Predation against birds with low immunocompetence

Differences in the phenotypic characteristics between individuals falling p rey to predators and conspecifics avoiding predation will reflect the inten sity of selection on prey. If prey are generally in poor condition, we pred icted that they should have an inferior health status in comparison to indi viduals dying for other reasons. We investigated this prediction fur prey a nd conspecifics that did not die from predation by comparing the size of th e spleen, which is an important immune defence organ reflecting one compone nt of immunocompetence, using 18 species of passerine birds and domestic ca t Felis catus predators as a model system. Prey had consistently smaller sp leens than non-prey, implying that they had weak immune systems. The data s et did not indicate that sex or age, month of death, body mass, body condit ion, liver mass. wing length or tarsus length differed significantly betwee n prey and non-prey. Thus there was little evidence of confounding factors affecting the results. These observations indirectly suggest that disease a nd parasitism may play an important role in predator-prey interactions.