Although nested species subset patterns and mechanisms promoting them have constituted the focus of a considerable research effort, little attention has been paid to the role of seasonality in generating or moderating these patterns. I conducted monthly censuses of 130 urban parks in the city of Valencia throughout an annual cycle to assess whether seasonality influenced nested patterns of bird species assemblages. Specifically I tested the hypothesis that an increase in the movement of birds among parks outside of the breeding season, may reduce nestedness of resident birds. Results obtained in this study showed that assemblages of resident bird species were nested during the breeding season but not outside it, thus giving some support to the hypothesis. However, a reduction in nestedness outside the breeding season was not explained by an increase in the occupancy of parks but with an increase in the number of absences from parks that had been inhabited during the breeding season, most likely due to the use of habitats outside parks by some species (finches mainly). Results suggest the importance of both selective extinction and colonization processes in contributing to the degree of nestedness during the breeding season. However, environmental stress in the form of human disturbance apparently had little effect on the distributions of individual species. Habitat nestedness did not appear to be a factor as parks of all sizes were similar in number and type of habitats. An assessment of seasonality in other landscapes is needed to further understand its general effects on nestedness.