Twice a year, a seabird census is carried out on Cousin Island to monitor the seabird population numbers. In particular, an emphasis has been placed on calculating the numbers of breeding pairs of tropical shearwaters as recent censuses have shown a significant fall in their numbers. The latest results will show a true representation of the current population but from personal observations their seems to be plenty about at the moment!
70 sampling plots were randomly selected and assigned to one of the three teams made up of wardens, volunteers and the science officer. Each team then visited their plots to record the number of nests/burrows belonging to each species. A rope extending from a central point was used to create a circle indicating different radius and therefore different sized areas. The Brown and Lesser Noddys, White-tailed Tropicbirds and Tropical and Wedge-tailed Shearwaters were counted within a radius of 9.77m which gave a total area of 300m2. The White Terns were counted within the 7.98 radius giving an area of 200m2. Where the shearwater burrows were dense, they were only looked for within a 5.64m radius which gave a total area of 100m2.
For the shearwaters, burrows were marked up in the day and revisited at night when they were back from fishing. A playback method was used to find the Tropical Shearwaters where a recording of a calling bird was played at the entrance of the burrow. If a tropical shearwater was present it would reply with the same call. The Wedge-tails were seen by looking for their presence in or at the entrance to the burrows.
The census is intense and is hard work, especially when surveying the hill areas and revisiting plots at night. However there is something very rewarding when you locate a potential burrow and revisit it to hear the sweet (!?!) sound of a Tropical Shearwater responding to the recording. Also for the first time we included wedge-tail shearwaters in the census; there are thousands of them returning to the island at sunset and their eerie calls can be heard all over the hill.
The census will provide us with the number of nesting pairs but not how many of the nesting attempts are successful so for this we carry out nest monitoring of a sample of nests to see how many eggs make it to the fledging stage and therefore the breeding success rate.
It will be interesting to see what the results show!