Seasonal availability of preferred foods may differ between species
One potential explanation for why colonizer species are less seasonal than native forest species may be differences in food availability. Translocation of birds to sites with high year-round food availability can lead to a prolonged or year-round breeding season (Komdeur 1996). While all audio recorders, and therefore all detected birds, were in forested sites, colonizer species are generally more abundant nearer the forest edge, in more open habitat patches and in parkland. Man-made parks have a different floral species composition as compared to natural forests, and it is possible that food availability does not correlate as closely with seasonal cycles in manicured parklands. Many genera of bird-dispersed fruit-bearing plants which are common in Singaporean forests, including Litsea, Myristica, Santiria , andTimonius , follow a masting pattern of reproduction (Corlett 1990). Mast fruiting most often occurs in June – August (Corlett 1990, Chong et al. 2016, Corlett 2019), coinciding with the fledging period of the native breeding season (Berman et al. 2023). In contrast, fruiting figs, another staple of the frugivorous diet, fruit continuously on the population level and provide a stable year-round food supply (Cannon et al. 2007; Corlett 2019). Figs are common in forest, but are especially abundant in parklands. It is possible that a lack of masting species in parklands may reduce the benefits of seasonal breeding among parkland-adapted birds. Two of the four parkland colonizers among our target species are primarily frugivorous (Straw-headed Bulbul, Lineated Barbet), and the other two are known to take fruit at least seasonally or occasionally (Asian Koel, Black-naped Oriole), supporting the potential importance of fruit availability for the phenology of these species.