Since 2011 we have been using different kinds of electronic tracking device to investigate the movements and behaviour of Bird Island’s Sooty Terns, greatly increasing our knowledge of the birds’ use of the surrounding ocean. Prior to that much of our knowledge of Sooty Tern survival, longevity, movements between islands and daily activities in the colony had been obtained from the ringing of adults and young birds while still in the colony, and searching for and recapture of the ringed birds on Bird Island and in other colonies in subsequent years.
Despite the increasing availability of new technologies, which tend to be very expensive, ringing still has a place in research on Sooty Terns, especially the ringing of chicks. The ringing of young birds prior to fledging provides us with hundreds, or ideally thousands, of birds of known age that we can follow through their lives.
Since my studies of Sooty Terns began in 1973, we have managed to ring large numbers of chicks in only four years – 1973, 1993, 1997 and 2002. In 1973 many of the rings were made of aluminium, a soft metal that wears down to become thin through abrasion with Sooty Terns’ legs (the rings wore down on the insides, leaving the ring numbers clearly visible) until eventually falling off. Rings manufactured more recently are made of stronger alloys and survive for the life of the bird. (In 1994, 1995 and 1996 I was advised to use coloured plastic rings to denote year of ringing, but these did not survive well on Sooty Terns and most fell off).
Recaptures of the ringed birds showed that chicks from 1973, 1993 and 2002 survived well after fledging but that those ringed in 1997 did not. The 1997 cohort of chicks fledged as the extremely powerful 1997-8 El Niño Southern Oscillation gathered strength, clearly leading to conditions that reduced the survival of these birds in the Indian Ocean.
The 2019 nesting season has proved to have had highs and lows for Bird Island’s Sooty Terns. Nesting began exceptionally early, in mid-May, but our GPS tracking studies showed that incubating birds were having to travel long distances to find food, involving long periods away from the nest to forage. After the eggs hatched the chicks fared well and on our return to the island on 20 August, we found the chicks to be heavy and strong. By 25 August, however, the chicks we were catching for ringing were lighter and four chicks that we had ringed on 22 August and recaptured on 25th had each lost 22-33 grams, suggesting that the birds had entered another period of food shortage. The chick’s weights have still not recovered but many have now reached their flying stage and are more difficult to capture. We have now ringed over 650 of this year’s chicks and we hope that future searches for them as they return to breed in about 5 years’ time will show whether their low weights at fledging have influenced their subsequent survival in the wider Indian Ocean.
As ever, we are most grateful to Bird Island for supporting our studies, and to the British Trust for Ornithology that allows us to use their rings.