biol. 6:377-381.

Emlen, J.T. 1971. Population densities of birds derived from transect counts. Auk 88:323-342.

Emlen, J.T. 1977. Estimating breeding season bird densities from transect counts. Auk 94:455-468.

Franzeb, K.E. 1981. The determination of avian densities using the variable-strip and fixed-width transect surveying methods. Stud. Avian Biol. 6:139-145.

Grue, C.E., R.P. Balda, and C.D. Johnson. 1981. Diurnal activity patterns and population estimates of breeding birds within a disturbed and undisturbed desert community. Stud. Avian Biol. 6:292-296.

O'Meara, T.E. 1981. A field test of two density estimators for transect data. Stud. Avian Biol. 6:193-196.

Ralph, C.J., and J.M. Scott (Eds.). 1981. Estimating numbers of terrestrial birds. Studies in Avian Biology No. 6. Cooper Ornithological Society, Lawrence, Kansas.

Reynolds, R.T., J.M. Scott, and R.A. Nussbaum. 1980. A variable circular-plot method for estimating bird numbers. Condor 82:309-313.

Robbins, C.S. 1981. Effect of time of day on bird activity. Stud. Avian Biol. 6:275-286.

Skirvin, A.A. 1981. Effect of time of day and time of season on the number of observations and density estimates of breeding birds. Stud. Avian Biol. 6:271-274.

Verner, J. 1985. Assessment of counting techniques. Current Ornithology 2:247-302.




Species Distance
1 Warbling Vireo 8m
2 Warbling Vireo 15m
  Warbling Vireo 45m
3 Warbling Vireo (2) 25m

Step 1 = Calculation of inflection point
  1. List the number of birds observed in each 10 m concentric circle

    • 0-10m = 1
    • 10-20m = 1
    • 20-30m = 2
    • 30-40m = 0
    • 40-50m = 1

  2. Calculate the density for each concentric band

    • 0-10m = 1 bird/314 m2 = 0.0032 birds/m2
    • 10-20m = 1 bird/943 m2 = 0.0011 birds/m2
    • 20-30m = 2 birds/1570 m2 = 0.0013 birds/m2
    • 40-50m = 1 bird/2827 m2 = 0.00035 birds/m2

  3. Plot Density vs. Distance from the Station Center

    As you can see, the density of birds observed begins to decline after 30 m. Thus, 30 m is the inflection point, so habitat density calculations will include all birds inside 30 m.

Step 2 - Habitat Density Calculations
  1. Sum the number of birds and the areas for each 10 m band inside the inflection point.

    1/314 + 1/943 + 2/1570 = 4 birds/2827 m2 = 0.001415 birds/m2

  2. Convert birds/m2 to birds/km2

    0.001415 X 1,000,000 m2/km2 = 1415 birds/km2

  3. Divide the density estimate by the number of stations censused

    1415/3 stations = 472 birds/km2

For the VARIABLE STRIP TRANSECT METHOD, the calculations are similar except that the inflection point graph will plot numbers of birds observed vs. distance, since all strips will have the same area (10 m X 600 m). Also, you do not need to divide by the total number of stations since you are walking only 1 line.