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AbstractUrbanization is expanding worldwide and can have major consequences for organisms. For example, urban areas can contain novel stressors that reduce the fitness of animals but may also have benefits, such as consistent human food availability. These trade-offs are particularly important in dynamic environments with unreliable annual natural food availability, such as in the Galápagos Islands. In our study, we compared the reproductive success of small ground finches (Geospiza fuliginosa; a species of Darwin’s finch) in an urban and non-urban area during a dry year (i.e. one with low natural resource availability) on San Cristóbal Island in the Galápagos Islands. We also documented the abundance of anthropogenic-related material, such as human hair and plastic, in finch nests and determined whether incorporating these materials into the nest led to entanglement- or ingestion-related mortalities. Overall, urban finches built more nests, laid more eggs, and produced more nestlings than non-urban finches. However, every nest in the urban area contained anthropogenic-related material, which resulted in four mortalities (out of 17 nests with nestlings) due to entanglement or ingestion. Non-urban nests did not contain any anthropogenic-related material. Our study showed that urban living has trade-offs: urban birds have overall higher reproductive success than non-urban birds, but urban birds can also suffer a negative consequence by using anthropogenic-related material in their nests. These results suggest that despite the potential cost of urban living, finches benefit overall from urban living and urbanization may buffer the effects of limited resource availability in the Galápagos Islands.

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Host-associated microbiota can be affected by factors related to environmental change, such as urbanization and invasive species. For example, urban areas often affect food availability for animals, which can change their gut microbiota. Invasive parasites can also influence microbiota through either competition or indirectly through a change in the host immune response. These interacting factors can have complex effects on host fitness, but few studies have disentangled the relationship between urbanization and parasitism on an organism’s gut microbial composition. To address this gap in knowledge, we investigated the effects of urbanization and parasitism by the invasive avian vampire fly (Philornis downsi) on the gut microbiota of nestling small ground finches (Geospiza fuliginosa) on San Cristóbal Island, Galápagos. We conducted a factorial study in which we experimentally manipulated parasite presence in an urban and non-urban area. Feces were then collected when nestlings to characterize the gut microbiota (i.e., alpha and beta diversity, community composition). Although we did not find an interactive effect of urbanization and parasitism on the microbiota, we did find main effects of each variable. Urban and parasitized nestlings had lower bacterial diversity and differences in relative abundance of bacterial phyla and genera compared to non-urban non-parasitized nestlings, respectively. Overall, this study advances our understanding of the complex effects of anthropogenic stressors on the gut microbiota of birds.