SOURCES OF VARIATION IN CLUTCH SIZE, EGG SIZE AND CLUTCH COMPLETION DATES OF SEMIPALMATED PLOVERS IN CHURCHILL, MANITOBA

Variation in clutch size, egg size and clutch completion dates was stu died in a population of Semipalmated Plovers (Charadrius semipalmatus) over 5 summers in Churchill, Manitoba. Clutch size variation was grea test in 1992, a year of unusually low summer temperatures, with almost 25% of clutches containing less than four eggs. More eggs were left u nhatched in the nest in 1992 than in any other year. Clutch completion dates were negatively correlated with the average temperature during the pre-laying period. Synchrony of nesting was greatest in 1994, the warmest year. Egg volume and mass were not reduced as a result of the cold summer, and the Variation in egg volume and mass attributable to differences among females was similar and significant in all years. Re peatabilities of all egg measurements were high. Egg volume was modera tely correlated with female body mass but not with female wing length nor female condition (mass/wing length), a result consistent with that found in other shorebirds. Egg size did not vary consistently with la ying order, suggesting strong selection for uniformity in egg size wit hin a clutch.