Patterns of Egg Laying and Variation in Egg Size in the Magpie Goose Anseranas semipalmata: Evidence for Intra-specific Nest Parasitism

SummaryThe precocial Magpie Goose Anseranas semipalmata nests colonially on seasonally inundated floodplains, producing a large conspicuous nest. As predicted from the hypotheses of Rohwer & Freeman (1989) regarding the ecological correlates of intraspecific nest parasitism, nests are parasitised by conspecifics. Criteria based on laying rates and sequences classed 35% (n = 22) of closely monitored nests (n = 63) as parasitised. Criteria were conservative, but interpretation is complicated by variable mating arrangements including a preponderance of trios comprising one male and two females. Parasitism rates may have been increased by rising water levels which resulted in flooding of most nests after about one third of the incubation period. Mean clutch size in nests classified as parasitised (10.9 ± 2.3 [s.d.]) was significantly greater than apparently unparasitised nests (8.5 ± 3.1). Parasitic eggs were similar in size and shape to other eggs in parasitised clutches. Clutch size, within-clutch variation in egg dimensions and degree of staining are unlikely to identify parasitism reliably. Improved assessment of the incidence and ecological importance of intra-specific nest parasitism in the Magpie Goose requires further study employing a combination of biochemical and observational techniques.