1. Reed warblers are not known to be parasitized by cuckoos in Wales, and are rarely parasitized in some English populations, despite the presence of cuckoos. Nine hypotheses are proposed to explain regional differences in parasitism frequencies. They are tested against national and international data from the BTO, RSPB and EURING, and data from four study sites. 2. The frequency of parasitism of reed warbler populations was found to be negatively correlated with habitat patch size. Thus the frequency of parasitism experienced by reed warblers and the number of cuckoos parasitizing a reed warbler population do not increase in direct proportion to host abundance. 3. Population sizes of two unparasitized (Llangorse Lake and Oxwich) and one rarely parasitized population of reed warblers (Pannel Valley) are neither too large nor too small to be suitable for parasitism by cuckoos. 4. Cuckoos are not excluded from these populations by lack of perches in the habitat, because of a lack of prey in the immediate area, or by territorial competition with cuckoos of other host preferences. 5. There has been no decline in frequency of parasitism of reed warblers in Britain since 1932. Thus, reed warbler cuckoos are probably not declining in numbers and contracting their range away from these three geographically marginal sites. 6. Cuckoos parasitizing reed warbler populations at Llangorse Lake, Oxwich and Pannel Valley would achieve reproductive success equal to or higher than that attained with other local hosts, so that any host switches away from reed warblers would be unprofitable. 7. Reed warbler cuckoos may have dispersed to all or most of these study populations in the past. However, the cuckoo populations would have been small and highly vulnerable to extinction. Cycles of increases, declines and extinctions of local populations could explain the variance in parasitism frequencies and number of cuckoos often observed. 8. The most important factors considered in this study that affect the presence and number of cuckoos at a site appear to be competition between females, habitat patch size and structure, rates of dispersal and demographic stochasticity.