This study was carried out to find what factors could affect bird diversity in green areas in an urban landscape. We selected 83 sites of different size and type of urban landscapes in Seoul, South Korea and surveyed bird diversity. Urban green patches were grouped into three subclasses: < 1 ha, 1–10 ha and > 10 ha. The cumulative bird diversity was greater in the subclass 1–10 ha than in < 1 ha or in > 10 ha. We suggest that bird diversity was closely related to habitat size, especially in the category 1–10 ha, and recommend this area be used to establish new bird habitats in urban landscapes. The number of bird species was significantly correlated with the number of insect species in studied patches, but was not correlated with the size of green areas or the distance to roads. Therefore, we suggest that the number of insect species is the most important factor affecting bird diversity within our urban study area.