AVIAN BROOD PARASITES possess an array of adaptations that help them to succeed at their alternative breeding strategy, and these adaptations reduce host fitness in several ways. Host eggs may not hatch because brood parasites have relatively short incubation periods that allow them to hatch sooner, and this may cause hosts to stop incubating their eggs (Briskie and Sealy 1990, Hauber 2003). If host eggs hatch, their nestlings may be outcompeted by parasitic nestlings that are often larger (reviewed in Lorenzana and Sealy 1999, Peer et al. 2005). Common Cuckoo (Cuculus canorus) nestlings eliminate competition by evicting host nestlings or eggs, and Honeyguide (Indicator spp.) and Striped Cuckoo (Tapera naevia) nestlings have specialized mandibular hooks used to kill host young (Davies 2000). Brood parasites also remove or puncture host eggs, and this reduces the host's clutch size and sometimes causes the host to abandon its nest (Davies and Brooke 1988, Sealy 1992, Peer and Sealy 1999a). Some parasites may occasionally depredate nests, thereby causing total nest failure (Soler et al. 1995, Arcese et al. 1996). Egg removal and egg puncture are among the least understood of these adaptations. In a study published in this issue of The Auk, Astie and Reboreda (2006) investigated the effects of egg puncture by Shiny Cowbirds (Molothrus bonariensis). Shiny Cowbirds punctured host eggs in parasitized as well as unparasitized nests, which reveals that the effects that parasites have on hosts are complex and cannot always be fully understood simply by comparing host reproductive success at parasitized and unparasitized nests.