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Assessing Breeding Populations of Ducks by Ground Counts

Appraisal of Factors Influencing Inventories


On the basis of field programs on the two study areas, a number of corrections to potential sampling errors and biases and solutions to problems of duck population estimation present themselves.

Unmated Males and the Census

Although the proportion of unmated males that remain through the breeding season on the pothole breeding habitat is unknown, there are observations that May sex ratios may be more distorted in large marshes where unpaired mallard and pintail males congregate (Hochbaum, 1944; Ellig, 1955). For parkland pond habitat, Diem and Lu (1960) discussed the errors involved in censuses because of distorted sex ratios and enumeration of unmated males as indicated pairs. For the present study, I assumed that the greater proportion of unmated males remained on the study areas until immediately prior to the brood season. Firstly, the assumption was based on some incomplete data on marked, unpaired drakes. Of 23 mallard, pintail, gadwall, widgeon, and blue-winged teal unpaired males marked in 8 years, 11 were subsequently seen on or around the study areas where they were marked, for periods up to 42 days. Furthermore, observation of marked pairs showed that as many as one in four may disassociate themselves from the home range because of the disturbance due to marking and capture. Utilizing a similar ratio for unmated drakes, I concluded that from 48 to 65 per cent of the unpaired drakes may also remain on or near a study area, at least during the laying and early incubation periods and be available for censusing. Secondly, in 1964 and 1965, at Waubay, South Dakota, Drewien (in litt.) established that unmated blue-winged teal drakes composed 15 to 20 per cent of the resident population of drakes of a study block and remained there throughout the breeding season. Unmated males showed varying degrees of site tenacity and localization of activity through the month of May to early June, as did the mated males and breeding pairs. Thirdly, Bossenmaier (1951: 61), who intensively censused a major moulting marsh Whitewater Lake in southern Manitoba, did not record the first flocks of 10 or more mallard drakes until May 28, 1950, and May 21, 1951. While these may have been either previously paired, postbreeding or unpaired drakes, he gives no records of earlier congregations of drakes. I suggest that the greater proportion of both mated and unmated drakes of most dabbler species do not leave the breeding ponds for the moulting lakes until the nesting season is well under way. They are thus located on the breeding grounds and can be erroneously assessed as potential breeding pairs.

On both study areas, marked, unmated males were seen to aggregate with mated males whose hens were incubating and to take part in GFAC and ARF. They may also associate with pairs as novice drakes (Hochbaum, 1944) or form groups of two, three or more. In early April, associations of two or more unmated pintail drakes were common but fewer small groupings of unmated drake mallards were observed. During the prenesting period, groupings of two unmated males of other dabbler species were also uncommon except for blue-winged teal.

As all males are counted as potential breeding pairs, the use of a correction factor to disregard unpaired drakes should be valid, in order to assess only true pairs.

With all species the number of unmated drakes observed in the prebreeding period varies slightly from year to year (Table 3). If sufficient counts are made to show a significant difference, then the yearly sex ratio correction factor should be applied to the indicated population based on all drake and pair counts. If no counts are available the average prebreeding male to hen ratios found in Bellrose et al. (1961) and in Table 3 might be used.

Sex Ratio Correction Factors

The problem of overestimation by enumerating unmated lone males as indicated mated pairs becomes acute in those dabbler and diving duck species which are regularly known to have distorted sex ratios on the breeding grounds (Bellrose et al., 1961). Since it is almost impossible to separate lone mated from lone unmated drakes on the basis of plumage or obvious behavioural characteristics, I propose that all drakes should be enumerated as breeding pairs 1 to 2 weeks before the first appearance of broods, and that a prelaying sex ratio correction factor should be applied. For yearly trend information, in those species whose sex ratio may not fluctuate widely, enumeration of all drakes should not distort measures of relative abundance. For more sophisticated and accurate measures of populations and the factors limiting recruitment on special study blocks some correction for the unmated male component must be made (cf. Table 9).

