A neglected cost of brood parasitism: egg punctures by Shiny Cowbirds during inspection of potential host nests

MASSONI, V; JUAN C. REBOREDA

The Condor

Año: 2002 vol. 104 p. 407 - 411

Parasitized hosts of the Shiny Cowbird (Molothrus bonariensis) suffer severalcosts, and among the most important is the loss of eggs through egg punctures inflicted bythe parasite. Unparasitized nests also have eggs damaged by cowbirds, but researchers usuallyignore these losses. To quantify this cost we compared three groups of nests of theYellow-winged Blackbird (Agelaius thilius): parasitized and unparasitized nests from an areaused by Shiny Cowbirds, and unparasitized nests from an area not used by the parasite.Because cowbirds puncture eggs as soon as the first host eggs are laid, we calculated theclutch size only for those nests found during construction. Unparasitized nests in the areaused by cowbirds had lower egg survival rate and hatching success and higher probabilityof nest desertion than unparasitized nests in the cowbird-free area. Our results indicate thatone must consider egg punctures at unparasitized nests to avoid underestimating the impactof parasitism.