Changing Rainfall Drives Locally Asynchronous Reproduction of Tropical Birds via Modular Trophic Pathways

Phenological shifts are a pervasive response to climate change but remain poorly understood in the hyperdiverse tropics. Combining comprehensive multitrophic datasets and in situ meteorological data, we test classic hypotheses linking reproduction to the timing and magnitude of rainfall across trophic levels in tropical birds. In low-latitude mountains, breeding was primarily seasonal and varied based on diet. Consistent with the regional timing of wet and dry seasons, bird species that consume primarily nectar or fruit timed breeding to dry season flowering or wet season fruiting with limited variation across elevation and rainfall gradients. In contrast, species that consume arthropods shifted breeding locally, five months in less than a hundred kilometers, as the intensity of the dry season increased. Spatially asynchronous reproduction was repeated in more than 30 insectivore species as the main nesting season switched from before to after the dry season at a threshold in dry-season insects. Reversed seasonality magnified the short-term effects of drought as insectivore communities that nested after the dry season reduced reproductive effort-skipping breeding during resource-limited dry years-whereas communities that nested before the dry season adapted by breeding up to one month earlier. Strong spatial to temporal variation at a ratio of 5:1 suggests limited short-term behavioral flexibility within restricted breeding seasons timed based on the long-term magnitude of seasonal rainfall. At higher trophic levels, similar within-group but different between-group responses to rainfall magnitude demonstrate quasi-independent trophic pathways for how tropical food webs link to rainfall. Cumulatively, these results support an ecological tipping point tied to dry season intensity in which rainfall-mediated ecological constraints compartmentalized functional groups into vertical trophic modules, which responded differently to changing rainfall. Compared with the seasonal stability of nectar-fruit consumers, the rapid response of insectivores provides an early warning for changing rainfall.