Northern Prairie Wildlife Research Center
Assessing Breeding Populations of Ducks by Ground Counts
From 1952 to 1959, various seasonal counts and census techniques were utilized
to obtain estimates of breeding populations of ducks on an 895-acre study block
in the parklands of Manitoba and a 6,720-acre area in the grasslands of Saskatchewan.
It was determined that:
- Pintails and mallards arrived first on both areas during the last week of
March or first week of April. Later arrivals through mid-April included widgeon,
green-winged teal, shoveler, gadwall, and blue-winged teal. Redheads, canvasback,
and lesser scaup generally migrated during mid-April. Ruddy ducks were the
last to arrive in late April.
- Spring sex ratio counts showed various degrees of imbalance toward males.
Of dabblers, widgeon showed the least disproportionate prelaying sex ratio,
108:100, while blue-winged teal showed the greatest, 120:100. All divers showed
marked ratios in favour of males, from a low of 134:100 for redheads to 172:100
for ruddy ducks. No marked yearly variations in sex ratios were noted, although
samples were small.
- The parkland block contained a 4-year average, i.e., 1952 to 1955, population
of 75 pairs of dabblers and 20 pairs of divers. It enclosed 129 basins per
square mile. The grassland area breeding population averaged 50 pairs of dabblers
and 2 pairs of divers on 11 basins per square mile for the 1956 to 1959 period.
Basins averaged 0.7 acre on the parkland block and 5.7 acres on the grassland
area. Mallards were the most common breeders on both areas. Nonbreeding lesser
scaup were recorded on the grassland block during drought years, 1957 to 1959.
- Each year, for every species, there is an optimum census period in which
the greatest proportion of potential breeding pairs show ties with specific
pond sites, i.e., activity localization. These periods vary yearly with time
of migration, spring weather, and nesting phonology. However, numbers of early
and late nesting pairs may either have abandoned or not yet settled on breeding
areas at the time of the optimum census interval. Therefore, counts during
this interval may not adequately measure the over-all seasonal population
breeding in a unit of habitat. A single spring count cannot adequately assess
all pairs of a multiple breeding population with asynchronous nesting periods.
Two and occasionally three counts may be necessary to enumerate early, intermediate
and late breeding species.
From 1956 to 1959, the optimum census period for mallards and pintails in
grassland habitat was May 8 to 20; for widgeon and shoveler May 20 to June
5; and for gadwall and blue-winged teal May 25 to June 10. During these intervals
indicated pair populations showed the least fluctuation. Estimates of diver
populations should be based on a nesting study, supplemented by counts of
observed pairs and lone drakes on waiting stations. A number of recommendations
are given elsewhere in the paper for conducting census on grassland and parkland
study plots. In grasslands, breeding waterfowl counts should be made from
0800 to 1200 hours, when winds are less than 15 mph and temperatures above
- The most important potential source of error in presently used waterfowl
census techniques is the nonenumeration of groups of mallard and pintail drakes
of five or less as indicated breeding pairs. Since drakes of other dabbler
species do not congregate until the mid-incubation period of their hens, this
error becomes minor. Another major source of error is the enumeration of unmated
males as potential breeding pairs. Pair, lone drake, and grouped drake components
should be enumerated as indicated pairs for all dabbler species. The resultant
pair figure should be corrected for the unmated male segment by applying a
prelaying sex-ratio correction factor. Grouped drake components were common
in mallard and pintail populations and reflect weak pair bond and site attachments
of drakes during the early incubation period of hens. For widgeon, shoveler,
gadwall,and blue-winged teal, rarely were grouped drakes seen until mid- and
late incubation as these species have stronger site attachment and pair bonds.
Replicate counts on the grassland area, whether taken by walking or from a
vehicle, showed marked consistency of estimates for mallards but large coefficients
of variability for other species.
- Pair, lone drake, and grouped drake components of a censused population
change seasonally and throughout the day. In a mallard population, in which
most pairs were laying or in the early incubation stages, counts made periodically
throughout a day showed differences in components enumerated with each census.
Few pairs were recorded on censuses started at 0530 hours but increasing numbers
were seen at 0800, 1300, and 1530 hours with a maximum after 1800 hours. During
the same censuses, there were decreasing numbers of both lone drake and grouped
drake components seen through the day, after morning peaks at 0530 hours.
As more pairs reached the early and mid-incubation stages, fewer lone males
and more grouped drakes up to five in number were recorded.
- There are no published, standardized methods recognized to enumerate indicated
duck pairs in all habitats, nor to estimate pair numbers accurately from direct
count data. Presently used aerial census methods, through which yearly trend
data are gathered, are not sufficiently refined for use in intensive studies
of population dynamics. Comparison of results of studies that use different
population components and have no sex ratio correction on estimates is nearly
impossible to make. An urgent need exists for more testing of estimate variances
and evaluating the magnitude of sampling errors and biases in pair population
- Waterfowl pair estimation techniques remain inexact. Presently used methods
crudely assess populations during a very narrow time period but do not measure
weekly turn-over rates or seasonal populations. An apparently insurmountable
problem exists in attempts to count all pairs of one species breeding on a
habitat unit from April through June. With an extended breeding season in
which first arriving pairs have nested and drakes have abandoned home ranges
before late arrivals localize their activity and commence nesting, assessment
of populations may ultimately be based on two separate counts that are added.
The presence of renesting pairs further complicates results obtained from
adding May and June counts. Late migrants or drought-displaced pairs moving
into an area in June are still not adequately assessed. A further need exists
for more intensive studies of the behaviour, biology, and physiology of pairs
and how these factors relate to activity, mobility, and visibility of birds.
What is ultimately required is a field method to separate and assess nonbreeding,
renesting, early breeding, and late breeding pairs.
- Where (1) spring weather affects or protracts nesting phenology, (2) late
arrivals do not initiate nesting until mid-June, and (3) vegetation makes
observation of pairs difficult, assessment of populations may better be accomplished
indirectly. Intensive nesting studies and accurate measures of brood production
can be used to estimate original pair numbers. A composite of methods to arrive
at population indices may be required in some habitats. In all, the precision
obtained in population estimates will depend on the intensity of coverage
and the degree of accuracy required to fulfill the objectives.
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