Assessing Breeding Populations of Ducks by Ground Counts
Recommendations for Block Study Area Census
Grassland
On the basis of the behaviour of marked birds, studies of the chronology of nesting and frequent direct counts of pairs, I propose that the following procedures be utilized for ground census on large, square-shaped study areas, e.g., 10 or more square miles, in grassland habitats, where emergent vegetation has little effect on visibility of ducks. Their use should allow estimates which more closely approximate absolute breeding numbers per unit of pond-type breeding habitat. The extrapolation of these recommendations to marsh, parkland, and wooded habitats or to transect counts might require further sampling and modification, as visibility and mobility factors vary among habitats. I assume that all field workers are able to differentiate between waterfowl species and are able to recognize various component groups: lone drakes, grouped drakes, aerial flights, migrating groups, etc.- Censuses should be conducted during an optimum interval of the breeding
season, i.e., when most pairs and drakes show maximum site attachment, an
indication that the greatest percentage of the population is in the prenesting
(including renesting), laying, and early incubation stages. Complementary
nest phenology data are required to determine optimum census periods. Breeding
chronology can also be crudely deduced from ratios of pairs to lone drakes
to grouped drakes taken during mid-morning censuses. For counts made between
0800 and 1200 hours I suggest a simple rule of thumb for optimum census time
of mallards and pintails. Ratios of pairs to lone drakes to grouped drakes
should be approximately 1:1:1 (i.e., one-third or less of the total indicated
population should be enumerated as pairs, with the remaining two-thirds or
more as lone or grouped drakes). Phenologically optimum census periods for
other dabblers, whose pair bonds and site tenacity are stronger, and whose
drakes do not aggregate until after the mid-incubation period, are those in
which the pair to lone drake ratio is approximately 1:1 (i.e., one-half or
less of the population is counted as pairs). To ensure that pairs or drakes
are truly resident and show site attachment, counts for each species must
be correlated with time of arrival on the breeding grounds and nesting chronology.
- Ground census should be conducted between 0800 and 1200 hours, local standard
time, when all species are least mobile and pairs and lone drakes are most
likely to be on their waiting sites. As few birds as possible should be flushed.
Any birds taking flight should be visually followed to locate their points
of landing. These birds are then subtracted from counts if they land on ponds
yet to be censused or added if they alight on already enumerated potholes.
Birds flushing at some distance from the observer should be recorded as unidentified
ducks.
- Census should be conducted on sunny or bright, but not heavily overcast
days, with temperatures above 40ºF and with winds not in excess of 15
mph, because rain, heavy cloud cover, low temperatures and high winds all
affect mobility, dispersion, and visibility of ducks (see Diem and Lu, 1960).
Winds increase in velocity in the Canadian prairies in afternoons. Mid-mornings
present more optimum counting and light conditions. Counts should be conducted
from the south or east edge of ponds to avoid difficulties in identification
caused by backlighting and water reflecting sunlight. Replicate counts should
be made at the same time of day during the optimum census period, over the
same route and under approximately similar weather conditions.
- To obtain adequate estimates more than one census should be taken at the
optimum period for each species and an average of four to six counts be used
rather than maximum or minimum counts of each species. Accuracy and precision
are both increased with multiple counts. Average population figures taken
from multiple counts give some indication of the magnitude of the error of
estimates due to mobility of drakes and the temporary absence of drakes from
their waiting sites.
- Because a single census can not adequately measure populations of early
nesters (mallard and pintail), intermediate nesters (widgeon and shoveler),
or late nesters (gadwall and blue-winged teal), a minimum of two different
censuses must be conducted in the grasslands to sample a multiple-species
breeding population. Under the conditions studied from 1956 through 1959,
censuses of mallards and pintails made between May 8 and 20 and censuses of
widgeon, shoveler, gadwall, and blue-winged teal made between May 25 and June
5 adequately sampled breeding populations for determination of yearly trends.
Counts made after these dates tended to underestimate breeding pairs when
drakes abandon home ranges or to overestimate populations when small, postbreeding
flocks of drakes moved into the region. Drakes whose hens are incubating congregate
and may wander away from their home ranges, beyond the boundaries of the study
area.
