Parasitism of maternal investment selects for increased clutch size and brood reduction in a host

The allocation of resources to young that will ultimately be left to die appears counterintuitive. Yet obligate brood reduction has evolved in a number of species, despite the waste of reproductive investment this may incur. Here we test whether brood parasitism could be one factor leading to the evolution of obligate brood reduction because surplus eggs in the nest during incubation offer some protection from the costs of parasitism. Surplus eggs could benefit females in two ways. First, additional eggs may protect against the direct costs of parasitism by facilitating recognition and removal of parasitic eggs with greater accuracy. Second, additional eggs may protect against the indirect costs of parasitism as parasites often damage or remove host eggs when entering the host nest; surplus eggs may be an essential insurance strategy against this damage. We test these possibilities in the Montezuma Oropendola (Psarocolius Montezuma), a species experiencing high levels of parasitism by Giant Cowbirds (Scaphidura oryzivora) throughout their range. Overall rejection rates of cowbird eggs were high (72%), and experimental addition of parasitic eggs to empty, one-, and two-egg nests demonstrated that recognition success was unaffected by the presence of additional host eggs for comparison. However, the value of surplus eggs when one egg was removed or damaged by a parasite was high; 31.6% of successful two-egg clutches lost a single egg during incubation and would have failed to produce a chick without a second egg. This was directly attributable to parasitism in at least 33% of all cases. Therefore, despite highly developed host defenses against direct costs of parasitism (recognition and removal of parasitic eggs), the associated indirect costs (egg damage and removal) could play an important role in selection for a clutch size that results in more chicks than can be raised.