3.1.6 Estimating numbers in large flocks
Every individual in a flock can be counted directly if flocks number no more than a few hundred birds. Direct counting is easy with large birds at close range, but becomes progressively more difficult with larger numbers, smaller species and greater distances (Bibby et al. 1992). Problems in counting waterbirds will occur during aerial surveys of large flocks of Western Grebes, Eared Grebes, and Great Blue Herons, and during ground or water-based surveys of large flocks of grebes that are diving or swimming, or herons that are actively feeding.
When the number of birds is greater than a few hundred birds, estimation procedures have to be used. The birds in a large flock may be estimated by counting a block of 10, 20, 50 or 100 birds and then estimating how many similar-sized groups make up the entire flock (e.g., Butler et al. 1992).
Colonial-nesting freshwater bird surveys follow a survey design hierarchy which is structured similarly to all RIC standards for species inventory. Figure 1 clarifies certain terminology used within this manual (also found in the glossary), and illustrates the appropriate conceptual framework for a ground-based survey for grebes. A survey set up following this design will lend itself well to standard methods and RIC data forms.
Figure 1. RIC species inventory survey design hierarchy with examples.
The table below outlines the type of surveys that are used for inventorying colonial-nesting freshwater birds for the various survey intensities. These survey methods have been recommended by wildlife biologists and approved by the Resources Inventory Committee.
Table 3. Types of inventory surveys, the data forms needed, and the level of intensity of the survey.
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