Aseasonality may be adaptive in newly established populations
Recent parkland colonizers may be less seasonal because they have not yet adapted their phenology to the local cycle. Breeding seasonally is adaptively advantageous (Ims 1990), but introduced species in tropical habitats sometimes do breed aseasonally (Rodda and Savidge 2007, Beard et al. 2009, Brodie et al. 2021). Introduced populations which initially bred year-round have been known to become seasonal breeders after several generations in a new habitat (Hengeveld 1994).
Year-round breeding may be adaptive for newly established species expanding into an unoccupied niche (Hengeveld 1994), but for populations at carrying capacity, the inability to synchronize breeding with the time of peak food availability may lead to lower survivorship of young and excessive metabolic stress on parents (Thomas 2001). A lack of synchronized breeding may also reduce the chance of finding a suitable partner that is at the same reproductive and hormonal stage in the seasonal cycle. For newly established populations, it may take time to attune the biological clock to a new set of environmental signals, especially if those signals are weak (Baker and Ranson 1938). If this hypothesis is true, we might expect the most recent colonizers to be the least seasonal, and for populations to become more seasonal over time. The data from our limited panel of species only partially supports this pattern (Figure 6). Among the four parkland colonizers in our panel, Black-naped Orioles have been present in Singapore the longest, first establishing a breeding population in 1925 due to the conversion of forests into plantations (Wells 2010). At the same time, Black-naped Orioles exhibit the strongest seasonality among our four parkland colonizers (Figure 4, Figure 6), suggesting that their extended residency may have provided them with sufficient time to converge on a new seasonal rhythm. However, there is no clear linear pattern between seasonality and establishment dates among the other three species (Figure 6), suggesting that a larger species panel may be needed to confirm this relationship.