February saw the completion of the sixth annual frigatebird survey at Aldabra Atoll in the SW Indian Ocean.
To estimate the breeding colony size of the two species of frigatebird at Aldabra, the team divided each of the four colonies into smaller sub-colonies, to limit overlap and possible double counting. These sub-colonies were then surveyed by three counters.
Two counters counted all Lesser Frigatebirds and Greater Frigatebirds on nests who were incubating eggs or brooding a small chick, and a third counted chicks on nests (separated by species). The survey was timed with the later stages of incubation to limit the disturbance to and desertion of nests.
While breeding at Aldabra is somewhat synchronised in both species, it is more prolonged and variable in Lesser Frigatebirds than in Greater Frigatebirds. Conducting the annual survey in January/February ensures that most breeding pairs of both species are likely to be in the later stage of egg incubation or already rearing their chicks.
Previous surveys of the population have shown marked annual fluctuations in numbers and considerable changes in the location and distribution of the main breeding colonies on the atoll. The data collected from the 2016 survey will now be analysed to obtain an estimate of the current population size and compare this to the previous years.
Aldabra holds the largest frigatebird colony in the Indian Ocean with at least 4400 pairs of Greater Frigatebirds (Fregata minor) and 6600 pairs of Lesser Frigatebirds (F. ariel) breeding annually. This estimate of population size was taken from the SIF survey in 2011 which showed a 10% increase from the population estimate in 1980. Mixed breeding colonies of these two species are only known from North Keeling (of the Cocos Keeling Islands, Indian Ocean) and Aldabra.
Estimates of the breeding population of frigatebirds on Aldabra were undertaken in 1967 (Diamond 1975), 1976–1977 (Reville 1980, 1983) 2000 (Burger) and most recently by SIF, in the form of annual surveys starting in January 2011. The population size estimates of the previous non-SIF surveys are not included here as they all used different methodologies and were conducted at different times of year, therefore the population estimates are not comparable to those estimated by SIF.
Since 2011 the four known frigatebird colonies at Grande Poche, Passe Gionnet, Camp Frigate and Middle Camp have been surveyed annually using a simple, consistent methodology that gives basic information about the number of nesting pairs in each area and population trends.
By April Burt
Aldabra Scientific Coordinator, SIF