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.