On the origin of brood parasitism in altricial birds

The probability that obligate interspecific brood parasitism (OP), among altricial birds evolved directly from the normal breeding (no parasitism, NP) mode or indirectly through intraspecific nest parasitism (INP) was examined by using maximum-likelihood and parsimony approaches. We examined the probability of ancestral states at 24 key nodes in order to test our hypotheses. The state of the most basal node in a tree of 565 genera of altricial birds is equivocal; however, the state probability of NP at this node is about 5.5-fold more likely than the state of obligate parasite. A similar trend was observed for basal nodes of most families examined. The INP state was supported only in the Hirundinidae. The high incidence of INP among martins and swallows explains this finding. Contrary to our predictions, even in other groups where there is a high incidence of INP and OP, such as in the tribe Icteri and the Old World finches, the probability of NP being ancestral was very high. We conclude that in all cases but one (Hirundinidae) obligate, and probably facultative, brood parasitism evolved directly from normal breeding mode rather than indirectly through some other form of parasitism.