Sooty Terns on Ascension Island South Atlantic

Sooty Tern Ringing Data

Sooty Tern (Onychoprion fuscata) ringing data is available for download in either Microsoft Excel of Microsoft Access file formats:

Microsoft Excel (3.63Mb)

Microsoft Access (6.08Mb)

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Ascension Island KML File For Google Earth

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Age categories P1 – P7 of Sooty Tern chicks

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Plasticity in breeding periodicity: the sub-annual cycle of Sooty Terns on Ascension Island

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Cats and seabirds: Effects of feral Domestic Cat Felis silvestris catus eradication on the population of Sooty Terns Onychoprion fuscata on Ascension Island, South Atlantic

The population of Sooty Terns (Onychoprion fuscata) breeding on Ascension Island in the Atlantic Ocean was monitored over 17 years (1990–2007). This period spanned the programme of feral Domestic Cat (Felis silvestris catus) eradication from the island, which commenced in 2001 with the last Cat recorded in 2004. We report on the abundance of Sooty Terns and Black Rats (Rattus rattus) before and after Cat eradication. The Sooty Tern breeding population in the 1990s averaged 368 000 and Cats were killing Terns at an average rate of 33 adults per night. Following Cat eradication, adult Terns are no longer predated. However, egg predation by both Rats and Common Mynas Acridotheres tristis has continued with Mynas destroying more eggs than Rats. Unexpectedly, we observed a change in Rat predatory behaviour. Following Cat eradication, Rats have become a major predator of Sooty Tern chicks. Despite this change, the Tern population has shown a season-on-season increase since Cat eradication, 48.8% in 2005, 8.2% in 2006 and 6.1% in 2007, and the breeding population increased to 420 000 birds in 2007. Incubation success improved from 66.0 to 84.4% during Cat eradication, before dropping down again to 67.9% after Cats were eradicated and Rat control measures were introduced. Index traplines were set for Rats and Rat numbers fluctuated widely immediately after Cats were eradicated but there were no significant differences that could be attributed to changes in Cat numbers. Ascension Island Sooty Terns breed every 9.6 months and juveniles defer breeding for seven seasons. Hence 2008 is the first year in which an increase in the breeding Sooty Tern population directly attributable to Cat eradication is likely to be detected. We conclude that long-term monitoring is essential to guide conservation practice even in this relatively simple predator–prey system.

Sub-lingual oral fistulas in Sooty Terns

Sub-lingual oral fistulas are a condition first reported in New Zealand Stitchbirds (Notiomystis cincta) in which a lesion develops on the periphery of the mandible in the oral cavity and a fistula develops through which the tongue protrudes. We report that it arises in another species, the Sooty Tern (Onychoprion fuscata), breeding on Ascension Island in the south Atlantic. We discovered five adults with oral fistulas out of a total of 13,664 adults and chicks ringed during 11 breeding seasons over 13 years. Compared with other threats to the Ascension breeding population of over 180,000 pairs of Sooty Terns, we report that oral fistulas are a minor threat to colony stability, especially because afflicted birds were incubating eggs. Nevertheless, our findings suggest that further investigations of whether the condition is sexlinked, of its pathogenesis, and of its causation would augment our knowledge of the biology of Sooty Terns; such information might also have conservation implications for our understanding of the condition in endangered Stitchbirds. We call upon ornithologists to report oral fistulas in other species, because it would seem unlikely that these are the only two species predisposed to the condition.

Sexing sooty terns on Ascension Island from morphometric measurements

Sooty terns Onychoprion fuscata are one of the most abundant seabirds but breeding populations in many colonies have diminished. Rapid sexing of sooty terns in the field could be crucial in advancing our understanding of their reproductive biology, and in promoting conservation. However, sooty tern males and females are identical in their plumage and, thus, difficult to sex in the field. Morphometric measurements were taken from 63 adult sooty terns breeding on Ascension Island in 2005. A small blood sample was taken from the brachial vein to determine the bird’s sex using standard PCR-based molecular techniques. Males were consistently larger in all morphometric measurements than females but considerable overlap between the sexes resulted in no single measurement being a useful discriminator of sex. A principal components analysis on a correlation matrix of seven morphometric measurements indicated that the first principal component (PC1) was a good ‘body size’ axis explaining 40.5% of the variance in the original matrix. The suite of head measurements all had high character loadings on PC1 and were, therefore, good indicators of the body size of sooty terns. Tarsus length and wing length were less reliable predictors of sex. Discriminant analyses revealed that a disciminant function incorporating head measurements and wing length allowed 77.8% of sooty terns to be sexed correctly based upon morphometric measurements alone. Further morphometric approaches to sexing should be explored with sooty terns captured in subsequent years.

Sooty Terns Onychoprion fuscatus on Ascension Island in the south Atlantic are a reproductively isolated population

Population size is determined by the balance of births and deaths, and of immigration and emigration. In order to investigate population dynamics of the Sooty Tern Onychoprion fuscatus population on Ascension Island in the south Atlantic, we focussed on the key component of immigration by measuring the proportions of philopatric and immigrant recruits to the breeding population. The Sooty Tern colony had expanded by 7% following the eradication of Feral Cats Felis silvestris in 2003. Between 1975 and 2002 potential recruits to the breeding colony were ringed as chicks on Ascension Island and in colonies elsewhere in the Atlantic. Searches for these birds as adults breeding on Ascension commenced in June 2002 and continued until December 2008. Of the 600 chicks ringed on Ascension, 36 (or 6%) were recruited into the breeding population and of the 9,482 ringed in other colonies, only one (0.01%) was found breeding on Ascension Island. A further investigation of morphometric measurements revealed that Sooty Terns on Ascension were significantly different from those from neighbouring colonies. The sample of birds from Ascension was drawn from a different population and is possibly of a previously unidentified, sub-species. We conclude that immigration to the breeding population on Ascension is minimal and that they are a reproductively isolated population.

The use of Google Earth satellite imagery to detect the nests of masked boobies Sula dactylatra

Masked boobies Sula dactylatra, like many other species of Sulidae, do not construct elaborate nests. However, their nest sites produce a characteristic 'nest signature'. We found that these nest signatures could apparently be seen in freely available satellite images (GoogleEarth) of the main island of Ascension in the south Atlantic. We verified that this was the case by comparing nest signatures detected on these satellite images with field reports of occupied nests. We found that the locations of these nest signatures determined from satellite images agreed closely with the coordinates of actual nests on the ground. We used this information to determine the position and size of a previously unreported masked booby colony on the island. Thus, we show that the presence and abundance of some species can be estimated using freely available satellite imagery if a suitable signature in the satellite image can be found. Regularly updated satellite imagery of target sites could also be used for population monitoring. While this would be expensive, initial evaluation of the technique for particular species or populations can be achieved using freely available images. We encourage wildlife managers to view their study sites on GoogleEarth for evidence of their target species.