The errors involved in assuming that all unmated drakes are indicated pairs have been recognized by many workers (Hochbaum, 1944; Murdy, 1953; Sowls, 1955; Diem and Lu, 1960; Bellrose et al., 1961; and Hammond, in litt., 1966). Hawkins, Gollop, and Wellein (1951) concluded that obtaining prebreeding sex ratio data offered a logical method of correcting aerial counts for hens not seen associated with drakes. However, Kiel (1951) cautioned against the use of sex ratio correction factors because egress of premoulting males might equal disappearance of nesting females from the transects, the result "being a constant sex ratio but a decreasing countable population". On the Yellowknife Study Area, Northwest Territories, Murdy (1962) enumerated all drake lesser scaup and ring-necked ducks and applied a sex ratio correction factor for the imbalance of males. The method assumes that sufficient prebreeding sex ratio data are available. Murdy (1964) later utilized counts of pairs and drakes on waiting sites to estimate breeding populations.

Late Influxes of Pairs

In May, the separation of late migrant pairs or drought-displaced birds from residents is nearly impossible to make. The criterion I used revolved around whether pairs were grouped or spaced and general behaviour, such as whether hens gave the persistent quacking call or drakes showed hostility toward hens of pairs. However, during the transitory, postmigration period pairs may be spaced from other pairs but do not yet show activity localization. Such pairs could be included as residents.

Late May and June influxes of breeding pairs nesting for the first time or influxes of renesting pairs are apparently common on some marsh and pond study areas (Jessen, Lindmeier, and Farmes, 1964; Hammond, 1959; Kirsch, in litt.). Such influxes give rise to a multitude of sampling problems in the accurate assessment of the seasonal or total number of pairs which attempt to nest on a specific area. Hammond (1959; letter June 11, 1967) noted influxes of birds into the marsh habitat of Lower Souris National Wildlife Refuge, when: (1) sheet water disappeared from fields surrounding the marsh, (2) mass destruction of nests occurred through farming practices, (3) drakes and hens made premoult movements away from their breeding grounds, possibly after only one nesting attempt. Yearly, I recorded early June influxes of mallard pairs on the Roseneath Study Area, where ponds were intensively surveyed, but could not differentiate late migrants from resident pairs on the Kindersley Study Area because of much larger populations of all species. At present, the sampling problems involved with adequate census of seasonal populations and the differentiation of residents from new arrivals appear insurmountable. Where influxes occur indirect methods, concerned with nest counts and brood numbers, may have to be used to estimate breeding populations.

GFAC, ARF, TBF and the Census

Both mated and unmated drakes join spring and summer aerial flights, i.e., group flights associated with courtship and attempted rape flights, and all drakes in such flights temporarily on ponds have been enumerated on the Kindersley Study Area. As such flights may extend for 1 to 4 miles and gather drakes from a number of ponds they would tend to distort population estimates from narrow one-quarter-mile roadside strip counts and therefore might be deleted. Observations of behaviour of drakes in aerial flights temporarily on ponds should be noted so that late season, postbreeding groups of males and pairs are not classed in error as breeding birds in flights. The number of flights observed is small in relation to over-all population numbers, except for ARF of pintails and GFAC of lesser scaup, where groupings of males with one hen are common through May. Attempted rape flights may be more frequent in all species during late afternoons, at a period when incubating hens are taking their recesses and are more prone to attract drakes. If censuses are conducted in the 0800- to 1200-hour interval the number of three bird flights seen is much reduced over the early morning (0530-hour) and late evening (1800- hour) periods. Therefore, sampling errors due to mobility should be minor. Most mallard ARF were observed in the early mornings and late afternoons, after hens started to incubate. Few were enumerated in the 0800- to 1200-hour census interval. Mallard and pintail GFAC are usually seen in early and mid-April and are commonly associated with migrating flocks. The number observed per day on pond breeding habitat, during any optimum census period in May, will be negligible, especially if transects or study blocks are located at some distance from habitat where migrants congregate. For other dabbler species few GFAC were recorded after May 15 or during the optimum census period.

