Early Infestation by Bot Flies (philornis Seguyi) Decreases Chick Survival and Nesting Success in Chalk-browed Mockingbirds (mimus Saturninus)

—Bot fl ies (Diptera: Muscidae: Philornis spp.) are a group of fl ies comprising mostly species with a Neotropical distribution. Their larvae parasitize several species of birds, living subcutaneously on altricial chicks. We investigated the eff ect of parasitism by bot fl ies (P. seguyi) on the reproductive success of Chalk-browed Mockingbirds (Mimus saturninus) in temperate grasslands near the southern limit of bot fl y distributions. We analyzed seasonal variation of bot fl y prevalence during three consecutive years and how the timing and intensity of bot fl y infestation aff ected growth and survival of Chalk-browed Mockingbird nestlings. Bot fl y prevalence was 58.3%, 30.7%, and 45.5% each year, and in all years, it increased with time of breeding. Most of the infested nests fl edged no chicks. In these nests, chicks had a lower tarsus growth rate than in noninfested nests and died 3–4 days a er parasitism. The average time from hatching of the fi rst chick until infestation was 4.4 days. The age of the chicks at the time of infestation was associated positively with nesting success and negatively with intensity of parasitism. Bot fl y parasitism also reduced the survival of Shiny Cowbird (Molothrus bonariensis) chicks present in Chalk-browed Mockingbird nests, but the presence of Shiny Cowbird chicks did not aff ect timing of infestation or fl edging success of Chalk-browed Mockingbird chicks. Our results show that an intermediate prevalence of bot fl y parasitism produces an important decrease in the reproductive success of Chalk-browed Mockingbirds and suggest that bot fl ies may play an important role as selective agents in the evolution of host life-history strategies. Received 14 November 2005, accepted 25 July 2006.