Point Counts of Birds: What Are We Estimating?¹

Figure 1 - Left: Simplified example of a breeding range of a bird, partially filled with territories. Top right: Simplified example of a rectangular study area containing portions of four territories. Bottom right: The same study area in which three stations for point counts (located at the x's) have been established.

 

Figure 2 - Top: The location of birds within the study area at one particular instant. Bottom: Hypothetical paths of birds in the study area taken during a period of time.

 

Figure 3 - Left: The area in which a bird can be detected by the observer at a particular instant is called the detection zone. Right: The cumulative detection zone for a bird by an observer during the period of counting.

 

Figure 4 - Cumulative detection zone of the same bird for a highly skilled observer (left) and a less-skilled observer (center). Right: Cumulative detection zone of bird increases with longer duration of counting period.

 

Figure 5 - Cumulative detection zones can be increased from the normal (left) by using playbacks of calls or other attractors (center), but such devices may also induce birds from beyond the study area to move into it (right).

 

Figure 6 - If stations are too close together, relative to the movement patterns of a bird, the bird may be double counted. The x's indicate stations at which the bird, whose path is shown, is counted.

 

Figure 7 - Compared with an area lacking roads (left), a road, indicated at the bottom of the area, may increase the count either by increasing the cumulative detection zones of birds (center) or by increasing the actual number of birds present (right), or both.

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