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Breeding Population Inventories and Measures of Recruitment

VI. Selecting a Method

The most important step in defining the objective is to answer the questions why, where, and how to measure breeding population and recruitment. Once the why and where are known, the how will be much more apparent. The purpose will usually have to be narrowly defined because resources available to the biologist are generally limited and methods that meet one purpose fail for others. Answering the following questions will aid the biologist in selecting appropriate methods: (1) Is primary interest in population dynamics or in comparison of nesting habitats or management methods? (2) If interest is in population dynamics, will indices be sufficient or are estimates of parameters required? (3) What are the geographic areas and period of time for which estimates are required? (4) What resources are available?

If interest is in population dynamics, a thorough understanding of biology of the individual species is essential. A survey designed to estimate numbers of breeding waterfowl or recruitment rate without regard for biological differences among species will probably yield data of questionable value. Survey data are most appropriate for obtaining information on status of populations. If the data are sufficiently detailed and accurate, they may be used to construct or to drive population models. Because of the mobility of most waterfowl, such applications usually apply to large geographic areas.

Several decisions must be made prior to selecting a method for estimating breeding population and recruitment rate. The first is whether parameter estimates are required or whether indices will suffice. Indices have many advantages because simplifying assumptions may often be made. In practice, indices are often the only option possible with the available resources. A pilot study may be required to determine necessary sample sizes. Making informed decisions on sample size can avoid future problems. If resources are not sufficient to accomplish the objectives, it may be preferable to abandon or at least modify the proposed survey or study.

Managers of local areas are frequently interested in the effect of specific management actions such as establishing certain kinds of nesting cover or building impoundments. Evaluating the impact of these types of treatments on the size of the breeding population or recruitment rate is difficult because of the many confounding factors that influence the results. It is preferable to narrow the objectives to questions such as the effect of the treatment on nest success or recruits produced. In addition, questions about the effect of individual treatments are best answered by designed experiments that may call for different types of measurements than those used in surveys.

Prior to estimating breeding population or measuring recruitment, the population being sampled must be defined. For example, if recruitment is being measured on a block study area and a breeding pair is counted on the study area but nests off the study area, do the progeny from this nest count as recruits?

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