Bird Island’s Sooty Terns 2019

Densely nesting Sooty Terns in early June 2019 (Photo: Chris Feare)

The Sooty Tern breeding season is progressing well. Laying began exceptionally early this year, with the first eggs being recorded around 10 May. This proved to be a false alarm, however, as the birds departed, only to return later when laying recommenced around 25 May – still early by normal standards but closer to the regular laying start in the first half of June.

Egg care involves a lot of incubation, but in hot sunshine eggs are shaded from the high temperature by the adult standing over the egg (Photo: Chris Feare)

In the reserve area on the island we are monitoring the duration of incubation shifts of incubating birds. This gives us an indication of the ease or difficulty that adults are having in finding food this year. We have also deployed GPS tracking devices on some birds, which will tell us where our adult Sooty Terns are flying to feed this year. This is to build upon data that we have collected over the last few years.  This information will provide guidance on the oceanic areas that the birds use to feed themselves and their young, and thus identify oceanic areas that we can propose for protection from exploitative activities that could harm the marine environment and its productivity.

As by far the most numerous seabird in the tropical Indian Ocean, and as top predators in the marine ecosystem, the Sooty Tern population and its stability, or otherwise, could be a good indicator of the health of the local oceanic environment. Monitoring of the numbers of Sooty Terns in breeding colonies of the western tropical Indian Ocean, their ability to produce future generations and the survival of juveniles and adults from year to year, could thus play an important part in our understanding of changes due to human impacts.

Sooty Terns in their true realm – at sea (Photo: Chris Feare)

The study of Bird Island’s Sooty Terns has been running for over 40 years. During this time the colony has undergone a series of changes in response to changes in management on the island. But for a bird that can live for over 30 years, and spends most of this time out at sea, it is less easy to tease out alterations to their biology imposed by pressures, such as climate change and intensive fishing of tuna and sharks, other top predators, while Sooty Terns are away from the colony. New techniques are beginning to allow us to investigate this phase of the birds’ lives but such studies must continue over the long term to explore the birds’ responses to changes in temperatures and food supplies.

Sooty Terns should not be studied in isolation, however; increasing knowledge of their behaviour and population changes must be integrated with increasing knowledge of other facets of the marine environment to enable us to identify, and if possibly mollify, deleterious events in the oceans.


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