Focal species: historically native versus parkland colonizers
The number of focal species was limited to nine due to logistical
constraints; training a species classifier takes a degree of time and
effort. These nine species were selected because theirs were the
vocalizations which appeared most frequently in the audio dataset. The
focal bird species from Singapore were divided post-hoc into two
main categories: recent parkland colonizers and historically native
forest species. Parkland colonizers are not what would typically be
considered invasive species; these parkland species are native to the
broader Sundaic region, but did not historically occur within Singapore
because they are not adapted to closed canopy forest. Much of the
Sundaic bioregion is climatically everwet and has historically been
dominated by rainforest. However, deciduous woodland or coastal scrub,
with their distinct avifauna, exist in scattered pockets throughout the
area, or have been the predominant habitat type in some peripheral
regions of the Sundaic region, such as eastern Java and northern
peninsular Malaysia. Parks and gardens in urbanized Singapore more
closely resemble woodland than rainforest and have been colonized by
such woodland species over the past few decades. While all the recording
stations in this study were inside forest, not parkland, focal species
recorded included both historically native species and more recent
colonizers. The focal species in this study include four recent
colonizers, all of which have had breeding populations in Singapore for
less than 100 years, and five forest species historically native to
Singapore. The recent colonizers in Singapore are Black-naped Oriole
(Oriolus chinensis, established ~1925), Lineated
Barbet (Psilopogon lineatus, established ~1997),
Straw-headed Bulbul (Pycnonotus zeylanicus, established on the
main island ~1980), and Asian Koel (Eudynamys
scolopaceus, established ~1950) (Gibson-Hill 1950,
Wells 2010, Lim 2019). The historically native forest species in
Singapore are Little Spiderhunter (Arachnothera longirostra ),
Drongo Cuckoo (Surniculus lugubris ), Rufous-tailed Tailorbird
(Orthotomus sericeus ), Pin-striped Tit-babbler (Mixornis
gularis ), and Short-tailed Babbler (Pellorneum malaccense ).