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Assessing Breeding Populations of Ducks by Ground Counts

Summary


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:
  1. 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.

  2. 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.

  3. 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.

  4. 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 40°F.

  5. 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.

  6. 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.

  7. 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 estimates.

  8. 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.

  9. 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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