Denis Island – a home for Sooty Terns?

When Denis de Trobriand discovered Denis Island in 1773 it was populated by vast seabird populations nesting on the ground and in the trees including species of Booby (Sula spp) and the Sooty Tern (Onychoprion fuscata). Sadly during the early years of human occupation the sea bird populations were largely extirpated due to a combination of: direct human predation, loss of habitat to agriculture and the inadvertent introduction of predators such as rats (Rattus rattus).

Since 1998 however the island owners have initiated a structured programme of environmental management and restoration that has seen the rehabilitation of large tracts of habitat, the eradication of alien rats and cats and the re-introduction of rare endemic bird species.

Denis Island is now turning its attention to the re-establishment of a sooty tern colony as the first step in a process of ecosystem restoration in the southern portion of the island. Denis is working with the Green Islands Foundation and Professor Chris Feare, a leading expert on the specie, on this ground breaking project.

Three and two dimensional models have been set out in a grid across an area of about 15,000 m2 which offers different habitat types. Colony noise is being played from four strategically positioned loudspeakers such that the different stimuli of: both kinds of model, the habitat and the colony noise can be assessed in terms of the response it engenders in passing sooty terns.

This first year is very much a learning experience, such work having not been undertaken before and the lessons learned this year will be applied next season in the hope of gaining improved results with the ultimate goal, in the coming years, of re-establishing a Sooty tern colony on Denis.

Chances of ultimate success?

Denis Island did historically play host to a population of Sooty terns and Denis’ two nearest island neighbours – Bird and Aride islands - still have colonies. As such during the breeding season Sooty terns regularly overfly the island. Also we know from Professor Feare’s work that individual birds will move from disturbed colonies to less disturbed locations and also that it is likely due to decline of other colonies that there may be many adults looking for new places to breed in any given year. It is therefore felt that with the testing and refinement of habitat and lures that there is a good chance that a colony may be re-established on Denis Island in the future.

The indications from the first few weeks of work are certainly very encouraging. Sooty terns have shown very definite interest with frequent over flights and low circling of the trial area and several instances of birds landing and demonstrating display behaviour before alighting. The longest period a bird has yet been observed on the ground is approximately 20 minutes but a very clear indication of their preference in terms of stimuli and habitat has already been received meaning that efforts next year can be better tailored to their preference.

In the meantime the work will continue through this season and the interaction of birds with the trial area will be observed and recorded in detail.

Let’s hope that in years to come all this hard work will bear fruit.