Where only a single census can be conducted, the optimum period in an "average" year (i.e., one with no bimodal peaks in nesting effort) could be described as approximately a week before the first mallard or pintail broods are observed and while most of the intermediate and late nesters are in the prenesting, laying, or incubation stages. At Kindersley May 15 to 25 seemed most suitable to estimate early breeders and late breeders. Single censuses conducted after May 20 will tend to underestimate the mallard and pintail segment as some drakes have left the breeding home ranges. For the intermediate and late nesters, single counts taken as early as May 15 may not assess late migrants. Censuses of mallard and pintails taken after May 31 tend to overestimate the population if small groups of five or less post-breeding drakes congregate on favoured loafing sites and are counted as indicated pairs and not as postbreeding males. Censuses may be biased in years of extended nesting. When breeding seasons are staggered, with the first broods appearing when the late breeding pairs are initiating their first nests, some drakes have already abandoned home ranges. Two separate censuses may have to be undertaken in years when cold spells protract the breeding season. - All censuses should be conducted from a vehicle which is driven to a point
overlooking each pond, but not close enough to flush birds. To assure that
all birds are visible and not sleeping on shore some minor commotion, slamming
of car door or sounding of horn, should be used to alert them. Counts should
be conducted in vegetated ponds before new growth becomes dense. In late June,
birds may have to be flushed by walking through ponds choked with emergents.
- All lone pairs and lone drakes should be considered resident, indicated
pairs if they are spaced 15 or more feet from other pairs. Before the start
of nesting in April and again in June, all aggregated pairs are to be considered
migrants, displaced birds, or postbreeding groups. Lone females are not to
be considered pairs. Practically all dabbler hens are paired and disassociation
of drake and hen is invariably temporary. Late June and July counts of lone
hens, after drakes have left the breeding grounds, may be used as evidence
of incubation and continued breeding but because of variable daily recess
periods among hens, no estimate of total number of hens incubating can be
made from single censuses taken during one time interval of the day. The enumeration
of lone hens as indicated breeding pairs should be restricted to uncommon
or rare breeding species, e.g., at Kindersley, green-winged teal made up less
than 1 per cent of the breeding population. Lone hens found over one-half
mile from the next nearest drake of an uncommon breeding species could be
assessed as a pair.
- All groupings of males from 2 to 10 should be considered indicated pairs
except for the following stipulations:
- Mallard and pintail grouped drakes of up to 10 should be considered
resident pairs until approximately 1 week before the first two to three
broods are observed, i.e., about May 20 at Kindersley. Thereafter, to
the first week of July, only groups of five or less should be considered
resident pairs. Stage of body moult and behaviour may aid in separation
of apparent breeding drakes and those in postbreeding flocks. Prior to
mid-incubation, groups of two to five, inclusive, mated mallard drakes
can be considered the same as lone males on waiting sites, as periodic
shifts toward aggregation and then dispersion occur during the day.
- Widgeon and shoveler grouped drakes of five or less in number should
be considered resident pairs until the first appearance of two or three
broods, i.e., approximately June 5 at Kindersley. Rarely do mated drakes
of these species associate before their hens are in mid-incubation. If
grouped drakes are observed before May 10 they are invariably unmated
but corrections for these individuals can be made using the prebreeding
sex ratio.
- Gadwall and blue-winged teal grouped drakes of five or less should be
considered resident pairs until the first appearance of broods, i.e.,
about June 15 at Kindersley. Again, rarely do drakes of these species
associate before their hens are in mid-incubation. Grouped drakes observed
before May 15 are usually unmated, although an unmated blue-winged teal
drake may occasionally associate with a pair or with a mated drake.
- Mallard and pintail grouped drakes of up to 10 should be considered
resident pairs until approximately 1 week before the first two to three
broods are observed, i.e., about May 20 at Kindersley. Thereafter, to
the first week of July, only groups of five or less should be considered
resident pairs. Stage of body moult and behaviour may aid in separation
of apparent breeding drakes and those in postbreeding flocks. Prior to
mid-incubation, groups of two to five, inclusive, mated mallard drakes
can be considered the same as lone males on waiting sites, as periodic
shifts toward aggregation and then dispersion occur during the day.
