Determining hatch dates for skuas: an egg density calibration curve

Key life-history events, such as breeding phenology, underlie much ecological research and inform conservation efforts. Simple methods that improve efficiency during breeding studies are valuable, particularly in remote locations and extreme climates. Building on an earlier study, we investigated the relationship between egg density and incubation progression in two Arctic- and subarctic-breeding seabird species, Arctic Skua Stercorarius parasiticus and Great Skua S. skua, to statistically test its application as a calibration method. Corresponding with the preceding study we found that the decrease in calculated egg density during incubation can be described by a quadratic relationship with egg development for our populations. In addition, we demonstrate that this relationship was not confounded by multiple egg clutches nor differences in measurement intervals. From this relationship, a calibration curve was constructed to predict hatching dates within an error of c. three days for Arctic Skua and c. four days for Great Skua, using a single measure of the length, breadth and mass of an egg. Furthermore, when combining the data generated in this study, we found model support for a calibration curve independent of species, suggesting that this calibration may have the potential to be extended to other species with similar ecology. This technique can be used to inform the timing of colony visits and thereby maximise research and monitoring efforts for these species with minimal researcher disturbance.