Resolving the population status of Ascension Frigatebird Fregata aquila using a ‘virtual ecologist’ model
The Ascension Frigatebird is an island endemic whose conservation status was subject to confusion owing to difficulties in counting and misinterpretation of data. Accurate nest counts for this species are problematic owing to their extremely prolonged breeding seasons, high nest failure rates, turnover of individuals at nest-sites, replacement laying and biennial breeding. We conducted repeated complete censuses of Ascension Frigatebird eggs at the species’ sole colony of Boatswainbird Island, and collected data on laying phenology and nest survival rates within sample quadrats, throughout the 2001 and 2002 breeding seasons. We used these data to develop an individual-based model that predicted the number of Frigatebird nests present on each day an actual census occurred assuming an arbitrary 1000 breeding females bred there. We then divided the number of nests counted in these virtual censuses by 1000 to quantify bias, and used this figure to correct real census totals. The model revealed that the population numbered c. 6250 breeding females and c. 9350 mature females in 2001–2, and that numbers have not changed significantly since the late 1950s. Productivity, at 0.34 chicks/pair, was high compared to previous studies of Ascension Frigatebirds and most of those of congeners elsewhere.
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