 But 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.What was probably this species' closest relative in Recent times, the Réunion kestrel, became extinct around 1700 for fairly mysterious reasons.The recorded population dropped to an all-time low of only 4 individuals in 1974 and it was considered the rarest bird in the world.