Between 27 and 30 August we deployed satellite tags on 15 juvenile Sooty Terns in order to follow their movements after fledging and discover where their parents took them as the fledglings learned to feed themselves. When attached the tags to birds that weighed a minimum of 180 grams, judging that chicks of this weight would be better able to carry a tag weighing 5 grams than lighter birds. We were concerned that it had become difficult to find chicks of our target weight as young chicks were losing weight as a result of their parents experiencing difficulty finding food. We were worried that this could affect the survival of our tagged birds during their early days away from the colony. Our main objective was to see where parents took their offspring, assuming the older birds would take them to oceanic areas where they would stand the best chance of finding adequate food.
A month later, however, we are gratified that locations recorded by passing Argos satellites show that thirteen of our fifteen youngsters left Bird Island between 6 and 17 September and are now out at sea. All of the thirteen fledglings have been taken by their parents north to an area around a submarine mountain range, the Coco de Mer Ridge, that lies about 350-600 kilometres to the north-north-east of Bird Island. This suggests that waters around the undersea ridge are known by adult Sooty Terns from Bird Island to be good feeding areas, probably enriched by nutrients brought to the ocean surface by deeper water that is forced upwards when undersea currents meet the mountain range.
In their first month some of our birds have already travelled well over 1000 kilometres, but two birds have failed to leave Bird Island. One of these appears to be moving around the Sooty Tern colony, between the area where it was originally caught and the beach, although it did venture over the sea to about 2 km north of the island, only to return to the colony by the next satellite fix. The other tagged juvenile has not transmitted information to the satellites since 4 September. We assume that it has died, or that the tag has failed in some way.
We have returned to the island at the end of September for three days on the remote chance of finding one or both of these birds. This might allow us to remove the tag from the bird that could have died and re-deploy it on another bird that met our body weight requirement, and to check the health of the bird that is moving around the island.
Bird Island’s Sooty Tern colony now has far fewer birds than when we left three weeks earlier, indicating that most chicks and their parents have left. While some of the remainers look healthy (we shall weigh a sample to check), others are weak or injured, most commonly with damaged wings or feet and in some cases being blind in one eye, injuries doubtless resulting from fights over space or food earlier in their lives. There are also many corpses of chicks that failed to survive, probably due to food shortage (see blog post of 31 August). We have examined all of those in the area where our lost chick used to be, checking visually and with a metal detector, but to no avail. If the chick has died, it could be deep in the burrow of one of the island’s scavenging Ghost Crabs, way beyond our detection ability! Nevertheless, we shall continue the search until we leave on 29 September.
But even if we fail to find this chick, we are pleased that we have at least thirteen birds that are providing us to exciting new information and hope they will continue to do so over the coming months and years.
Rachel Bristol, Christine Larose and I are grateful to SeyCCAT for funding this study, and to Bird Island for their continuing support for our work, which on this visit includes the loan of and permission to use their metal detector.