Using the presence and condition of eggshells to determine nest success in the magpie goose, Anseranas semipalmata

Abstract

In a sample of 104 closely monitored magpie goose nests, all of 66 successful nests contained substantial numbers of eggshell fragments (52–1000+), many embedded deeply in the nest matrix. Detached shell membranes were also found in all successful nests. All but one of 38 unsuccessful nests contained no shell fragments. The exception contained about 60% of the shell from one egg, but fragments were clearly distinguishable from shells of hatched eggs because they were contaminated with egg yolk. Weight of eggshell in individual nests could be related to the number of hatchlings (excluding failed nests: r² = 0.67, P < 0.0001, n = 39), but the time required to retrieve and process shell fragments is perhaps too long to justify routine use to estimate hatchling production for individual nests. Rather, we suggest that the presence of substantial quantities of eggshell fragments (>12 g or the equivalent of more than 1 egg) embedded in the nest matrix be regarded as diagnostic of hatching success. We interpret the clear discrimination between successful and failed nests as a direct result of consistent features of magpie goose biology and the breeding environment. The most important of these are: (i) the prolonged hatching and on-nest brooding period, during which eggshells from hatched eggs are crushed and their movement into the nest matrix promoted; (ii) the continuous addition of vegetation to the nest, often covering shells; (iii) the removal of shells from damaged (but not hatched) eggs by attending adults; and (iv) the dominant egg predator being a large snake (the water python, Liasis fuscus) which consumes eggs whole and hence leaves no shells at the nest site. We suggest that post-hatch surveys of nest structures should be used to improve understanding of spatial variation in nest success within colonies, and to compare success between colonies of different size.