Anthropogenic nesting substrates increase parental fitness in a Neotropical songbird, the Pale-breasted Thrush (Turdus leucomelas)

The failure of breeding attempts is a major hindrance to bird reproduction, making nest site choice under strong selective pressure. Urbanization may offer lower risk of nest predation to certain bird species, but the impact of using anthropogenic structures as nesting sites on parental fitness is seldom studied. We studied the effect of anthropogenic substrates and brood parasitism by the Shiny Cowbird (Molothrus bonariensis) on the nest success of a Neotropical songbird, the Pale-breasted Thrush (Turdus leucomelas). We monitored 263 nesting attempts between 2017 and 2020 to estimate daily survival rate (DSR), which represents the probability of a given nest survive until the next day. DSR was modelled as a response variable in function of substrate type (plants as “natural” or human buildings as “artificial”) and brood parasitism as fixed factors, using as covariates year, a linear and a quadratic seasonal trends. Additionally, we tested the effect of these same explanatory variables on the number of fledglings per nest using a generalized linear mixed-effects model. Most nests (78.7%) were placed in artificial substrates and apparent nest success (i.e. the percentage of nesting attempts that produced at least one thrush fledgling) was higher in artificial (50.2%) than in natural substrates (37.5%). DSR was higher for nests in artificial than in natural substrates regardless of cowbird parasitism, whereas the number of fledglings per nest was higher both in artificial substrates and for nests without cowbird parasitism. We highlight that nesting in buildings significantly increases parental fitness in Pale-breasted Thrushes, which may favor their settlement in cities and potentially drive the evolution of this breeding behavior in urban birds.