Cousin Island Special Reserve is an important breeding site for seabirds. Nature Seychelles’ staff working on Cousin Island carry out seabird census at least once a year to determine the size of the breeding population as well as the long term trends. From 2009 to date, the censuses have been carried at least twice yearly.
Cousin Island is a remarkable model of ecosystem restoration and continued conservation efforts in safeguarding natural wildlife habitats. In addition to being a protected area under Seychelles law, Cousin is also an Important Bird Area (IBA) that is recognised globally in bird conservation. Indeed, Cousin is one of the sites of highest ornithological interest in Seychelles.
In a recently published Nature Seychelles report of the census of White-tailed tropicbird (Phaethon lepturus), White tern (Gygis alba) and Tropical shearwater (Puffin iherminieri) carried out in February, the results indicate that not only has there been no decline in the populations of these species since the last census in July 2014, but that their numbers are in fact stable on Cousin Island.
These three species of seabirds breed all year round and therefore at least two censuses are done each year to determine the population size. One census is done during the south-east trade winds and another during the north-west trade winds. The recently published census report is for the count carried out during the north-west trading season.
Nature Seychelles staff working in groups of three or four individuals carried out the census, counting adult birds, active nests and shearwater burrows. Active white terns were determined by counting eggs and chicks only. The presence of adult White-tailed tropicbirds or Tropical shearwaters in obvious nesting sites(cavity or burrow respectively), was used to identify active nests.
Prevalence and occupancy varied greatly between the 70 plots that were surveyed. The results of the report indicate that the White-tailed tropicbird was the most prevalent across the entire island. However, the most abundant breeding seabird is the White-tern or Fairy-tern with over 1,800 breeding pairs, followed by the Tropical shearwater with more than 1500 breeding pairs and then White-tailed tropicbird with more than 1200 breeding pairs.
The White-tailed tropicbird population on Cousin has been monitored since 1987 providing 28 years’ worth data to determine the breeding population trends. Although there have been fluctuations from one year to the next, there has been a general upward trend in the three decades.
In the case of the White terns, population estimates are available since 1999. The population has remained stable over the years despite fluctuations from one year to the next in the last 15 years.
The Tropical shearwaters also have population census data from 1999 which indicate an overall downward trend. However, It is presumed that the breeding population has been underestimated over the last few years due to the lack of a correction factor applied to the estimate, to account for the proportion of Shearwaters especially Audubon’s shearwater, that do not respond to the call-playback method.
The call-playback method, according to the “Seabird Monitoring Handbook for Seychelles” published by Nature Seychelles is the standard method for censuring seabirds that nest in cavities like Shearwaters and has been found to be most effective in the case of Wedge tailed Shearwaters. Taped calls of the birds are played at the entrance to potential cavities and adults and sometimes chicks respond.