As mentioned in my blog of 28 July, Dylan Savy, the 11-year old son of the Bird Island’s general manager and grandson of island owner Guy Savy, was due to count the number of Sooty Tern chicks in the squares we had marked in the colony before our departure. Dylan duly undertook these counts on 31 August. This revealed that an average of 56% of the chicks that hatched from eggs that were present at the end of incubation had survived to the stage of fledging. This compares with only 21% in 2015, which was altogether a bad year. Coupled with the high density of nesting in 2016, this high fledging success indicated that Bird Island’s Sooty Terns had a good breeding season on the island. But what next?
After leaving the colony once they have learned to fly, young Sooty Terns have to learn to feed themselves. We know that parents can feed their flying chicks in flight but we do not know how long chicks remain with a parent before embarking on their independent lives. In fact, we know little about the lives of young birds between leaving the colony and returning to breed, usually five years later. We have two ring recoveries from juveniles, one in Sri Lanka and the other in Northern Australia. These suggest a wide dispersal, as we have also found with adults that we have tracked while they are away from the colony. However, the technique used for tracking adults, using geolocators that must be recovered by recapturing the returning marked adults the following year, is unsuitable for studying the five-year travels of the youngsters. With young birds being away for five years, and only about a quarter of them surviving this period of their young lives, we need a different tracking method.
Devices that record the movements of the young birds in real time using satellites are currently the most promising technique from investigating the wanderings of young Sooty Terns. However, we first need to find a way of attaching trackers like these in a way that will enable the tracker to function properly on the birds without affecting their behaviour or survival. We have made good progress in our research on attachment methods but satellite transmitters and satellite time are expensive and we shall need to find more funding to discover where young birds go.
These five years “away from home” are undoubtedly important times of learning for the youngsters. In addition to learning how to feed themselves (they are a different colour from adult Sooty Terns and may initially feed in a different way from adults), they need to learn how to discover where they are most likely to encounter fish, how to avoid predatory fish when they are feeding, how to avoid storms and cyclones, and how to maximise their flying skills in order to achieve all of these.