Age Determination of American Black Ducks

Principal Investigator:
Paul Ashley, MSc. Graduate

Supervisors:
Dr. Scott Petrie, Long Point Waterfowl Executive Director
Norman North, Canadian Wildlife Service
Dr. Bob Bailey, University of Western Ontario

Background and Project Design

While it is known that juvenile American black ducks (Anas rubripes) moult some tertials and tertial coverts during their first fall and winter, descriptions are incongruous and the timing, extent and sex-specific differences in moult patterns were largely unknown. We studied moult of these feathers from fall to spring using captive, harvested, and trapped wild birds, as well as specimen wings. During fall and winter, juvenile males moulted earlier and replaced more tertials and tertials coverts than females. We attribute these differences to their primary mating system of annual monogamy, with females being the limiting sex.
The early acquisition of adult tertials and tertial coverts are likely selectively advantageous for males because these feathers are prominently displayed during courtship and pair bond formation. Juvenile birds that have acquired adult-like feathers may exhibit a higher degree of fitness, facilitating breeding in the first year. All juvenile black ducks retained some juvenile tertial coverts until the end of April. This knowledge of wing feather replacement is useful for ageing birds as either second year or after second year. Age-specific studies pertaining to survival and productivity of American black ducks (Anas rubripes) are constrained by the fact that no technique has been developed to reliably age them as second year or after second year from late winter to late spring. We developed a qualitative age-class scoring technique that can be readily used in the field. When tested on 5 independent observers, known-aged birds (n = 106) were correctly classified with 94 – 98% accuracy. To reduce subjectivity and provide an objective corroboration of age estimates we also developed multivariate models from measurements of wing feather variables (weight and length of greater secondary covert 9, and width of tertial covert 5) that determined age with = 90% accuracy (n = 255). There was 94% agreement between qualitative and quantitative age assignments of wild birds caught in spring (n = 172). The application of these ageing techniques should be useful in a host of life-history studies conducted on wintering, spring staging and nesting grounds.

3 April: American Black Duck wing June 2004: American Black Duck wing

Management Implications

Moulting Study

Knowledge of the timing of moult facilitates studies linking wintering areas, breeding areas and migratory pathways by means of stable isotope analysis. This technique has been used to link wintering and breeding habitat in several species of birds. Origin of breeding may be determined from analysis of feathers moulted at breeding/natal areas. Using a dual feather approach on staging birds it is possible to link breeding and wintering areas, and also at least one other point along the migratory path. For example, if a second year female black duck is captured on a staging area in spring, isotope analysis of a primary or secondary feather will infer the general location of hatching and the same analysis of tertial covert 3 or 4 (if replaced) may infer the general location of wintering since they are generally replaced in late winter. Isotope analysis as part of a large-scale monitoring program may also be potentially used to determine the contribution of breeding areas to wintering populations and vice versa. This knowledge, although known to some extent for black ducks by means of banding, is limited by the lack of banding and band recovery in much of the northern, forested breeding area.

Ageing Study

The development of accurate postseason aging techniques is a Black Duck Joint Venture research priority partly in response to fewer birds being banded annually during the preseason banding period. Band recovery models assume that banded individuals within each identifiable age-sex class have the same annual survival and recovery rates, thus the ability to age black ducks as second year and after second year instead of just after hatch year may increase precision of model estimates if vital rates of second year and after second year birds differ. Temporal changes in survival and recovery rates of postseason banded black ducks where all birds are identified as adults (after hatch year) are similar to those of adults banded preseason. It is known that black ducks, like most (if not all) species of ducks have lower annual survival and recovery rates and higher harvest rates in their
first year of life than in subsequent years specific studies to determine differences in annual survival and recovery rates of second year and after second year winter banded black ducks have not been conducted because birds could not be reliably aged. Whether these rates for second year birds are more similar to hatch year or after second year birds, or somewhere in between, are unknown. Although it has been shown that second year mallards have similar vital rates to after second year birds demographic parameters may differ between these species and warrants investigation in black ducks.
  There may also be differences in age-specific productivity rates of black ducks. Pairings within and between age-classes may produce significantly different reproductive outcomes due to such factors as courtship, breeding and nesting experience, and hormonal regulators in both the male and female. With the ability to determine age of each sex, studies of black duck productivity can include an age component.

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