More trouble for Bird Island’s Sooty Terns?

Sooty Terns over Bird Island
Sooty Terns over Bird Island

In 2011 and 2012 we marked 85 of Bird Island’s breeding Sooty Terns with geolocators, tiny devices that record geographical location during their migration. Geolocators have to be recovered the next breeding season to download the data and in 2012 and 2013 we manged to find 40 of the marked birds. The data showed that the main areas used by Bird Island’s Sooty Terns out of their breeding season are the Bay of Bengal and the ocean around Sri Lanka and straddling the Laccadive-Chagos Ridge to the south-west of India.
The extreme weather in the western Indian Ocean during this year’s breeding season appears to have been the factor, that by some mechanism, led to food shortages and a resulting very poor survival of chicks in the colony, such that only about 20% survived to leave the colony (see post of 31 October).
Our geolocator results showed that by now, late November, Sooty Terns should be arriving in the Sri Lanka and Bay of Bengal areas. These areas are currently being battered by storms that have led to serious flooding in Chennai and other areas. We do not know what effects these conditions might have on Sooty Terns. Does it affect their ability to feed? Does heavy rain when they are out at sea affect their survival? Do they have the flexibility in their migration to avoid these conditions and forage elsewhere? Are this year’s young still with their adults or do juveniles migrate elsewhere? How long with the stormy conditions last?
Two other snippets of information could be relevant. First, during a die-off (thought by Sri Lankan vets to have been to Newcastle Disease, an infection normally of poultry) of adult Sooty Terns in Sri Lankan waters in 2011 two ringed birds were found, one ringed while breeding in Seychelles, the other when breeding on Juan de Nova, the western Indian Ocean’s largest Sooty Tern colony. This suggests that the Sri Lanka/Bay of Bengal area could be an important migration destination for Sooty Terns that breed over a wide area.
Second, Sooty Tern chicks that we ringed on Bird Island in 1997 experienced very poor survival compared with chicks that had been ringed in the 1993 and 2002 breeding seasons. In 1997-1998 the Indian Ocean Dipole attained a strong positive value, just as it is now! (see http://stateoftheocean.osmc.noaa.gov/sur/ind/wtio.php) We do not know how long this will last, but predictions of severe coral bleaching in the Indian Ocean in February-May 2016 (http://www.noaanews.noaa.gov/stories2015/100815-noaa-declares-third-ever-global-coral-bleaching-event.html) suggest that Sooty Terns in Seychelles might experience difficult conditions into their next breeding season.

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