A potential consequence for urban birds' fitness: Exposed anthropogenic nest materials reduce nest survival in the clay-colored thrush.

Urban bird dwellers survive and reproduce in highly urbanized ecosystems. Some individuals adjust to these novel conditions by changing natural nesting materials for artificial materials, thus making nests more conspicuous in the environment. The consequences of using artificial materials for nesting remain poorly understood, especially from a nest-predator perspective. We studied if exposed artificial materials on bird nests affect the daily survival rate in a common dweller species, the clay-colored thrush (Turdus grayi). We used previously collected nests with a different area of exposed artificial materials and placed them with clay eggs on the main campus of the Universidad de Costa Rica. During 12 days of the reproductive season, we monitored the nests using trap cameras located in front of each nest. We found that nest survival decreased as the proportion of exposed artificial materials increased in the nest and, unexpectedly, that the principal predators were conspecifics. Thus, artificial materials used in the outer layer of nests make them more susceptible to predation. The use of artificial materials likely reduce reproductive success and population size in urban clay-colored thrush, but further field experiments measuring the effect of waste in birds' nests on the reproductive success of urban birds are needed.