Our trial of the attachment method for fitting satellite tracking devices to juvenile Sooty Terns later this year (see post of 20 June) reached its first milestone when we left Bird Island on 30 June. Both of the birds fitted with dummy tags had alternated between incubating the single egg and going out on foraging trips, with their untagged mates reciprocating, ensuring that both members of the pairs shared these duties. Although other evidence suggests that food is not plentiful for Bird Island’s Sooty Terns this year, both pairs completed incubation successfully with their eggs hatching on 23 and 26 June.
Each time a tagged bird returned from a foraging trip it was caught using a small hand net. It was thoroughly examined for any signs of abrasion to its feathers or skin where the tag or its harness come into contact with the birds’ bodies. In addition, the birds were weighed on each return from foraging. We were pleased to discover that there were no signs of abrasion, that the tags remained perfectly positioned on the birds’ lower backs, and that both birds maintained their body weights throughout this part of the trial.
We thus feel reassured that the attachment method is suitable for attaching the working tags to juvenile Sooty Terns just before they fledge in late August.
Now that most eggs in the nesting colony have hatched, it is dangerous for people to enter the colony. Human disturbance can be damaging or fatal to young chicks, which are often viciously attacked if they stray into the small nest territories of neighbouring Sooty Terns. The next phase of the trial therefore involves monitoring the nest sites of our two tagged birds from a distance using binoculars. Each nest is marked with a plastic label and the two tagged birds have been fitted with lightweight plastic rings. One of these is white with black individual lettering, while on the other leg one bird carries a yellow ring, the other an orange ring. Bird Island’s Conservation Officer, Joana Soares, will visit the colony in the evening twice weekly from 15 July (by which time chicks will be big enough to avoid much of the damage inflicted by neighbouring adults). She will look for the tagged birds at their nest sites, and also check for the presence of the chicks, both of which carry a metal ring on one leg. This will hopefully allow us to determine if the chicks are surviving, and if their tagged parents are continuing to provide parental care.
We are aware, however, that the birds might be facing difficult times this year in terms of their food supply. The sea surface temperature in the tropical western Indian Ocean remains higher than the average and this might affect food availability for Sooty Terns, which could affect the chances of survival of the chicks of our two birds fitted with dummy tags. When it comes to fitting working tags on chicks on the point of fledging later in the year, we shall select some of the chicks that are the earliest to reach fledging stage, and among these select the heaviest, since earlier studies have shown that these are the most likely to survive into adulthood.
We are grateful to SeyCCAT (Seychelles Conservation and Climate Adaptation Trust) for funding this project, and to Bird Island for providing all facilities to make the study possible.