Number of close spatial and temporal neighbors decreases the probability of nest failure and Shiny Cowbird parasitism in colonial Yellow-winged Blackbirds

We investigated whether the synchrony and proximity of nests of Yellow-wing ed Blackbirds (Agelaius thilius) provided protection against nest predation or brood parasitism by Shiny Cowbirds (Molothrus bonariensis). We analyzed the effect of the temporal aggregation of nests on the daily probability p er nest of predation, desertion, egg punctures, and parasitism throughout t he breeding season. The probabilities of nest predation and nest desertion increased through the breeding season. The temporal aggregation of nests wa s negatively associated with the probability of nest desertion, egg punctur es, and parasitism, but there was no association with the probability of ne st predation. We also analyzed the effect of the number of close neighbor n ests on the daily probability per nest of predation, desertion, egg punctur es, and brood parasitism. The spatial aggregation of nests was negatively a ssociated with the probability of nest failure and brood parasitism, but th ere was no association with Cho probability of egg punctures. We discuss wh ether dilution effect or group defense, two mechanisms proposed to explain the antipredatory advantages of colonial nesting, are likely to apply to ou r system.