The Peregrine Fund has been involved in conservation of the Mauritius Kestrel ever since we sent Stan Temple over to the Indian Ocean in 1973 to determine the status of this island endemic, almost as long as we have been working with the Peregrine Falcon. Later, we supported the work of Carl Jones and his associates, and for a number of years one of our falcon breeders, Willard Heck, spent his falls and winters on Mauritius helping Carl to breed kestrels and hatch their eggs in captivity to support a highly effective reintroduction effort.
For a time in the 1970s, the Mauritius Kestrel was the most endangered bird of prey in the world, with a surviving population in the wild of only two known pairs in the Black River Gorges of southwestern Mauritius where a remnant of the original subtropical, evergreen forest still exists. As a result of captive breeding and release of the progeny, careful husbandry of wild eggs and young, control of exotic predators, supplemental feeding, and provision of nest boxes in areas lacking suitable nesting sites, the kestrel population increased—dramatically so in the 1980s and 1990s.
Recently, The Peregrine Fund has again provided support to help defray the cost of a complete, island-wide survey of nesting falcons to assess the current status of the species, which had been estimated using mathematical models to consist of as many as 200 to 250 nesting pairs and 800 to 1,000 individuals in the immediate post-nesting period. Now we have two reports that illuminate the situation.
By 1997 the kestrel had expanded its range considerably and existed in three separate populations: 3 or more pairs in the Moka Mountains inland from Port Louis (not certainly known in 1997), 61 pairs in the western population (Black River Gorges and surrounding areas), and 35 in the eastern population (Bambous Mountains and surroundings). In his note on the 2007-08 survey, Richard Dale reports no pairs in the Moka Mountains, 43 pairs in the western population, and 42 pairs in the east. For the species, that is a difference of 99 observed pairs in 1997, compared with 85 in 2007. It is not a huge difference, but concern lies in the details of the surveys in east and west, which are considered to be separate populations, and in the methods used to estimate total population size from a much smaller number of actually counted birds.
The eastern population has remained stable at around 40 breeding pairs for the past 10 years or so, apparently being limited by available nest sites (mostly nest boxes and a few cliffs and tree hollows). Of concern in 2007, in the western population there were no kestrels present at 38 previously occupied sites and the number of observed pairs was only 72% of the pairs counted in 1997. Most of the unoccupied nesting sites were in marginal habitats peripheral to the Black River Gorges, where the best habitat occurs. In many cases dilapidated nest boxes were a problem. There has been no supplemental feeding of these pairs for a number of years, and this reduction in food could have decreased the breeding success of some pairs. More study will be needed to puzzle out these problems, but it appears that an unmanaged population in this region will not be as large as one that receives annual support.
The Moka Mountains were last investigated in 2001–02, when only a single pair was seen. A thorough search this season revealed no birds at all, and it seems likely that this population, always marginal, no longer exists. In the past 10 years, the habitat has visibly deteriorated owing to heavy invasion of exotic vines and privet, which produce difficult conditions for foraging kestrels. Habitat deterioration will no doubt continue to be a problem for kestrels throughout their range on this heavily over-peopled island.