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

In: Wildlife research, Band 25, Heft 6, S. 603

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.