Northern Prairie Wildlife Research Center
Assessing Breeding Populations of Ducks by Ground Counts
Because of poor visibility of pairs in the heavily vegetated parkland ponds, and
because of the relatively large (10.5 square miles) study block in the grassland,
two different census methods were adopted to assess the abundance of breeding
Roseneath Study Area
Each pond on the 895-acre study block was visited a minimum of four times during
a 7- to 10-day period when most of the early nesting pairs of mallards and pintails
were in the laying or early incubation stages. Because of yearly variations in
phenology, the census period varied, but was usually between May 5 and May 25.
The exact locations on ponds of pairs, lone drakes, and groups of five or less
drakes were plotted on a base map. If a pair or lone drake was observed on or
near the same pond on three out of the four counts, a breeding pair of that species
was "assigned" to that area. This method was similar to the one proposed by Evans
and Black (1956) to test "constancy" of pond use. It is also similar to a method
in use by Kirsch (in litt.) on the Woodward Study Area, North Dakota. Counts
were conducted in morning and late evening hours when many hens were off their
nests for recess periods. A comparable census was conducted 2 to 3 weeks later
when most of the late nesting dabbler species-widgeon, gadwall, blue-winged teal,
shoveler, and green-winged teal were also in the laying or early incubation stages.
Again pairs were assigned to a particular pond or localized area. Where the number
and species of pairs breeding in a locality was doubtful, 2 to 3 hours of observation
on 4 or 5 consecutive days helped resolve the questionable count. In short, the
accuracy of the census depended on an intimate seasonal knowledge of the pairs
continually utilizing a localized area and on the assessment of these birds as
indicated breeders. Because the study block was small and home ranges of many
pairs would encompass all or parts of it (Sowls, 1955; Dzubin, 1955), I also periodically
censused pairs in the quarter-sections surrounding the study area to determine
populations. Pairs were arbitrarily assigned to the study area only if the drake's
or pair's waiting area (Hochbaum, 1944; owls, 1955; Dzubin, 1955) was located
within boundary. Censuses of a small block-type area, such as the 895-acre Roseneath
Study Area, do lend themselves to close approximations of breeding pair numbers
as the ponds, upland and even pair populations form an integral part of a much
larger complex of habitat surrounding the block. As such, the assigned population
is an estimate of the pairs utilizing ponds on the study block as waiting areas
and does not include the pairs breeding in its immediate environs and using the
study area ponds periodically.
Early in the study I concluded that ground census of diving ducks-canvasback,
redhead, lesser scaup, and ruddy ducks-utilizing various ponds would not adequately
estimate breeding members. Diver pairs, except ruddy ducks, tended to aggregate
on particularly deep ponds that I named "primary waiting areas" (Dzubin, 1955),
and to fly to surrounding smaller ponds for nesting, feeding, and loafing. On
the study area, two such congregating ponds served 15 to 25 pairs that nested
on the block and ponds surrounding it. A census of pairs and lone, unmated drakes
on such primary waiting ponds could not be used to estimate pairs actually breeding
on the block. Canvasback and redheads have maximum home range sizes of 2 to
4 square miles. Such mobile species with large ranges do not lend themselves
to adequate census on a small block. Lesser scaup are not as mobile while ruddy
ducks tend to be sedentary. Some redhead hens were semi-parasitic, some were
completely parasitic, while others laid normal clutches (see Weller, 1959).
The distorted sex ratio in all divers and especially scaup (Bellrose et al.,
1961) made counts of diver lone males to indicate pairs, meaningless. The secretive
habits of ruddy ducks also made analysis of observational data difficult. Therefore,
diver populations were censused through a nesting study, wherein all emergent
cover was periodically searched for over-water nests. The maximum number of
viable, destroyed, and deserted nests found during the peak breeding period
was used to estimate the breeding population on the study block.
Several basic assumptions were made in arbitrarily assigning breeding pair
numbers to the study block:
- That lone drakes or pairs of dabbling ducks localize their breeding activity
to one or more ponds and are consistent in their use of waiting areas. Previous
studies on marked birds had shown that all breeding pairs restrict their activity
during the prenesting, laying, and early incubation periods (Sowls, 1955;
Dzubin, pers. obs.). However, much individual variation in activity localization
within any species occurs, and species differ in hourly and daily mobility
and home range sizes (Sowls, 1955; Dzubin, 1955; Evans and Black, 1956). Sowls
(1955: 54-57) has reported that on a ditch near a marsh much interchange of
pairs between the same waiting site occurred through the day. I noted more
interchange of waiting sites under dense pair populations of the grassland
than under less dense populations of the parkland.
