ACCURACY OF EGG FLOTATION IN DETERMINING AGE OF CANADA GOOSE NESTS

Floating bird eggs has become a common means of determining the incuba tion stage of eggs in the field. However, relatively little attention has been paid to potential sources of error in this technique, and few researchers have validated the flotation method for their study speci es. We compared the actual ages of eggs from 351 Canada goose (Branta canadensis interior) nests with the ages predicted by flotation. Mean observed and predicted ages were not different, and 82.3% of predicted hatch dates were +/-4 days of those observed. The accuracy of estimat es varied with the age of eggs floated; flotation tended to overestima te nest age early, and underestimate it later, during incubation. This flotation method was sufficient for our purposes, but we suggest the method be modified if accuracy >+/-4 days is desired. Researchers shou ld consider all inherent sources of error and validate flotation when reporting results using this method.