Angelique Pouponneau, CEO of SeyCCAT, and Vania Robert, its Executive Assistant, visited Bird Island for two days, 26 to 28 August, to see for themselves the progress Rachel, Christine and I are making on the SeyCCAT-funded project involving the satellite tracking of juvenile Sooty Terns. We were able to show them the procedures we followed for testing all of the satellite tags prior to deployment on Sooty Terns, as recommended by the manufacturers. We prepared transmission schedules for the tags, activated the tags in an open area of the island to instruct them to send location data to the Argos satellite system, and then we received data from the Argos satellites, showing that they had recorded the co-ordinates of the tags on Bird Island. This confirmed that they were working satisfactorily.
We were then able to demonstrate to Angelique and Vania the fitting of the activated tags to 2-month-old terns that were approaching their departure from the breeding colony.
Angelique and Vania watched, listened and questioned, but also actively participated in the capture, weighing, ringing and recording of data on the tagged juvenile Sooty Terns. We had begun this process on 22 August, when we found the large juveniles, that were frequently exercising their wings, to be heavy and healthy and concluded that we would have no problem in finding birds that weighed at least 180 grams. We regard this as the minimum body mass for the safe deployment of the tags, with little risk of adversely affecting the birds’ behaviour. In this first sample of weighings of birds that had reached flying stage, 69% of the youngsters that we caught exceeded this weight.
It therefore came as a surprise on 27 August when we found that most birds that we caught, expected to be approaching flight capability, were well under this weight criterion, with some individuals weighting as little as 115 grams. Since then, only 21% of chicks that we have weighed have attained the critical 180 grams. We have recaptured four of the birds that we had ringed and weighed on 22nd of August and found that they had all lost weight, representing about 15% of their body. In addition, we are now finding freshly dead well-grown chicks in the colony. On dissection, these birds have empty gizzards, indicating that they had starved. For an unknown reason we seem to have entered a period of food shortage for Bird Island’s Sooty Terns.
Unpredictable food shortages are not unusual in tropical seabird colonies and it was a risk factor acknowledged at the outset of this project. But the suddenness of its onset has been a surprise. However, our selection of the heaviest birds for tagging will hopefully provide us with the birds most likely to survive.
During the evening of 30 August we deployed the last two of our 15 satellite tags, and during the session of ringing and tagging we were pleased to see many adults returning to the colony and feeding their chicks small sardine-like fish. Hopefully, this indicates a relaxation of the food shortage and we now eagerly await receipt of location data telling us where our 2019 juveniles are as they learn to fend for themselves in the tropical Indian Ocean.