Ecological specialization can explain the declining status of many species in the face of current global changes. Amongst specialists, nomadic predators present conservation biologists with many challenges, mainly because of the difficulty of studying highly mobile individuals over time and across very large areas. For these species, the relative influence of prey abundance, habitat heterogeneity and arrival time at the breeding grounds on breeding parameters remains poorly understood. We studied the factors influencing variation in breeding numbers and performance of a declining nomadic specialist raptor, the Pallid Harrier , in north-central Kazakhstan. During a 5-year period, we recorded large inter-annual variation in vole abundance in the main study area, and differences between habitats. We also recorded a strong numerical response of breeding Pallid Harriers to inter-annual changes in local vole abundance. From a 13-year dataset on breeding harriers in the same area, harrier numbers appeared to vary cyclically, with an interval between peaks of approximately 6 years. At a broader, regional scale, variations in Pallid Harrier abundance appeared asynchronous, suggesting a regional redistribution of harriers between years. Reproductive success depended on local vole abundance, but also on timing of breeding and nesting habitat. Clutch size, nest success and fledged brood size increased with vole abundance. Late breeders had smaller clutches and apparently lower hatching rates than early ones, possibly as a result of the interplay between their probable poorer body condition and habitat-specific variation in predation rates. In true nomadic specialist predators, such as Pallid Harriers, breeding success may therefore depend on a complex interplay between spatial variation in prey abundance, habitat composition and timing of breeding attempts. One of the factors influencing the start of breeding is the length of time taken to prospect between different breeding sites, which in turn may depend on the predictability of spatial and temporal variation in vole abundance. These results have important conservation implications, as changes in climate and habitat could affect spatial and temporal variations in vole abundance, with possible consequences for timing of breeding, food availability and, ultimately, the reproductive success of this declining nomadic predator.< Leer menos