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Home » Projects » Protection of white-tailed tropicbirds and Audubon’s shearwaters in the Bahamas: Addressing the effects of rats, avian predators, and lack of nest sites

Protection of white-tailed tropicbirds and Audubon’s shearwaters in the Bahamas: Addressing the effects of rats, avian predators, and lack of nest sites

Cavity nesting seabirds in the Bahamas and other tropical archipelagos have declined sharply since humans arrived. Although biologists have attempted to find and protect the remaining populations, even on uninhabited and formally protected islands scientists have recently documented large declines in local populations of Audubon’s shearwaters (Puffinus lherminieri lherminieri) and white-tailed tropicbirds (Phaethon lepturus catesbyii). Halting the declines of these cavity-nesting birds requires a diverse approach that accounts for the major limiting factors of survival and reproduction for both of these species in the Bahamas: rats, native avian predators, and a lack of nest sites.

The greatest threat to both species is the presence of introduced mammals (primarily Rattus rattus) on many of the islands where the birds formerly nested or where sparse populations persist.  Secondly, native predators including barn owls (Tyto alba) and wintering Peregrine falcons (Falco peregrinus) have been observed killing breeding adults and chicks that are near fledging. The magnitude of the effects of avian predators has not been quantified, but steps to understand and minimize their effects on these remnant populations are needed. Finally, in the large colonies, many pairs appear to have difficulty finding appropriate limestone crevice nest sites. Increasing the number of high quality nest sites could potentially increase recruitment at both large and small colonies where spaces are limited, but simple and effective methods to build and place sites are needed.

This project proposed to begin a long-term restoration effort by focusing efforts on Audubon’s shearwaters and white-tailed tropic birds at the Exuma Cays Land and Sea Park (ECLSP), addressing each of the three major threats that the seabirds face.  Rat trapping occurred on numerous cays in the park to determine rat presence/absence and relative abundance and nests were monitored for evidence of rat predation; seabird carcasses were evaluated to determine the relative impact of different avian predator species, and pellets were collected from falcon and owl nests; and nest site limitation was tested by building artificial seabird nests and using playbacks of courting vocalizations and decoys to attract seabirds.

Researchers discovered that predation effects varied across colonies. Rats were present at some very small colonies and predation rates suggested threat of location extinction. The project’s trapping efforts continued in order to protect those small populations until large-scale rat eradication programs are launched. Data on rat presence/absence will be critical for informing those efforts. Results also indicated that at some key colonies, barn owl predation rates were extremely high, and that this co-occurred with large mouse populations, indicating an interactive effect: populations of introduced mice support high barn owl populations, increasing the severity of owl predation on seabirds. Interestingly, falcon predation did not seem to be a major factor for seabird populations on any of the cays studied. More than 40 artificial nests were installed and visitation and nesting at those nests varied across colonies, in part relative to colony history and the availability of existing natural sites. Because introduced mammals were newly discovered on some cays that were candidates for artificial sites (small population, limited nesting sites), nests were not installed at those locations to avoid additional predation by and growth of predator populations. Birds responded positively to acoustic playbacks, retuning calls and searching crevices, indicating that this technique could be extremely useful for growing and re-establishing colonies once cays are free of introduced predators. Mackin is helping to mobilize an archipelago-wide rat eradication effort and continues to work towards a long-term seabird conservation program in the area.