Behavior and survival of parasitic Shiny Cowbird ( Molothrus bonariensis ) fledglings

Abstract

For songbirds, the post-fledging period is critical for development and survival, as young must learn to get food, practice flying, and identify predators to become independent from their parents and disperse or migrate. Obligate avian brood parasites, like cuckoos and cowbirds, lay eggs in nests of other bird species that provide parental care until the parasite’s young become independent. The information on the post-fledging period in songbirds is limited due to the difficulty in following and observing fledglings and it is even scarcer for brood parasites. We studied the behavior, survival, and age of independence of Shiny Cowbird (Molothrus bonariensis) fledglings raised by 2 hosts that differ markedly in body size: the House Wren (Troglodytes aedon) and the Chalk-Browed Mockingbird (Mimus saturninus). We radio-tracked individuals from a few days before they left the nest until they disappeared from the natal territory. We found: (1) chicks left the nest when they were 11–15 days of age and the age of fledgling did not differ between the 2 host species, (2) after leaving the nest, they spent most of the time perched in a hidden place without begging but as they grew, the proportion of time perched without begging decreased and begging in hidden and exposed places increased, (3) we observed approaches between adult parasitic females and young in host territories, (4) young abandoned the host territory and joined conspecific flocks when they were 35–39 days of age, and (5) the estimated post-fledging survival was between 12.5% and 20.8%.