The intra- and interspecific relationships between abundance and distribution in helminth parasites of birds

1. Positive correlations between local abundance and distribution on a larger spatial scale are commonly observed among related species.
2. Within parasite species, the same relationship may be expected between prevalence and intensity of infection across host species used. Across parasite species, a positive relationship is expected between average abundance in a host population and the number of host species that can be exploited based on the resource breadth hypothesis. Trade-offs between the ability to exploit many host species and the potential for heavy infections, however, could result in a negative relationship.
3. Intraspecifically, using data on 51 helminth species parasitic in birds, prevalence and intensity of infection among host species used are generally only weakly correlated. Only in nematodes is there an overall positive relationship between prevalence and intensity.
4. A comparative analysis was performed on data from 389 species of cestodes, trematodes and nematodes parasitic in birds to determine how host specificity covaries interspecifically with abundance, measured both as prevalence and intensity of infection.
5. After controlling for phylogenetic influences and sampling effort, the number of host species used correlated positively with prevalence in all three parasite taxa, and with intensity of infection in trematodes only.
6. These results do not support the existence of a trade-off between abundance and the use of many host species, as has been found for fish parasites. Instead, whatever makes helminth parasites of birds abundant within a host population may facilitate their successful colonization of new host species.