“Paradise for birds?” Unveiling the Avian Populations of Aride Island

Aride Island Nature Reserve stands out within the Seychelles’ archipelago for its incredible bird population, especially its land birds. But how can this claim be verified?

Nothing can prepare you for a first visit to Aride Island. It is a sensory overload similar to attending a chaotic concert where performers care little for harmony and volume. Among the cacophony, identifying each call and distinguishing between the birds, whether it’s a seabird or land bird, is almost impossible. Aride Island hosts one of the most important seabird colonies in the Indian Ocean, with around half a million birds of up to 10 species. Additionally, it is home to at least six land bird species, creating unparalleled avian density and diversity, almost unmatched in Seychelles or even globally.  Or is it?

To confirm this, Alex Koehler, a biology student from the University of Constance in Southern Germany, has made it his three-month mission to survey the land birds’ population of the island as part of his thesis for his Master’s degree. A mammoth task, as it is estimated that the island of 68 hectares (more than twice the size of Cousin Island) has over 10,000 avian land inhabitants. Out of this 10,000, the endemic Seychelles Fody (Foudia sechellarum or ‘Tok tok’ in Creole) surpasses all other species with an estimated 5 to 7 thousand individuals. Other species such as moorhens, doves and the Seychelles sunbirds (Cinnyris dussumieri or ‘Kolibri’ in Creole) are also present but in small numbers. 

Alex is, however, more excited to establish the population of the Seychelles Warbler (Acrocephalus sechellensis or ‘Pti merl de zil’ in Creole) since Aride is said to hold the most abundant population of this species, which had been introduced from Cousin in 1988.

“As an endemic species that was on the brink of extinction in Seychelles, this is a remarkable story of recovery for Seychelles, considering it can now be found on five islands,” says Alex. It is estimated that Aride now holds the largest population of this species, as previous studies have shown that, unlike other islands, on Aride they also breed throughout the year, perhaps due to good habitat quality with a high food availability.
 
“This census is essential to establish the population of each species and monitor against sudden changes,” says Alex, adding that ‘island populations are more threatened by catastrophic events, diseases and genetic impoverishment’.

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Counting birds on an island where they occur in high densities without any unique features to differentiate between different individuals poses unique challenges. For the land bird population estimate, Alex will use point counts to estimate the number of individuals of each land bird species.
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​Approximately 100 point counts are conducted systematically, across the whole island using a grid system.  At each location across the grid, a point count is conducted, and all birds that can be seen or heard are recorded.

While less abundant species that conquer bigger territories, like the Seychelles Blue-pigeon, can be sufficiently monitored using a broad grid of point counts that covers the whole island, species occurring in high densities are harder to estimate. Each Seychelles Warbler family inhabits an average territory of 200 m². A feasible point count grid cannot catch every single bird on the island because it is not fine-grained. Therefore, Alex will first ring as many Seychelles Warblers as possible in a restricted area to estimate the densities and get an impression of the total number of birds in that area. This knowledge helps to put the point counts into perspective and ensures a more robust estimate for the whole island.

Counting birds seems simple enough, but this survey is not without difficulties. Dense and high vegetation in certain areas of the island may limit visibility and thus decrease the detection probability. Further, the limited track system on the island poses challenges to accessing all points where the point counts are conducted.

“The steep terrain makes it difficult sometimes, and you need a good sense of orientation to not get lost,” says Alex, adding that luckily, the island is not very big and at some point, you will always reach the shores.
Using data from as far back as 2000, Alex hopes his census will assist in establishing the population trend and extract key features that have made it favourable for this species to increase abundantly on Aride.
The project will run for three months, but Alex is happy to spend time on Aride among birds and close to the ocean. ​