Acoustic phenology of tropical resident birds differs between native forest species and parkland colonizer species

Most birds are characterized by a seasonal phenology closely adapted to
local climatic conditions, even in tropical habitats where climatic
seasonality is slight. In order to better understand the phenologies of
resident tropical birds, and how phenology may differ among species at the
same site, we used ~70,000 hours of audio recordings collected
continuously for two years at four recording stations in Singapore and
nine custom-made machine learning classifiers to determine the vocal
phenology of a panel of nine resident bird species. We detected distinct
seasonality in vocal activity in some species but not others. Native
forest species sang seasonally. In contrast, species which have had
breeding populations in Singapore only for the last few decades exhibited
seemingly aseasonal or unpredictable song activity throughout the year.
Urbanization and habitat modification over the last 100 years have altered
the composition of species in Singapore, which appears to have influenced
phenological dynamics in the avian community. It is unclear what is
driving the differences in phenology between these two groups of species,
but it may be due to either differences in seasonal availability of
preferred foods, or newly established populations may require decades to
adjust to local environmental conditions. Our results highlight the ways
that anthropogenic habitat modification may disrupt phenological cycles in
tropical regions in addition to altering the species community.