[6] After considerable pioneering conservation efforts by Carl G. Jones and Abdool Wahab Owadally the numbers had increased to circa 400 birds in 2019.

However, the most severe decline was in the 1950s and 1960s due to indiscriminate DDT use and invasive species like cats, mongooses, and crab-eating macaques which killed the kestrels and their eggs.

Stanley Temple from Cornell University studied this species for two years and the first attempt in 1973 to breed the birds in captivity failed because the hatchling died when the incubator had a breakdown.

Though conservation measures were immediately undertaken with the help of a breeding program by the Jersey Zoo (now Durrell Wildlife Park), the efforts to rescue this species initially failed because the eggs were not fertile.

With the help of Gerald Durrell, the Welsh biologist Carl Jones established a wildlife sanctuary on Île aux Aigrettes.

In addition, Carl G. Jones supplemented their diet to enable the laying a new egg after the first one had been removed, thereby averting any negative impact on the wild population.

Since 1994, the programme serves only as a safeguard, should some catastrophe befall the wild population, and other rare endemics are now being cared for at the station (such as the pink pigeon or the Mauritius fody).

While some apparent inbreeding depression was noted in the captive birds,[citation needed] it was certainly lower than might be expected given that the effective population size was maybe 5 individuals during the mid-1970s.

[citation needed] However, the debilitating effects of DDT accumulation on the birds' health, and not inbreeding, are considered to have been the major cause for the failure of Temple's breeding program.

As near-panmictic conditions were sustained for many generations, alleles that might cause inbreeding depression were steadily removed by means of natural selection.

The phenomenon that effective population sizes as low as 4–5 can be tolerated without pronounced inbreeding depression is also known from other small-island birds, such as Petroica traversi or the Laysan duck.