BODY-SIZE, FITNESS AND COMPATIBILITY IN BARNACLE GEESE BRANTA-LEUCOPSIS

Large-sized Barnacle Geese Branta leucopsis of both sexes had a higher probability of breeding successfully in any particular year and produ ced more goslings than did smaller birds, Large females paired at an e arlier age, suggesting that they were preferred as mates and were like ly to have entered the breeding population earlier, The relative sizes of the pair bond members also affected fitness, Most birds were able to maximize their breeding performance by mating with relatively simil ar sized partners; the greater the size disparity of mates, the lower the breeding performance. This supports the idea that compatibility of mates may be important in determining fitness of the pair. The succes s of different pair types was also affected by environmental condition s, with certain size combinations doing better in some years and poorl y in other years.