Urbanization and phenology
Phenology is an aspect of a species’ ecological niche which is dynamically associated with both habitat and climate, and vocal seasonality is typical of most birds globally, even in equatorial regions where the climate is nearly aseasonal (Thomson 1950, Hau et al. 1998, Beebe et al. 2005, Berman et al. 2023). Among the panel of nine focal species in this study, some sang seasonally as expected, while others did not. Historically native species adapted to forest interiors sang seasonally, while species which have only established breeding populations within the last few decades, and which are adapted to more open habitats, sang aseasonally. Singapore has undergone dramatic urbanization over the last 200 years. That urbanization has led to changes in land cover, including the introduction of open parkland habitat resembling woodlands, which in turn has led to a shift in species composition and the colonization of woodland-adapted birds. Our results suggest that this conversion of forest into parkland and urban areas has not only altered the species composition of Singapore, but in doing so may have also led to an altered community phenology. While it is clear from the data that historically native forest species are more seasonal in their song than the more recently established species, it is less obvious why these two groups of species should have distinct phenologies. Some potential explanations include dietary differences, or an initial transitional period of aseasonality for newly established populations that may last for decades. In the following paragraphs we discuss the potential factors driving these phenological differences.