Study area
Data were collected in Singapore, a small Southeast Asian island nation (1.3˚N, 103.7˚E; 734 km2) located within the Sundaic biogeographic region close to the equator with a tropical rainforest climate. It is hot and rainy year-round. Climatic seasonality is weak, but on average it is slightly sunnier in February-March and rainier in November-December (Figure 1, Berman et al. 2023). Historically this island was dominated by mixed dipterocarp everwet forest. Singapore, however, has undergone dramatic urbanization in the last 200 years: only 0.28% of its original primary forest cover remains (Yee et al. 2011) and about 30% of the native bird species have gone nationally extinct (Chisholm et al. 2016, Chisholm et al. 2023). Currently, 20% of the country’s land cover is secondary forest (Yee et al. 2011), which ranges from native-dominated old secondary forest within nature reserves to open woodlands dominated by primarily exotic tree species.
While the urbanization of Singapore has led to the local extinction of many forest interior species, it has also led to the colonization of regionally local species adapted to more open habitat. The four recording stations in Singapore were in Sungei Buloh Wetland Reserve (1.441586 N, 103.735308 E), Dairy Farm Nature Park (1.358419 N, 103.777492 E), Central Catchment Nature Reserve (1.355488 N, 103.804549 E), and National University of Singapore Campus (1.295020 N, 103.779385 E). These recording stations were 3-10 km apart from one another. All recorders were placed in mixed dipterocarp tropical rainforest habitat. The recording stations in Central Catchment Nature Reserve, Sungei Buloh Wetland Reserve and Dairy Farm Nature Park were all in mature secondary mixed dipterocarp tropical rainforest, while the recording station on the National University of Singapore campus was located in early-stage successional forest. These four recording sites were close enough to one another, and located in sufficiently similar habitat, that we would not expect phenology to differ among them. Data from these four recording stations were amalgamated into a single dataset to maximize sample size for each species.