Pair Bond Duration and the Census

Much variation of pair bond duration exists between and within species of Anatidae. The duration of the pair bond, size of home ranges, and period of localized activity on a waiting site all affect accuracy of spring pair censuses. Hochbaum (1944) recorded the early abandonment of hens by drake mallards and pintails, usually after laying of the clutch. Sowls (1955: 101) noted that blue-winged teal and shoveler drakes did not abandon their hens until late incubation, while mallard, pintail, and gadwall did not associate with the hen after clutches were complete. Similar observations were made by Smith (in Bellrose et al., 1960: 427). Leitch (1952) noted that mallard and pintail drakes remained on territories the shortest time, while blue-winged teal, widgeon, and shoveler drakes remained on their territories much longer and population figures for these species, taken from ground counts, should be quite accurate. Bue (1952) pointed out wide variations in the times that species and individual drakes abandon home ranges. Similar variations in stage of incubation at which drakes desert hens were also reported by Oring (1964). McKinney (1965) summarized the available literature on pair bond duration in North American Anatidae. He showed that many diver and dabbler drakes did not associate with their hens after the first week of incubation. However, blue-winged teal, shoveler, gadwall, and lesser scaup generally abandoned their hens only after the second or third week of incubation. Gates (1962) reported that drakes of renesting pairs of gadwall abandoned their hens sooner than drakes of initial nestings, a conclusion I have substantiated in following renesting mallards, pintails, and blue-winged teal. In Maine, Stotts and Davis (1960) noted that seven drake black ducks attended hens from 7 to 22 days, averaging a minimum of 14.3 days, during early, first nesting attempts. For eight examples of late renesting, drake attendance varied from 4 to 16 days and averaged only 9.1 days. I noted that the pair bond may also be retained through a renesting attempt. Nine hens, of dabbler pairs which renested, were trapped and marked over an 8-year period. Five of the nine renesting hens were subsequently seen with their original drake while four had reformed pair bonds with another unmarked drake. Kirsch (in litt. ) suggested that some mallard and pintail pairs retain the bond for periods of up to 2 months and renest several times.

I also noted wide yearly variations in time of breaking of the pair bond. In 1958, an exceptionally early breeding season with hatching starting on May 15, 22 mallard hens and broods were observed with associated drakes, whereas in years when hatching peaked after May 25, fewer than five were recorded annually. In 1955, 1956, and 1958, I have recorded newly hatched broods associated with pairs of blue-winged teal and shoveler in late June, 2 weeks after the first broods of this species were recorded. Kirsch (letter, June 27, 1967) noted "many" blue-winged teal broods with associated drakes on the Woodward Study Area, North Dakota. Most drakes were seen with early season broods but seldom remained with the brood beyond the first few days. At Roseneath, I regularly saw drake ruddy ducks associated with hens and broods up to 10 days old. In all cases of drakes with broods the hen did not take the "repulsion posture" and it was therefore assumed to be a mated pair. Thus, some drakes have stronger hen attachments and are associated with their waiting areas for longer periods than other drakes. Therefore, they are more readily seen and available to be censused as indicated pairs.

Pair bond duration is related to strength of site tenacity by the drake and the daily re-use of the activity centre by the hen and drake. Pair bonds are periodically re-enforced through the early and mid-incubation period as long as the drakes return to the waiting sites while hens are on recess. Tight pair bonds are maintained by frequent and joint activity through the migration, postmigration, and laying period but progressively weaken as the pair is associated less and less during incubation. Strong pair bond attachment in males is associated with strong attachment for the waiting site of the home range. Site tenacity to the breeding home range leads to faster pair bond re-establishment whenever the hen returns to the home range to feed, bathe and preen during her infrequent recesses. Pintail drakes appear least attached to the waiting site; other dabbler drakes return regularly to it or to favoured feeding areas in early morning and late afternoon to rejoin the hen on her recesses during these periods. Other dabbler drakes, especially mallards, whose hens are in the same period of incubation do not disassociate themselves from drake aggregations and are therefore found less frequently on the waiting sites. Such differences appear due to individual behaviour.

