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.