- Drakes in groupings of 2 to 30 males and one hen, in group flights associated
with courtship (GFAC) or attempted rape flights (i.e., aerial flights temporarily
on ponds) should be considered resident pairs as both mated and unmated drakes
join such flights. Groupings of several pads or aggregations of five or more
males and two females in apparent postbreeding groups (usually after June
1) should not be enumerated as resident pairs. A drake initiating a three-bird
flight ("territorial chase") from a pond should be considered a resident pair
even though he may land elsewhere (see Dzubin, 1957; Lebret, 1961; and McKinney,
1965, for descriptions of all aerial flights). Drakes or pairs flying over
an area are not to be counted as resident pairs. 10. Because an unknown, but
apparently large, proportion of unmated males remain on the breeding grounds
and because sex ratios of all dabbler species are unbalanced toward males,
a correction factor should be used to reduce the error arising when all drakes
are considered potential pairs. Provided all lone and grouped drakes are counted
as pairs (under 8 and 9, above) a correction factor to account for unmated
males should be applied. Such factors based on prelaying sex ratios for each
species as given in this paper (Table 3) and by Bellrose
et al. (1961) should be utilized to obtain a "sex ratio corrected population".
Sex ratios may fluctuate yearly and may also be different in pond and large
marsh habitats. Hammond (in litt.) has shown that sex ratios of mallards
and pintails can vary yearly, especially following a poor production year,
e.g., 1961. An attempt should be made to gather these ratios yearly in each
census region. Sex ratio corrected populations are important in determining
accurate productivity rates of pairs. If sex ratios do not appear to vary
yearly in one habitat type, uncorrected indicated pair figures can be used
for determining trends on transects. In summary, all dabbler drakes should
be enumerated as indicated pairs but a sex ratio correction factor should
then be applied to account for the unmated segment of the breeding population.
- Enumeration of populations of divers (i.e., Aythya sp.) is complicated
by unbalanced sex ratios and the congregation of breeding pairs on deep ponds
used as preferred waiting sites. A1though dispersal, through periodic movements
of diver breeding pairs to nearby ponds to nest, does occur, divers show contagious
and non-random distribution patterns. For portions of any day through the
breeding season, diner pads and drakes are found loosely associated. Canvasback
and redhead (and to a minor extent, lesser scaup) are also highly mobile during
the breeding season, the maximum extent of the home range being some 2 to
4 miles. Unless counts of all preferred waiting sites are made on a study
block and consideration given to home range sizes, census of divers will be
inconsistent. Also, as Weller (1959) has pointed out, a varying proportion
of redhead hens are completely parasitic and do not lay in their own nests.
They might, therefore, not be considered true breeding pairs. With divers,
only visible pairs or lone drakes known by their behaviour to be on waiting
sites should be enumerated as indicated pairs. Nest cover searches should
encompass all pond-edge and upland habitats for lesser scaup and all pond-emergents
plus nearby shrub uplands for canvasback, redhead, and ruddy duck. For block-type
study areas I concluded that a better estimate of diver populations could
be made during the mid- to late incubation period of the early nesting pairs,
through enumeration of nests, including all those which are viable, hatched,
or destroyed.
As with divers, ruddy ducks are best enumerated by nest counts as their secretive habits do not lend themselves to accurate ground census. Also, some pairs appear to be nonbreeding, summering birds. It is acknowledged that nesting studies can also tend to distort estimated population sizes per unit area as localization of prime nesting cover may attract hens from 1 to 2 miles away. Also, all nests may not be located and some late nesting pairs may be considered renesters. Enumeration of observed pairs and lone drakes on waiting sites should be used to complement nest counts and aid in estimating pair numbers. I suggest that census errors are smaller using nest estimates than those obtained from a ground count of indicated pairs which includes all drakes. Where nesting studies are not feasible the enumeration of all diver pairs and drakes and subsequent correction of these estimates with prebreeding sex ratios may give crude population estimates useful in measuring trends (Murdy, 1962). On block areas, lone diver hens need not be assessed as indicated pairs as they are invariably mated with a nearby drake, which may be enumerated as a lone male on a waiting site.