- That any ingress of pairs onto the block was counterbalanced by a similar
egress of the same number and species of pairs out of the study area. This
is the most difficult assumption to assess, in the light of the wide home
range size of pintails, mallards, and divers. However, under the low population
levels with which I worked, assumption (1), above, was considered to be valid.
- That all pairs counted on the block remained there to nest and that all
species present bred. I noted that pairs were occasionally displaced by other
pairs or remained on the study area for 1 to 2 weeks without any nesting attempt.
This was especially true of a small number of late nesting gadwall and widgeon
pairs that tended to move off the area as soon as the nest site was chosen
away from the study block. There was no evidence of nonbreeding pairs of any
species in the parkland, except in the case of parasitic redheads.
- That turnover of pairs was minimal. Hochbaum (1944: 158) and Smith and
Hawkins (1948) discussed the possibility of late nesting pairs moving into
an area and not being enumerated by a census conducted during one interval.
Almost yearly I noted an influx of five or six mallard pairs in late May or
early June. Drakes of such pairs were brightly plumaged, unlike the drab males
that had been seen in the area for the previous 2 weeks. These pairs appeared
to be late breeders, nesting for the first time. In the population assessment,
they were not considered to be breeding pairs over and above those censused
in mid-May. As I could not determine if they were new breeders or renesters,
I again assumed that an equal number of renesting mallards left the area to
breed elsewhere. Acceptance of this assumption probably led to underestimation
of seasonal population sizes each year.
- That all diver nests were located in the emergent vegetation of study area
ponds. This assumption was considered valid as all emergent vegetation was
checked for nests at 2-week intervals. Any diver pairs that utilized the study
area as a part of their home range, and nested on a pond immediately off the
area, were not counted.
At best, estimates taken from direct ground counts of pair numbers breeding
on a small block of parkland habitat should be considered relatively imprecise
approximations of seasonal breeding pair populations. The estimation of absolute
breeding pair numbers per unit area remains an inexact technique, subject to
many vagaries of species behaviour, visibility, mobility, and seasonal nesting
Coding of Population Components--Kindersley
All data were coded by a system adopted after a U.S. Bureau of Sport Fisheries
and Wildlife ground-census code, i.e., 1/0 = pair, 0/1 = lone male, 3/2 = 3 pairs
and 2 lone males, 3:0 = 3 grouped males, 5:5 = 5 grouped pairs, 0/F = lone female,
0:4 = 4 grouped females (W.H. Kiel, in litt.).
Although Hochbaum (1944: 85) recommended that only territorial pairs and lone,
waiting drakes be censused as breeding birds, he described seven categories
of ducks found in a breeding marsh from April to early July: (1) unmated ducks
not yet courting, (2) unmated ducks in prenuptial courtship, (3) mated pairs,
(4) novice drakes, (5) sexually active unmated males, (6) summering drakes,
and (7) unsuccessful or nonbreeding females.
For counts of dabbler species on the potholes of the Kindersley Study Area
the following 10 categories were adapted for use after Hochbaum's components:
- Resident pairs-lone pairs on ponds, or pairs spaced over 15 feet
apart along sections of shore line. These were apparently settled, dispersed
- Grouped pairs and drakes-aggregated pairs or pairs and drakes that
behaved as flocks and were not spaced. They were apparently migrating individuals,
not yet settled or dispersed.
- Breeding-season groups-aggregations of 2 to 20 + males and one hen
on ponds. Birds were assigned to either (a) "group flights associated with
courtship" (GFAC), in April and early May, if males were giving displays and
hens were "inciting", i.e., "spring courting flight" of Dzubin (1957); or
(b) "attempted rape flights" (ARF), if drakes harassed lone hens which gave
the repulsion call (Dzubin, 1957; Raitasuo, 1964; McKinney, 1965). In mallards
and pintails these latter flights were seen after May 5, at a time hens start
to incubate. For other species they were generally recorded after May 25.
