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 ).