Parkland
As previously noted, two different methods were used to arrive at dabbler population estimates on the grassland and parkland study areas. In the grassland, periodic counts were averaged and a sex-ratio correction factor applied to account for unmated males. In the parkland, fewer direct counts were made but many ponds were visited daily, which led to a more intimate knowledge of pairs and species using a particular portion of the block. Pair numbers were assigned to the block if pairs were observed on or near a particular pond during three out of four censuses.The minimum block size, its configuration, pair numbers, species make-up, pond numbers, vegetation, etc., required to obtain statistically adequate estimates for accurately measuring yearly changes in population size are still largely unknown. Small blocks of 1 to 2 square miles have the advantage of being quickly censused and are most amenable to replication of counts but the assessment of pairs is subject to wide sampling error and various biases because of bird mobility, overlapping home ranges, and small sample sizes of each species. On larger blocks of 10 square miles or over, sample sizes are increased, but mobility on the edges still persists and no differentiation between late migrants and residents can be made.
Many of the above grassland recommendations can be modified for use on small sample plots of parkland habitat, i.e., 600 to 900 acres containing 50 to 100 pairs of 10 species per square mile. A weekly census of dabbling ducks on all ponds of the area, throughout the breeding season, can be used to determine peak indicated numbers of each species. Optimum census periods can then be calculated for the early, intermediate, and late breeders. For each of these three groups, counts might be conducted daily, from 0800 to 1200 hours, for 4 or 5 successive days and all pairs, lone drakes, grouped drakes, aerial flights on ponds, lone hens, and aggregated pairs or mixed-sex groups be plotted on a base map. Such data can be used to determine any localization of activity of indicated pairs. As noted under grassland recommendations, migration and nesting chronology vary with species. Counts of mallards and pintails should be conducted 2 weeks before those of other dabblers, usually in early or mid-May, i.e., about 2 to 3 weeks after the first hens start to lay. All counts can then be used to aid in estimating breeding pair abundance of dabblers, if the assumption is made that the average indicated population taken from four or five counts during the optimum census period is in fact an accurate representation of the seasonal breeding population. If late migrant pairs are noted moving into the area through the summer, periodic counts from April through July may be necessary.
It should be recogmzed that any counts conducted over a 5- to 7-day period will not adequately assess the seasonal turn-over of pairs on a block and will not measure late season migrants or breeders. Peak indicated populations may occur in June, after late season influxes of pairs (Jessen, Lindmeier, and Farmes, 1964; Evans and Black, 1956; Kirsch, in litt.). More accurate estimates of breeding pairs on small study areas can be made by use of the above census recommendations supplemented by an intimate seasonal knowledge of the behaviour, biology, and nesting success of each pair using the block. By observing pair behaviour and mobility and by locating nests, a better estimate of resident pairs can be made. Such intense field work requires that a worker confine his summer activities to only one block area. Objectives of any study will determine the degree of accuracy or precision required of any population estimate.
As in the grasslands, counts of diver breeding pair populations are complicated by irregular congregations of drakes and pairs on favoured areas. Seasonal counts of viable, hatched, or destroyed diver nests obtained through periodic nest searches, supplemented by periodic counts of pairs and lone drakes on waiting stations, can be used to assess diver populations on a small block. Census of ruddy ducks is difficult because of their secretive habitat and use of congregating ponds by mated and unmated drakes. A number of hens arrive on the breeding ground unpaired. Early morning (0400 to 0600 hours) and late evening (1900 to 2100 hours) counts on preferred congregating and nesting ponds, in which the observer quietly watches one pond for one-half to 1 hour, can be used to assess breeding pair numbers. Again, a nesting study should complement any census.
