Two hypotheses have been proposed to explain the abundance-occupancy relationship (AOR) in parasites. The niche-breadth hypothesis suggests that host generalists are more abundant and efficient at colonizing different host communities than specialists. The trade-off hypothesis argues that host specialists achieve high density across their hosts’ ranges, whereas generalists incur the high cost of adaptation to diverse immuno-defense systems. We tested these hypotheses using 386 haemosporidian cytochrome-b lineages (1894 sequences) recovered from 2318 birds of 103 species sampled in NW Africa, NW Iberia, W Greater Caucasus, and Transcaucasia. The number of regions occupied by lineages was associated with their frequency suggesting the presence of AOR in avian Haemosporidia. However, neither hypothesis provided a better explanation for the AOR. Although, the host-generalist Plasmodium SGS1 was over 3 times more abundant than other widespread lineages, both host specialists and generalists were successful in colonizing all study regions and achieved overall high prevalence
