Northern Prairie Wildlife Research Center
Breeding Population Inventories and Measures of Recruitment
I. Introduction
B. Uses of Population Estimates
1. Goals and Objectives
Waterfowl managers assume that population parameters
such as the size of the breeding population and fall flight
can be manipulated by management actions. The goals and
objectives of management are therefore stated in terms of
numbers of birds; consequently, populations must be
inventoried in order to determine if the goals and objectives
have been attained. In fact, prior inventories are usually
required in order to set reasonable objectives.
2. Population Trends
Declining populations of waterfowl have been cause for
concern for many years. Lincoln (1936) discussed low
numbers of ducks and outlined the first systematic attempt
to estimate the North American duck population. An
unbiased index to actual population size is often all that is
needed, but trends are difficult to establish because of
variation due to climate (Johnson and Shaffer 1987). Not
only must the data be long-term, but also the methods used
for collecting the data must be consistent. Even when long-term data sets are available for assessing population trends,
interpretation of the data is extremely difficult.
The breeding population in any year is a function of both
recruitment and survival rates in previous years, which are
themselves functions of factors such as weather, predation,
and hunting. Peaks and valleys in the trend line, such as
that for the Mallard (Figure 13-1), may reflect variation in
habitat conditions. The trend therefore may be different
depending on the year in which the data set begins. For
example, if the Mallard breeding population estimates
(Figure 13-1) had begun in 1965 rather than 1955, there
would be no apparent decline. This fact reinforces the need
for long-term data. Johnson and Shaffer (1987) reviewed the
Mallard trend data with this in mind and attempted to
determine whether an apparent decline in Mallard breeding
pairs resulted from changing water conditions or from
changes in survey methods, or whether it represented a true
decline. They concluded that population had actually
declined.
1955-87 Mallard Breeding Population Estimates
 |
| Figure 13-1. Mallard breeding populations for North America from 1955 to 1987. |
3. Evaluation of Habitat
There is an obvious though complex relation between
habitat quantity and quality and population density.
Counts, representing the number of birds on an area, or
indices, assumed to vary in proportion to the number of
birds, therefore serve as a measure of habitat quality. In
general, areas with abundant wetlands and good-quality
nesting habitat are considered good for waterfowl. The
Canada Land Inventory (Perret 1969) used this relation to
map waterfowl habitat according to several classes. The
maps were based primarily on landform and other physical
land characteristics as determined from aerial photographs.
A similar approach has been used in the United States.
Stewart and Kantrud (1973) related observed waterfowl
density to glacial landforms and then prepared a map of
waterfowl habitat in North Dakota (unpublished map, on file
Northern Prairie Wildlife Research Center, Jamestown,
North Dakota).
4. Harvest Management
Management of harvest by means of regulating hunting of
waterfowl is a traditional waterfowl management method.
The need for population estimates on which to base annual
hunting regulations led to the establishment of the North
American cooperative breeding ground surveys. Although
the impact of harvest on annual survival of Mallards has been questioned (Anderson and
Burnham 1976, Nichols et al. 1984), annual regulations
remain one of the primary operational tools used by
waterfowl managers in North America. Each year, estimates
of breeding populations and recruitment are used in the
process of setting annual regulations. In addition,
population inventories and trend data are used to determine
when populations have reached levels that require
restrictive regulations. Recommendations for population
levels that will require specific harvest management
action have been published for North America (U.S. Fish
and Wildlife Service and Canadian Wildlife Service 1986:15).
Harvest of ducks has usually been restricted when
populations reach a predetermined low number. Some local
goose populations have been increasing to the point where
they cause depredation problems or exceed the carrying
capacity of refuges. Goose management plans may call for
changes in local regulations or management techniques
designed to move geese when local populations exceed
specified levels (Hunt et al. 1962, Craven 1978).
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