Denis Island’s breeding population of Fairy terns counts nearly 2000 breeding pairs

Denis Island’s breeding population of Fairy terns counts nearly 2000 breeding pairs
Between the 17th and 24th of January 2016 a census of breeding Fairy terns (Gygis alba) was conducted by Green Islands Foundation (GIF) staff and volunteers on Denis Island. The goal was to gain an estimate for the size of the breeding population during the north-west monsoon. The Fairy tern (otherwise known as white tern) is a small seabird found across many tropical oceans. They are the most widespread breeding seabirds in Seychelles with highest breeding populations found on rat-free islands such as Aride, Cousin and Cousine.

There are at least 14000 breeding pairs on the granite islands alone, with several thousand pairs seen on the outer islands. Fairy terns lay a single speckled egg usually on thin branches with no surrounding nest; although this may be a strategy to keep the chick out of reach of predators (such as rats), the young chick can often fall from the thin branches, particularly in strong winds.

Figure 1. GIF staff Jennifer Appoo and Volunteer Jeremy Waters counting fairy terns
GIF staff Jennifer Appoo and Volunteer Jeremy Waters counting fairy terns

The census was carried out using the Seabird Monitoring Handbook for Seychelles 2nd Edition (Burger & Lawrence 2003) as a guide. Given the Fairy tern colony on Denis is rather large and dispersed, 70 randomly positioned plots across the island were chosen to count the birds (50 random points and 20 coastal points known to be favoured habitat). Using the 2014 habitat map (R. Bristol) each census point was allocated to one of the 16 recorded habitats on Denis Island. Three habitats did not have census points in them but are either not suited (bare ground) or form only a very small part of the island (<3 ha).

At each location a virtual circle was created with a 20 metre diameter (approximately 314m2) using a central point and a 10 metre long rope as a guide. For all trees in the plot, the number of individual birds and ”nests” (egg or chick) was recorded. At 32 locations (45%) no Fairy terns were present at all, and on the other 38 locations a total of 73 active ”nests” were identified. Of those, 50 were in the egg-stage and 23 in chick-stage.

Figure 2. Habitat map for Denis Island adjusted from R. Bristol (2014) and the locations of the census point counts.
Habitat map for Denis Island adjusted from R. Bristol (2014) and the locations of the census point counts.

The total surface of habitats used for the calculation is 109 hectares (out of 144 ha) resulting in a total of 4235 resting / roosting birds and 1869 nests or breeding pairs. The breeding success of Fairy terns usually is only around 20% and we are yet to monitor the breeding performance. The current estimate of nearly 2000 breeding pairs, however, shows that Denis Island provides a suitable and well-occupied breeding area for Fairy terns. We are very glad to report that the Fairy terns are doing so well on the island, many guests of the hotel enjoy watching a pair preening or raising their chick in the backyard of the bungalow which is a beautiful and romantic sight.

By Nick Burnham & Janske van de Crommenacker

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