In short, the period of drake desertion of the home range varies with the species, the individual pair, the nesting phonology of a season and perhaps density of pairs. There is no sharp break but only a general waning of the pair bond and attachment to home range. Therefore, no accurate and predictable period of drake desertion can be given for a species or for any breeding season. In general, mallard and pintail drakes should be censused before their hens are in their second week of incubation, whereas widgeon, shoveler, gadwall, and blue-winged teal can be accurately censused by counting drakes through the second or third week of incubation. The correlation of census periods with breeding phenology is imperative.

Grouped Drakes and the Census

In spring the period of strongest pair bond attachments is associated with periods of maximum pair spacing and maximum drake intolerance. These periods extend from the time of dissolution of the migrating flock to the beginning of incubation. Paired mallard drakes rarely associated with each other in the prenesting and early laying periods. Unmated drakes of all species remain gregarious while even some mated pintail drakes may associate with each other during the laying period (Smith, 1963). At Kindersley, there was a strong tendency for groups of four to seven drake pintails to associate, although this may have been a reflection of the higher percentage of unmated males found in this species (Table 3). At Kindersley, more groups of two to eight drakes with a single hen, in GFAC or ARF, were seen of pintail than of mallards (Tables 8a, 8b). In mallards the period of drake intolerance is followed by a period of drake sociability. Mated drakes form small aggregations, usually two or three but up to ten. Such morning and mid-day drake associations are characteristic of mallards and pintails toward the end of the egg laying period and through incubation (Tables 8a, 8b). However, mated drake associations were rarely observed in widgeon, shoveler, gadwall, and blue-winged teal until mid- and late incubation periods (Tables 8c, 8d, 8e, 8f). Bue (1952) and Evans and Black (1956) noted that grouped mallard and pintail drakes were conspicuous before broods appeared. These authors noted that with blue-winged teal, whose drakes stayed with hens until nearly hatching time, grouping of drakes coincided with the appearance of broods. Thus, as the drake-hen bond wanes, drake intolerance for other drakes also wanes and males form aggregations for varying periods of the day. A number of inter-related phenomena occur with drakes after the hen begins incubation: (1) the hostility of drakes toward hens and other drakes decreases, (2) the tendency to behave sexually toward all hens except his own mate increases, (3) drakes disperse widely from their waiting area sites, (4) they begin to associate with other drakes in larger and larger groups. Most drakes make a final break with the breeding ground from 2 to 5 weeks after the hen has started to incubate when they form large premoulting flocks and migrate to moulting lakes. Few drakes of any species, except small groups of blue-winged teal, were recorded moulting in the parkland or grassland study area ponds.

Most mallard drakes with incubating hens tend to form associations with other drakes during the mid-day hours, 0800 to 1600. Before and after this interval drakes either avoid association with other drakes or remain isolated on their waiting areas. Morning counts, i.e., 0800 to 1200 hours, showing a breeding population of predominantly pairs and lone drakes would indicate that the greater portion of the pairs are in the prelaying and laying interval. Censuses which show a preponderance of lone males and grouped males would indicate that most pairs are laying or incubating.

I made observations in parkland, grassland, and large marsh habitats which suggested that males associated more readily under dense than under sparse population situations. The opportunity for drake association is increased or drakes may be forced to group by lack of surface waters or common loafing spots. Also, if large marsh areas serve as congregating areas for unmated males, they may be seen associated throughout the breeding season (Hochbaum, 1944; Ellig, 1955).

