POLYMORPHISM IN RINGED PLOVERS

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Summary.The rather similar ringed plovers Charadrius hiaticula and C. semipalmatus represent essentially allopatric, circumpolar populations which overlap in the breeding season only in a narrow zone in the eastern Canadian Arctic.In eight fiords on eastern Baffin Island, semipalmatus was the more numerous by a factor of almost three. Both plovers occurred together, although not in equal numbers, in the same wide range of habitats.Pair formation in both plovers apparently took place at the same time, but for more than half of the individuals, not in the same geographic area. The majority of the hiaticula individuals arrived after most semipalmatut, and the late arriving hiaticula appeared to be paired upon reaching Baffin Island. There was an overlap in their arrival times of about five days. Mixed hiaticula‐semipalmatus courtship groups occurred during this period. Pair forming activities of hiaticula and semipalmatus appeared similar but were not studied closely.Analysis of the non‐overlapping territories of two populations suggested that the plovers did discriminate one type from the other for there was a tendency for unlike pairs to nest side by side. The significance of this odd pattern is not known.Analysis of the mensural and plumage characteristics of adult plovers from the overlap zone seemed to indicate that gene flow between the two was absent and that they were behaving as distinct species. However, mixed pairs occurred in five out of eight fiords. Considering the populations of all fiords, the number of mixed pairs found (20) was significantly greater than theoretical expectation, which allowed for an occasional mixed mating since a choice of mate was not always possible.All mixed matings produced mixed broods. The young were either hiaticula or semipalmatus. No intermediates occurred. Moreover, 10 pairs in which both birds were pheno‐typically “semipalmatus” produced both semipalmatus and hiaticula‐type chicks. No intermediates were detected. Egg and chick mortality probably prevented the detection of more such cases. All hiaticula x hiaticula matings produced only hiaticula‐type chicks. The inheritance data indicated that hiaticula and semipalmatus are morphs of a single polymorphic species.If a switch gene is operating, it may have originally controlled only toe‐webbing, and the other differences may have accumulated as the result of geographic isolation. That differences in plumage pattern and body size are greatest where separation was probably longest supports this hypothesis.One possible model considered semipalmatus‐type individuals to be either SS or Ss and hiaticula‐types to be ss. Accordingly, all semipalmatus‐types involved in mixed matings with hiaticula‐ types would have been Ss individuals because all such matings produced chicks of both types.Circumstantial evidence suggested that during the last 80 years, the relative number of the two types in the overlap area has shifted in favour of the semipalmatus‐ type. This might have been the result of selective factors intrinsic to the polymorphism.