Mode of development and interspecific avian brood parasitism

Avian interspecific brood parasites differ considerably in their commitment to parasitism; 87 species are obligate brood parasites, whereas 35 species are known to be facultative brood parasites. This variation is strongly related to mode of development. Obligate parasitism is found almost exclusively in altricial species, whereas facultative interspecific parasitism is predominant in precocial birds. We propose that the association between mode of development and form of parasitism reflects a fundamental difference between altricial and precocial birds in the relative benefits of emancipation from parental care after laying. We argue that altricial brood parasites obtain such a large increase in realized fecundity by avoiding the costs of parental care that obligate parasitism is favored over facultative parasitism. In contrast, precocial brood parasites gain relatively little in terms of increased fecundity via obligate parasitism, and much of this increase could potentially be gained by facultative parasitism. Thus, obligate interspecific brood parasitism will not be favored in precocial birds. Three factors influence this difference between altricial and precocial species: (1) altricial birds have relatively more energy and nutrients with which to lay additional eggs, (2) altricial birds can produce more eggs for the same amount of energy and nutrients, and (3) altricial birds realize a greater relative gain in fecundity for each additional egg laid. We suggest further that facultative interspecific parasitism in birds may originate simply through a carry over of intraspecific parasitism; 29 of 33 facultative interspecific parasites also parasitize conspecifics. Facultative parasitism of other species would provide a greater range of potential host nests and could be maintained as an evolutionarily stable end point by the same mechanisms that maintain intraspecific brood parasitism. [Behav Ecol 1991;2:309–318]