In summary, on block-type study areas, enumeration of groups of males of five or less, especially mallards and pintails, before the appearance of first broods in mid-May, is a valid measure of indicated breeding pairs. They should be enumerated on large sample plots (4 square miles or over) but further investigation of their distribution and activity patterns should be made before all such groups are tallied on narrow transects. For other dabbler species in which groupings of drakes are generally less than five, i.e., two's and three's, all such drakes should also be enumerated as indicated pairs on block areas prior to recommended cut-off dates.

Optimum Census Periods

I have recommended that optimum census periods be established yearly for each species. These should be based on migration chronology and nesting phenology. Counts should be conducted when the greatest proportion of any species is in the prenesting (including renesting), laying, or early incubation stages. For 1956 through 1959 at Kindersley, Saskatchewan, the interval from May 8 to 20 was considered optimum for censusing mallards and pintails, from May 20 to June 5 for widgeon and shovelers, and from May 25 to June 10 for gadwall and blue-winged teal. In years with no April cold snaps, counts for mallards and pintails could be initiated 2 to 3 weeks after the first few hens start to lay or are seen dropping into nesting cover. Egg laying may start from 10 days to 3 weeks after the first pairs migrate into an area. For other dabbler species, counts should start 1 to 2 weeks after the first eggs are noted. All censuses should terminate before the first few broods appear or better still when the first nesting hens are in their third week of incubation. Censuses taken in the above intervals will assess populations of pairs in their first nesting attempt at a relatively stable level. Exceptionally late migrants or drought-displaced birds moving into an area in mid-June would still not be adequately enumerated without later periodic counts.

Murdy (1953) concluded that optimum census periods occurred after the migrants had left and before emergent vegetation and pair behaviour changed. For 1951, he pointed out that the optimum time for the annual duck survey in South Dakota was during the week of May 13 to 23 when mallard, pintail, shoveler, blue-winged teal, and gadwall pairs were all in residence and populations were relatively constant. The optimum survey period for mallard and pintail pairs extended from April 28 to May 28 while for the remaining three species it extended from May 15 to 28. Hammond (1966) noted that optimum census periods vary annually by 7 to 10 days. He recommended a May 7 to 17 census period in North Dakota for mallards, pintails, canvasbacks, and wood ducks and a May 25 to June 7 census period for gadwalls, blue-winged teal, redheads, lesser scaup, and other species. For a study block in the forested habitat of the Northwest Territories, Murdy (1964) recommended two censuses, one immediately after ice break-up between May 20 and 25 for all puddle ducks except shovelers (but including canvasback) and the second between June 6 and 12 for late nesting divers. Where possible, single censuses should be avoided, although Salyer (1962) felt a single valid census could be conducted in early June at the period of nest initiation of blue-winged teal.

Number of Seasonal Counts Required

Intensive studies of waterfowl breeding chronology and optimum census periods show that all duck species do not migrate into a region or initiate nesting at the same time (Kiel, 1949; Lynch, 1951; Murdy, 1953; Bue, 1952; Stoudt, 1952; Smith, 1956; Evans and Black, 1956; and others). All dabbler species show peak indicated populations at different times, usually associated with the period of strongest site attachment, i.e., prenesting, laying, and early incubation periods. Mallards, pintails, and canvasback are generally early breeders; widgeon, shoveler, redhead, and ring-necked duck intermediate; while gadwall, blue-winged teal, lesser scaup, and ruddy ducks are late breeders. Regional differences in start of nesting may occur as Kirsch (in litt.) noted that blue-winged teal on the Woodward Study Area, North Dakota, could be considered intermediate breeders. Generally, one census cannot accurately assess peak populations of all species. I concluded that a minimum of two, and possibly three, counts may be necessary to assess pair numbers of a multi-species breeding population with asynchronous nesting periods.