Birds in "three-bird flights" (TBF), i.e., territorial pursuits, were also
noted. If the pursuing drake returned to the pond under observation, he was
recorded as a lone drake. If the chased pair landed in an already censused
pond, it was also recorded.
- Postbreeding-season groups-aggregations of males and two or more
hens which behaved as a unit. These groups were usually observed after June
15. Such flocks of drakes and pairs were considered to be in postbreeding
condition and not part of the breeding population.
- Lone drakes-drakes that were spaced over 15 feet from other drakes
on waiting stations. These drakes were generally observed through the laying
and early incubation periods. The distance separating drakes varied with phonology
of season and the species.
- Grouped drakes, five or less- drakes associated with other drakes
in small, cohesive aggregations of two, three, four, or five. These groups
were observed in mallards and pintails from late in the laying period through
the mid-incubation period, April 24 through June. In other dabblers they were
seen from mid- to late incubation, beginning May 20 through June. Small groups
of unmated drakes, two to five in number, of all species, were occasionally
recorded through April and early May.
- Grouped drakes, more than five-aggregations of more than five drakes.
They were usually observed in the late incubation or postbreeding periods.
- Lone Hens-hens not associated with drakes. This category included
hens that had just left their nests, after laying or during incubation recesses,
and had not yet rejoined their drakes. They made up less than 4 per cent of
any population count in May and early June but were more common in late June
after drakes had left for the moulting grounds.
- Grouped Hens-two or more hens in aggregations that behaved as units.
They were observed in the postbreeding period after June 15 and to July 10.
They included hens that had either lost clutches or had abandoned near-flying
broods. Rarely would two or more incubating hens on recess from the nest be
seen together. They were not considered pairs, even though their drakes may
have already abandoned the home range.
- Drake-hen Ratios-where divers, and occasionally migrating dabblers,
were associated in loose aggregations, and pairs and lone drakes could not
be separated, counts were lumped and recorded as a ratio of drakes to hens,
Kindersley Study Area
Direct counts of all ducks were used to estimate population levels on the grassland
study block Because of staggered breeding seasons and differential times of migration
and nesting of each species, a number of counts were conducted through April,
May, and June. Few breeding pairs or lone drakes of any species were recorded
after July 15.
In 1956, the 10.5-square-mile study area was divided into three sections. All
of the "indicated" pairs on the ponds of each section were counted by two men
who walked together to each pond. Although an attempt was made not to flush
pairs, it was unavoidable on small, open ponds where some pairs tended to flush
as far as 200 to 300 yards away. Pair counts made on ponds in the western third
of the study area showed that from 20 to 45 per cent of the pairs were flushed.
Pairs or drakes seen to land in a pond not already tallied were subtracted from
the pond totals. However, some censusing of previously counted pairs undoubtedly
occurred. The magnitude of the duplication error was unknown and variable. However,
the consistency of counts taken in mid-May was high, and comparison with nests
found, especially for the major breeding species (i.e., mallard), showed no
wide discrepancies. Counts were conducted from 0530 to 1100 hours, M.S.T., at
approximately 7-day intervals from May 3 to June 11.
From 1957 to 1959, inclusive, breeding pair counts were made with binoculars
or a 20x telescope from a vehicle that was driven to a point overlooking each
pond. Fewer than 5 per cent of all ducks were hushed. Approximately 22 miles
were driven during the census of the 10.5-square-mile block. Censuses were generally
conducted on bright days when wind velocities were below 15 mph. Two censusers
working together were able to survey all wet ponds between 0800 and 1200 hours.
I had determined that pairs of most species were least mobile during this period.
Fewer than 15 of the 114 pond basins contained dense stands of dried emergents
(Carex, Scirpus, Beckmannia, or Glyceria) in which pairs could
secrete themselves. It was generally after May 20 when emergent vegetation grew
to a 6- to 8-inch height, and therefore no attempt was made to "beat-out" ponds.
Near emergent-rimmed ponds, the slamming of the car door or sounding of a horn
was sufficient to alert the birds enough to make them visible for censusing.
Again counts were made at 5- to 10-day intervals from early April to mid-June.
As on the Roseneath Study Area, census of pair and drake divers was not considered
an adequate measure of the number of pairs breeding on the study block. Therefore,
data on breeding divers were obtained from an associated nesting study. A viable,
hatched, or predator-destroyed nest was considered evidence of a breeding pair.
