Tracking Sooty Terns at sea

In recent blog posts I have often referred to our work in the Sooty Tern colony, without explaining what we are doing. Our main objective this year is to discover where Bird Island’s Sooty Terns feed during incubation and early chick rearing.

As mentioned in the more technical post of 14 December 2015, we have been investigating ways of attaching small (weighing about 3 g) GPS tags  to Sooty Terns (a bit like miniature sat-navs that record where the birds are every 10 minutes, rather than guiding them to a location!). In 2014 we tried various methods, eventually settling for attachment to the central pair of tail feathers, but after six days the birds broke off the these feathers and we lost all of the tags. In 2015 we used dummy tags to investigate what we thought would be a more secure form of attachment, using a harness to attach the tag to the lower back. The harness is secured around the top of the bird’s thighs. This proved very successful: the dummy tags remained in position for up to 25 days with no detected adverse effects on the bird carrying the dummy tags, which we were able to remove easily at the end of the trial.

DSCN1923
A Sooty Tern fitted with a Pathtrack GPS tag. This bird was recovered after it had spent six days at sea (hristine Larose)

On Bird Island in 2016 we have therefore been using this attachment method, making the harness from a very flexible elastic used in angling. This appears to allow unhindered movement of the birds’ legs, is easy for us to use (Christine undertakes this part of the process), and will degrade under UV light if we are unable to catch the bird on its return to the colony. This means that tag will fall off after 3-4 months. Pathtrack, the UK manufacturers of the tags, have adapted the tags they have made for us to accommodate this kind of attachment and this appears to have worked very well.

After our arrival on Bird Island on 8 June we marked groups of nests where egg laying had just begun and ringed the adult that was incubating at the time. Thereafter we monitored the identity of the incubating bird (ringed or unringed) at each nest each morning; from these data we can determine the number of days that each member of the pair incubates during its duty shift. We waited until eggs had been incubated for around 10 days before catching some adults and fitting them with GPS tags.

Track n Google Earth from three Sooty Terns fitted with GPS tags. They all went south to an area of seas south-west of Silhouette Island, but the third bird then travelled further to the north west before returning to Bird Island
Track n Google Earth from three Sooty Terns fitted with GPS tags. They all went south to an area of seas south-west of Silhouette Island, but the third bird then travelled further to the north west before returning to Bird Island

During our six weeks on the island we managed to obtain more than 20 tracks from foraging birds. Early during the investigation Bird Island’s Sooty Terns were travelling south to feed, close to Silhouette Island and about 120 kilometres from the nesting colony. Later, they fed more to the north and west of the colony, doubtless visiting areas where their food was most abundant at the time. Detailed analysis of the tracks awaits but it appears that the attachment method we have developed is appropriate for Sooty Terns and that we can learn a lot more about their foraging behaviour during the breeding season.

We are most grateful to Guy Savy and his family for their continuing support for our work on their wonderful island. In addition, we thank the British High Commission (Seychelles), The Percy Sladen Memorial Fund and James Cadbury for their contributions towards the cost of some of the GPS tags. This is allowing us to undertake these fascinating studies and to learn for the first time where Bird Island’s Sooty Terns feed during the breeding season. This information will hopefully be useful in advising on the location of Marine Protected Areas in Seychelles.

 

 

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