The migration of Bird Island’s Sooty Terns revealed

High density nesting in Bird Island’s Sooty Tern colony (Photo: Chris Feare)

On Bird Island, and in other colonies throughout Seychelles, Sooty Terns breed seasonally during the south-east trade wind, roughly April to October. Outside this time, the millions of birds involved disappear from Seychelles waters and, apart from five recoveries of ringed birds (3 in Sri Lanka/southern India and one each in northern Australia and at sea off Somalia), their whereabouts have been a mystery.

Bozena Summers, Ron Summers, Chris Feare and Christine Larose  deploying the first geolocators in 2011
Christine Larose, Chris Feare, Audrey Jaeger and Camille Lebarbenchon deploying geolocators in 2012

In 2011 and 2012, Christine and I, in collaboration with Audrey Jaeger, Camille Lebarbenchon and Matthieu Le Corre from the University of Reunion and Ron Summers from the RSPB in UK, fixed tiny geolocators to rings on the legs of 86 Sooty Terns that were breeding on Bird Island. During the breeding seasons of 2012 and 2013 we searched the Bird Island colony for these marked birds and managed to find and catch 43 of them. In the interval between marking the birds and recapturing them, the geolocators recorded data that allowed the geographical locations of the birds to be followed, providing migration tracks and revealing for the first time where Bird Island’s Sooty Terns seek their food when not breeding.

A geolocator attached to a plastic ring on the leg of a Sooty Tern (Photo: Chris Feare)

The results of this study have just been published in the journal Frontiers of Marine Science* and show that Bird Island’s Sooty Terns disperse widely over the Indian Ocean, embracing most of the ocean north of 25°S. However, within this broad area certain areas were favoured. The Bay of Bengal and seas around Sri Lanka were most used but other preferred areas were (in order of importance) a region straddling the Chagos-Laccadive ridge, a region straddling the equatorial 90-East ridge, and a region in the north of the Mozambique Channel near the Comores Islands.

The main areas (A-D in order of importance) used by Bird Island’s Sooty Terns during their non-breeding migration. The area shown in pink is that used during the breeding season, and the area in green is that used during the pre-breeding exodus.

Furthermore, the tracking data showed that after return from their migrations, Sooty Terns undertook a pre-laying exodus to foraging areas to the south-east of Bird Island. A pre-laying exodus is more typical of Procellariform birds (petrels, shearwaters and allies) and this is the first time that this behaviour has been demonstrated in terns.

In addition to geolocation, the tags recorded contact with sea water. This confirmed the long-held supposition that Sooty Terns remain in flight for the duration of their 4-5 month absence from Bird Island and suggested that the very minimal contact with salt water that was recorded occurred when the birds were actively feeding.

The Sooty Tern’s real home: in the air over the ocean (Photo: Chris Feare)

While breeding in Seychelles, eggs from parts of some colonies are harvested as a delicacy for local human consumption. Our data on non-breeding movements indicate that these birds are likely to experience further threats elsewhere, e.g. through over-fishing and pollution. In particular, the Bay of Bengal, identified in our study as the main non-breeding area used by Bird Island’s Sooty Terns, is one of the world’s seas most heavily polluted by plastic microparticles, which birds can ingest while feeding. Perhaps even more critical, an article in the 11 November 2017 issue of New Scientist magazine highlighted that the Bay of Bengal is a water body destined to become deprived of oxygen as sea temperatures rise. This will limit this area’s ability to support fast-swimming pelagic predatory fish such as tuna, upon which Sooty Terns depend for driving their small prey fish and squid to the surface, rendering them accessible to the birds. Despite the large numbers that currently grace the islands with their presence, we must not be complacent about the future of Seychelles’ Sooty Terns whose threats extend well beyond their nesting colonies!

*The paper is free to access on the journal’s website: https://www.frontiersin.org/journals/marine-science/sections/marine-conservation-and-sustainability

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