Egg-puncturing behaviour by shiny cowbirds increases nest desertion in chalk-browed mockingbirds

Abstract

When brood parasitic shiny cowbird (Molothrus bonariensis) females visit chalk-browed mockingbird (Mimus saturninus) nests, they peck the host eggs trying to puncture them. These punctured eggs are later removed from the nest by the host during the nest sanitation. We analysed the effect of cowbird punctures on the probability of nest desertion by mockingbirds. We evaluated the association between nest desertion and: (1) the number of mockingbird eggs lost by punctures, as a proxy of the cost of parasitism, (2) the number of host eggs laid before the first event of egg-puncture, as a proxy for the energy invested in the current nesting attempt, (3) the number of unpunctured host eggs, as a proxy of the benefits of continuing the current nesting attempt. We included as co-variable the number of cowbird eggs in the nest. The probability of nest desertion was positively associated with the number of eggs lost by punctures and negatively associated with the number of eggs laid before the first event of egg-puncture, and with the number of unpunctured eggs. The number of cowbird eggs was negatively associated with the probability of nest desertion. In addition, we evaluated experimentally the effect of the number of punctured eggs on nest desertion. The experimental 4 host eggs treatments were: none punctured eggs, 2 punctured eggs, and 3 punctured eggs. All nests with none punctured eggs were not deserted, all nests with 3 punctured eggs were deserted, and half of the nests with 2 punctured eggs were deserted. Our results indicate that clutch reduction due to egg-puncturing behaviour by shiny cowbirds increases nest desertion in chalk-browed mockingbirds.