Northern Prairie Wildlife Research Center
Breeding Population Inventories and Measures of Recruitment
I. Introduction
C. Uses of Measures of Recruitment
Uses of measures of recruitment are similar to uses of
population inventories, and the two are often used in
combination.
1. Evaluation of Habitat
In the previous section we pointed out that the size of the
breeding population may be used to evaluate habitat. The
same breeding population will produce different numbers of
recruits, however, if the recruitment rate varies. The
recruitment rate, as well as breeding population, may be a
function of the amount and type of habitat. The number of
recruits produced or an index to it can be used to compare
the production capability of waterfowl habitats among areas
or years.
Management of breeding habitat is designed both to
increase recruitment rate and to attract birds, hence a
measure of recruitment is required to determine whether
management is successful. The number of recruits
produced is a measure of effectiveness of management
techniques designed to increase waterfowl production but
is difficult to measure. Indices to recruitment rate, such as
daily nest survival rate, are often used to evaluate habitat.
2. Harvest Management
The fall flight, composed of adults and recruits surviving
from the spring population, is a function of the size of the
breeding population, summer survival rate of adults, and
recruitment rate. Fall flight is a population parameter of
great importance to managers and is used in some
population models. When breeding population is high and
recruitment is good, the fall flight is large and should permit
a larger harvest than when
recruitment is poor. It is seldom possible to measure actual
recruitment within the time frame required for setting
regulations. For example, the USFWS relies on indirect
indices.
3. Population Models
Recruitment estimates are required to construct population
models and to verify results obtained from them. Models
have been developed to predict the result of harvest
management strategies (e.g., Anderson 1975, Hammack and
Brown 1974). These models require estimates of both
recruitment and survival. Other models deal with
management techniques that are designed to change the
recruitment rate (Cowardin and Johnson 1979, Johnson et
al. 1986, 1987a).
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U.S. Geological Survey