Behaviour and census
Waterfowl census, although requiring an appreciation of statistical methods and an adequate knowledge of species biology, also requires an intimate knowledge of species behaviour under a variety of population densities and environmental conditions. The decision whether to include a group of drakes and a single hen of an aerial flight temporarily on a pond or a group of pairs, in the potential breeding pair column of a census sheet, will depend on how well the census-taker knows the seasonal behaviour of each species. Waterfowl census then becomes, in part, an art for those knowledgeable workers who can appreciate key attack, sexual, and escape patterns in drakes and hens and apply such observations to decisions as to whether to include a bird, an aerial flight, or group of birds in a count. Similar sentiments have been echoed by Stokes and Balph (1965) who stress the need for an appreciation of species behaviour for a better understanding of all population ecology phenomena and by Diem and Lu (1960) who point out that "accurate interpretation of census data requires more basic knowledge of the behaviour and physiology of the individual bird".Today, waterfowl censusing remains highly subjective. Counts made by two workers are not strictly comparable for the same or different areas: Census-takers, themselves, can help infuse more objectivity into counts. Recognition of ducks must be instantaneous. They should also be able to determine by sample counts of pair, lone drake, and grouped bird ratios, the optimum census periods. They should readily recognize any biases and potential sampling errors involved. Accuracy and precision of estimates can, in large part, be a reflection of the knowledge and mental alertness of the individual conducting the census.
Seasonal counts and periodic replication of censuses tend to increase accuracy of estimates. A high degree of accuracy and precision is justifiable on special study areas from which pair population estimates are used to calculate the true number of pairs successful in producing broods. Also, increased precision leads to the better detection of the effects of various limiting factors, however minor, on final production. Valid comparison of results of work in various habitat types is facilitated by data with known sampling errors and variability. Statistically describable estimates of seasonal breeding populations based on proven census techniques still remain a basic need of most waterfowl research and management programs.
Supplementary data required
Waterfowl breeding pair census will become more meaningful as data become available on the following topics:- The proportion of any pair population which is nonbreeding and the climatic, density, or habitat factors which lead to nonbreeding: do all yearling lesser scaup and late-hatched mallards of the previous year nest? If they do breed, are they late nesters?
- Duration of the pair bond in seasons of varying phonology: how long does the "average" drake of each species remain on his activity centre and when do drakes abandon home ranges, i.e., how long are drakes available for counting as indicated pairs? Does site attachment strength change with increasing density?
- Mobility radii and home range sizes of lone drakes, pairs, and grouped drakes: how does pond density and availability of breeding requisites in each habitat affect home range and activity centre size? What are the daily and seasonal patterns of activity for each species and each population component? How do daily activity patterns of lone or grouped drakes affect spatial distribution and census?
- Yearly prelaying sex ratios of all species on the breeding grounds: what is a statistically adequate sample to describe prelaying sex ratios of a species? How long is the period of residence of unmated drakes on nesting grounds and when do they leave for moulting marshes? Do unmated drakes migrate earlier or later than mated ones?
- Turn-over rates of local populations and the adequate census of total seasonal populations: should indicated pair population estimates obtained in early May be added to mid-June estimates to account for delayed nesting by late migrants, yearlings, and shifting populations? Does the maximum or mean pair population counted during one 3-week period of a 2½-month-long breeding season adequately assess breeding populations actually breeding on a block or transect?
- The effect of nonrandom and contagious distribution patterns of ponds, pairs, lone drakes and aggregated drakes, on sampling procedures: how do periods of intense spacing activity followed by increased sociability of drakes affect pair and drake distribution and their countability on strips or blocks of habitat?
- Length of breeding home range residence of pairs which have lost clutches: preliminary observations indicate that some mallard pairs may remain on or near their home range for periods up to 3 weeks after their clutch is destroyed. These pairs make no attempt to renest in this interval, and may never renest, but they are enumerated as indicated breeding pairs.
Thirty years after the publication of Bennett's (1938) census recommendations, waterfowl biologists are still using methods which are not being constantly challenged, improved, and tested. Field workers have commendably adapted recommended census methods to varying habitat conditions but too few of them have published or tested their counting techniques. Meaningful comparisons of results of studies by two workers using different methods, with varying error estimates, are almost impossible to make. Apparent differences of yearly estimates may be due more to variations in census methods than to actual population status (Diem and Lu, 1960). Although trend data obtained from population indices are sufficiently accurate for yearly management purposes (Crissey, 1957), trends are not adequate for precise measures and descriptions of species population dynamics.
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