Very early, Mendall (1948) had noted that more than one count might be necessary to enumerate early hatching black ducks and golden-eyes and late hatching ring-necked ducks and teal (A. carolinensis; A. discors). Kiel (1949) conducted as many as four breeding pair censuses on 12 Manitoba transects. The highest mallard and pintail populations were recorded in the April 21 to 25 count while the highest indicated populations of widgeon, shoveler, gadwall, and blue-winged teal were noted from May 14 to June 6. Lynch (1951) recognized that late breeding blue-winged teal and lesser scaup could not be adequately surveyed in mid-May at the time of the mallard and pintail aerial census. For over 10 years in the parklands of Saskatchewan, Stoudt (1964) used two censuses to assess breeding populations, one in May for early breeders and the second in June for all other species. Similar double censuses were conducted on various Alberta study areas by Smith (1957). Evans and Black (1956) made periodic censuses, April through August, on the Waubay Study Area, South Dakota. All authors utilized the peak number of indicated pairs for any one period as the estimated breeding population for the respective study or transect area. However, as discussed later, I suggest four or five replicate counts and a mean population estimate would better describe species pair numbers than a maximum count taken from one census. To establish trends, and where time and manpower is limited, a single count can be successfully used to estimate total populations, as has been proposed by Hammond (1959; in litt.).

Time of Day for Counts

I have recommended that in grassland areas where emergent vegetation does not affect visibility of ducks, ground counts be conducted between 0800 and 1200 hours. Between these times, pairs and drakes localize their activity and are least mobile; most laying and incubating hens are on their nests, winds are low, and light is favourable. Diem and Lu (1960) recommended that for censusing mallards on transects in parkland habitat, counts be conducted in morning hours after 0530 but before 0930 hours as both pairs and lone drakes are more visible in early morning and leave transect ponds in the forenoon. They showed no statistically significant differences in indicated numbers of blue-winged teal and lesser scaup for counts conducted at 0530, 0930, and 1330 hours. They further concluded that aerial counts made at mid-day may be from 30 to 50 per cent lower than early morning counts and recommended all censuses be made from 0600 to 1200 hours. Smith (1956) concluded that for parkland areas of Alberta mid-day aerial counts, when pairs were most inactive, did not give accurate coverage. Numbers of paired ducks observed in mid-day decreased from early morning counts while numbers of single males increased. In Manitoba, Rogers (1964) began lesser scaup counts between 0800 and 0900 hours, completing them in 2 to 3½ hours. For marshes, pond-habitat blocks, and transects, Hammond (1966) recommended all-day counts starting after 0900 hours, as some duck pairs were in nesting cover prior to this time. Late afternoon counts in large marshes were to be avoided. Kirsch (in litt.) censused ducks from 0800 to 1500 hours on the Woodward Study Area. For aerial surveys of ducks of prairie small pond habitat, J.D. Smith (1964) points out that transect counts should be completed by noon of each day. Optimum daily and seasonal times for aerial pair surveys are also discussed by Stewart et al. (1958) who point out that winds generally rise toward mid-day, while light is poor in the early morning and evening.

Daily patterns of use of marsh areas or ponds may affect optimum census times. In North Dakota, Lacy (1959) noted that peak use of ditches by nesting pairs occurred 2 hours after sunrise with decreasing evidence of pair use thereafter, as birds retired to an adjacent large marsh. By 1300 hours fewer than half the pairs enumerated in the morning were assessed. In 1967 counts, on part of Lacy's study area, Hammond (letter, June 11, 1967) found substantially more pairs present in the morning than afternoon. Many pairs retired to a nearby marsh after egg laying and drakes of incubating hens also tended to fly to the marsh in afternoons. Hammond pointed out that late afternoons were a good time to find redhead drakes on open bays, especially during the laying period. Sowls (1955: 54) recorded maximum populations of pairs in a roadside ditch from 0400 to 0800 hours, decreasing pair numbers through mid-day, and minimum numbers from 1600 to 2000 hours. He also stressed that sharing of a single loafing spot by several pairs of blue-winged teal and gadwall may occur at different times of the day. A pair located in one spot during one census period need not be the same pair in the spot at a later period. For any one species, a turn-over of pairs occurred with the early, intermediate, and late nesting hens and their drakes using the same spot for varying portions of the morning. Heavy vehicular traffic on transect routes may also tend to flush birds, forcing them away from well-travelled roads after the mid-morning hours.

