Precocious breeding by yearling giant Canada Geese

Many species of waterfowl are capable of breeding as yearlings. In the subfamily Anserinae, however, reproduction normally does not commence until individuals reach two to four years of age (Rohwer 1992). Most published accounts indicate that Canada Geese (Branta canadensis) conform to the pattern typically found in other geese, deferring reproduction as yearlings and initiating breeding when birds are two years of age or older (Hanson 1962, Brakhage 1965, Bellrose 1980, Moser and Rusch 1989). The few documented exceptions (Brakhage 1965, Cooper 1978, MacInnes and Dunn 1988) suggest that the likelihood of a successful breeding attempt is higher for yearling males than females and that when early breeding occurs, one member of the pair is often at least two years old. Only two records of nesting attempts by yearling female Canada Geese have been published (Hall and McGilvrey 1971, Mickelson, 1975). In each case, the female produced some fertile eggs but deserted the nest before the eggs hatched.

The observations noted above demonstrate that reproduction by yearling Canada Geese is physiologically possible for both sexes, but they also raise a number of interesting questions regarding why most individuals defer breeding during their first year. Energetic or nutritional constraints, and the inability of yearlings to secure and successfully defend a territory, are likely physiological (Elder 1946) and social (Brakhage 1965) impediments to early breeding. From an evolutionary perspective, reduced reproductive success of yearlings relative to adults, and increased mortality associated with early breeding, also may contribute to deferred sexual maturity in waterfowl (Rohwer 1992).

A nine-year study of the demographics and ecology of Giant Canada Geese (B. c. maxima) in central Missouri provided an opportunity to monitor the incidence and success of nesting attempts by yearling males and females in a local population that was established by translocation of wild geese 20 years ago. In this paper, we present data on the incidence of breeding by yearlings in this population and offer several potential explanations for the occurrence of precocious reproductive activity.

Methods. -Because the age of adult geese cannot be determined accurately from plumage or morphology, estimation of age-specific breeding rates requires data from geese marked during their first summer of life. To obtain a sample of marked individuals, geese were captured by drive trapping each year from 1989 through 1998 during the summer flightless period. Age (hatching year or after hatching year) and sex of captured geese were determined using cloacal and plumage characteristics (Hanson 1959). Geese were marked with standard U.S. Fish and Wildlife Service aluminum leg bands and uniquely coded white neckbands that could be read with a spotting scope from a distance of 200 m (Malecki and Trost 1986).

Nesting data from marked yearling geese were classified as nesting attempts or as successful nests. The category "nesting attempt" included all observations of marked yearlings that were known to have established a nest. These data were derived through observations of marked pair members incubating during spring nesting surveys, observations of marked pair members with goslings during June brood surveys, and capture of marked yearling females with incubation patches during June banding (Hanson 1959). Nesting attempts were classified as "successful nests" only when hatching was confirmed by observation of young.

Results. -Between 1989 and 1997, 4,161 geese were neck-banded in central Missouri. Of these marked individuals, 1,549 were hatching-year males and 1,420 were hatching-year females. During seven years of monitoring, we obtained 1,062 records of nesting attempts (415 involving known-age individuals) by marked adult and yearling Giant Canada Geese. Nesting by yearling geese accounted for 19 of these records, and the incidence of yearling nesting was equally divided between males (10 records) and females (9 records). Six of the yearling nesting attempts were confirmed from multiple observations, and the remaining 13 involved single observations.

Of the seven nesting attempts by yearlings that were known to produce young, four involved yearling females and three involved yearling males. In instances where we knew the age of both members of a pair, males and females breeding for the first time as yearlings were always paired with older mates. Although the other 12 breeding attempts by yearlings also may have been successful, we were unable to confirm the success of these attempts by observation of goslings.

Discussion.-Although previous accounts indicate that Canada Geese typically defer reproduction until individuals have reached two to four years of age (e.g. Hanson 1962, Craighead and Stockstad 1964, Raveling 1981, Moser and Rusch 1989), our data demonstrate that yearlings of both sexes can reproduce successfully in wild populations, and they provide the first records of successful reproduction by yearling females. These results confirm assertions of earlier investigators that reproduction by yearlings probably is not deterred solely by physiological capability (Hall and McGilvrey 1971).