To ensure that dabbler censuses were conducted at an optimum period, a similar
nesting study was simultaneously conducted to determine breeding season chronology.
Census dates were arbitrarily chosen to ensure coverage during optimum breeding
periods for early nesters, mallards and pintails; intermediate nesters, widgeon
and shovelers; and late nesters, blue-winged teal and gadwall.
Analyses of Census Results
Census results from each survey made during the optimum breeding interval for
each species (i.e., when the greatest proportion of hens of pairs were laying,
incubating, or in revesting breeding phases) were lumped and an average breeding
pair figure calculated. All mallard and pintail pairs, lone drakes, grouped
drakes, and grouped drakes in group flights associated with courtship and attempted
rape flights, counted before May 20, were considered breeding pairs. For widgeon
and shoveler, all of the above categories were considered breeding pairs if
censused before June 5, while for blue-winged teal and gadwall all of the above
categories were considered breeding pairs if noted before June 10. Lone hens
were not added to the population counts as I assumed all hens to be paired to
one of the lone or grouped drakes already enumerated on the study block. Since
fewer than four lone hens of any species, or less than 4 per cent of the indicated
pairs counted from 1956 to 1959, were ever encountered on any census, any error
in deleting this component would be small. The average number of pairs was termed
the "mean indicated breeding population".
Sex ratio data from Bellrose et al. (1961) had shown that among most
dabbler species there is a preponderance of drakes immediately prior to the
breeding season. I collected similar data during the prenesting interval. To
ensure that pair census data were not weighted with the unmated-male segment,
a correction factor using the average prelaying drake-to-hen ratio for each
species was applied to the indicated population. The breeding-pair population
figure was then termed the "sex-ratio corrected population". Sex ratio corrections
were applied only to dabbler figures as diver populations were assessed from
nesting studies. Murdy (1962) has more recently applied such corrections to
spring censuses of lesser scaup and ring-necked ducks at Yellowknife, N.W.T.
In 1956 and 1959, in order to test stability of population. and reproducibility
of census counts, four or five censuses were made during the period when the
greatest proportion of pairs were in the prenesting, laying, or incubation periods.
In 1959, periodic censuses were conducted three times a day on May 11, 15, and
16 to determine daily variability in the census and percentages of pairs, lone
males, and grouped males during any one time interval.
To clarify further what population components were used in breeding-pair counts,
the following definitions will be followed throughout this paper:
- Assigned breeding population-The dabbler breeding population assigned
to a unit of habitat, usually 160 acres. The population was determined by
plotting the location of pairs and lone drakes on a map during four or five
censuses. If a pair or lone drake of any species was seen on a particular
pond three or four times, it was assigned to that pond or quarter-section.
- Indicated breeding population-The population estimated from counts
of various components.
- Mallards and pintails: Prior to May 20, all lone males, pairs, all
grouped males, and males in aerial flights temporarily on ponds censused
were considered pairs. Thereafter, all lone males, pairs, grouped drakes
of five or less, and aerial flights temporarily on ponds were considered
evidence of breeding pairs. After May 20, groups of six or more males
were considered postbreeding birds.
- Widgeon and shoveler: Prior to June 5, all lone males, pairs, all grouped
males, and all males in aerial flights on ponds censused were considered
pairs. Thereafter, only grouped males of five or less in number, lone
drakes, drakes in aerial flights, and pairs were considered breeding pairs.
Groups of mated males, usually in two's and three's, were rarely observed
until mid- or late incubation. Groups of unmated males were, however,
- Gadwall and blue-winged teal: Same as widgeon and shoveler except cut-off
date of June 10 was used. Again, associations of mated males were uncommon
- Divers: All viable, hatched, destroyed, or deserted nests found, which
were initiated prior to June 15, were considered evidence of a breeding
pair unless field observation indicated a nest to be a renest. With ruddy
ducks the cut-off date was extended to June 20.
- Sex-ratio corrected population-This was the mean indicated breeding
population taken during four or five censuses, during the time most breeding
pairs were in the prelaying, renesting, laying, or incubation stages, to which
a sex-ratio correction factor as found in Table 3
was applied. The correction factor was determined from spring counts of the
sex ratio of each species before the first eggs were found. It reduced the
indicated pair population by the proportion of unmated drakes found in the
species. The sex-ratio corrected population was considered to be the best
estimate of the absolute breeding population of a study area.
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