Ecological restoration: the fight to save the Cahow

Cover Photo: Bermuda Petrel from the Surfbirds galleries © Bjorn Anderson -

By Alex Lees


The Cahow (Pterodroma cahow) is a member of the gadfly petrel family, most closely related to the Atlantic P. molIis/feae complex, which are also critically endangered (Bretagnolle 1995). Its sole nesting ground occurs on the castle Harbor Island Group off Bermuda, with the birds feeding out over the Gulf stream over 400 miles to the east (Nilsen, 1991).

Islands often appear to be stable. In some cases, organisms have persisted for many millions of years, and having an oceanic climate, extreme climatic events may be comparatively rare. However, when subject to disturbance they do not appear to be stable, but often suffer catastrophic ecological change (Cronk 1997). Such was the case with the Cahow; the species continued existence was only based on hearsay until the type specimen was collected in 1906. A tideline corpse found in 1932 was the only record between 1906 and the bird’s famous rediscovery in 1951 (Nilsen 1991).

As is common with several other Pterodroma species (Probst, et al 2000), predation by introduced mammals (dogs, cats and rats) has been a major cause in the decline in the Cahow. Feral pigs have also contributed to habitat destruction which is however predominantly an anthropogenic effect. Rat predation was limiting the population growth of the extant Cahow population. Rat predation on Grey-faced petrels (Pterodroma macroptera gouldi) in New Zealand was found to reduce breeding success by up to 35% (Imber, et al 2000). Warfarin baiting was used to eliminate the rat population and give the birds a chance to recover (Nilsen, 1991). Another major problem facing the population was nest site competition with White-tailed tropicbirds (Phaethon aethereus).

The remaining Cahow population had been forced into sub-optimal habitat (into natural crevices and holes in cliffs) away from the interior forest which had either been denuded, or where introduced mammals halted any breeding attempts. The tropicbirds, which arrive to breed later in the year than the Cahows, would kill any petrel chicks they found and then use the nest site as their own. This intra-specific competition problem was solved by placing a baffler over artificially constructed nesting burrows (Nilsen 1991). A similar technique was used to protect the endangered Gould's Petrel (Pterodroma leucoptera leucoptera) (Priddel and Carlile, 1995). An additional technique used was to artificially construct nest sites on the vegetated tops of the nesting islets where tropicbirds are less likely to find them. Over half of the Cahow population now uses artificial nest sites.

A miniature camera system used to view Bonin Petrel (Pterodroma hypoleuca) burrows in order to assess the status of the petrels' breeding activity could prove equally useful in Bermuda for studying the Cahow’s reproductive success (Seto and Jansen, 1997). Despite these successes, the population’s fortunes turned for the worse in the late 1960s as the insidious effects of DDT started to take hold. The resultant enzymatic imbalances led to egg shell thinning and decreased productivity by 35%. Global cut downs on the use of the chemical had alleviated the loss by the 1988s (Nilsen, 1991). Seabirds are excellent subjects for examination of heavy metals because they are long-lived, feed at different distances from land, and exhibit different trophic levels. No one has of yet studied tissue metal levels in the Cahow, but in a study on the related Bonin petrel (Pterodroma hypoleuca), the mean values for mercury were higher than the levels known to cause adverse reproductive and behavioral effects. Such problems may well already be affecting Cahow populations and solutions can only be sought at a global level (Gochfield, et al 1999, Burger and Gochfield, 2000).

(Harney, 1989) has documented that remote sensing techniques such as radar altimetry, synthetic aperture radar, coastal zone color scanning, and infrared radiometry provide effective, instantaneous, and relatively inexpensive means for characterizing critical habitats of marine birds – such techniques could be used to monitor the Cahow’s habitat at sea. Restoring the breeding habitat of such seabirds on the breeding grounds is just one facet of their conservation; discussion of the problems they face from longliners and other pelagic threats is beyond the scope of this paper however.

In a study carried out by (Brook and Kikkawa, 1998), it was found that increased mortality due to the introduction of new predators or diseases, inbreeding depression or frequent severe storms greatly increased the extinction risk to Capricorn silvereyes. Habitat reduction, if not very severe was deemed is unlikely to have a large impact. Comparing this information directly to the Cahow’s fortunes, the first two problems pose obvious threats but as long as quarantine regulations are maintained then the problem is only a minor one. Storms are in fact an ally of the Cahow as they create nesting habitat by uprooting trees (Nilsen, 1991). The curse of inbreeding depression, although not documented in the literature must be one of the greatest threats to the Cahow. If the breeding population was reduced to 36 birds in the 1950s with an unknown floating population of non breeders (failed breeders or birds not of breeding age – which they may be up to six years) (Warham, 1990), then such a population bottleneck must have deleterious effects on genetic fitness.

(Probst et al, 2000) found that the burrows of the Barau's petrel (Pterodroma baraui) of the Mascarene archipelago were very susceptible to disturbance. A thick, undisturbed, humus layer was found to be a prerequisite to burrow establishment implying that the species may be especially susceptible to trampling by large vertebrates. A similar situation is likely to be true of Cahow’s nesting in natural sites and care should be taken by restorationalists when they are working in the habitat.

Alex Lees is an undergraduate in Biological Sciences from UEA, England and is currently studying at UCI in California.


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