Generally, high post-noon temperatures and winds tend to affect mobility, visibility, and therefore, countability of ducks. On the Kindersley Study Area many pairs, lone, and grouped drakes rested on shore lines whenever temperatures exceeded 60°F and winds were low, a situation also reported by Diem and Lu (1960). This behaviour and their general inactivity made them difficult to locate visually during the afternoon, from 1200 to 1600 hours. Activity increased after 1700 hours. For parkland and marsh habitats estimates made from early morning counts, 0400 to 0800 hours, when all ducks are most active and visible, may more closely approximate absolute breeding populations. Although visibility is increased, estimate biases will occur because of mobility of pairs and absence of some pairs in nesting cover or distant feeding grounds. Standardization of census times between habitats need not be important if statistical testing of counts shows little hourly variation in countableness or if correction factors can be used. Most studies suggest that more consistent and accurate counts are obtainable in the early and mid-morning than in the afternoon and evening.

Duck and Pond Distribution

Ducks and ponds are not regularly or randomly distributed over the parkland and grassland pond habitat. Potholes do not occur "in neat patterns or regular numbers" (Smith, Stoudt, Gollop, 1964). Pond distribution and numbers change seasonally as some small, temporary potholes dry through April and May. Ducks themselves tend to be found in aggregations on favourable potholes or portions of any habitat block, i.e., a contagious distribution (Grieg-Smith, 1964; Southwood, 1966). Pairs of some species, e.g., divers and blue-winged teal, appear more social and are found more closely associated on "primary waiting areas" (Dzubin, 1955), than other species. In grasslands, pair and drake pintails are more closely aggregated than mallards. Other dabbler species tend to be more dispersed because of inherent behavioural spacing mechanisms. Since the degree of spacing and intensity of coactions varies with the breeding phase of each pair and the density of pairs (Dzubin, pers. obs.), the spatial distribution of pairs, lone drakes, grouped drakes, and grouped prebreeding and postbreeding birds, throughout the breeding season, is a constantly changing phenomenon. In spring, newly arrived pairs are aggregated. With the advent of nesting, pairs space themselves from other pairs. Spacing mechanisms promote regularity of distribution while sociability leads to contagious arrangement of indicated pairs over the habitat. As early as 1951, Lynch (1951) had recognized seasonal spatial distribution as a major sampling problem for aerial transect counts. He noted that when drakes and pairs start to group in late May the optimum period for census has passed, especially since enumeration of "ganged" drakes on transects biases the resultant density figures.

Any inferences or predictions of population densities based on single, seasonal counts during an extremely complex period of spatial distribution are subject to wide error, unless sampling conditions are intimately known. The accurate measurement of density of a multi-species population in any habitat is extremely difficult (Odum, Cantlon, and Kornicker, 1960; Preston, 1948, 1962; Williams, 1953) and requires further investigation in pond-type waterfowl habitat.

The statistical concepts underlying strip intersect methods as measures of bird density have been discussed by Moore (1955) and Davis (1963). With waterfowl, transect censuses over one-quarter- or one-eighth-mile-wide strips dissect a large number of home ranges. Ponds on which ducks are enumerated may be resting, feeding, loafing, nesting, waiting, or social congregation areas. The hourly, daily, and seasonal use of ponds varies but replication of counts can be used to pool data and determine average densities of breeding pairs. Density of pairs affects the distribution pattern of pairs or lone drakes, as do favoured feeding or loafing spots. All these factors should be weighed to better plan transect surveys and predict the accuracy and precision obtainable from any strip census.


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