Phenological Plasticity without Directional Change: Tropical Hornbills Track Temperature but Maintain Reproductive Stability

Climate change is altering breeding phenology in birds worldwide, yet tropical systems remain understudied despite harbouring the greatest avian diversity. We drew upon 26 years (1998-2024) of breeding phenology and reproductive success data on three sympatric Asian hornbill species (Great Hornbill Buceros bicornis, Wreathed Hornbill Rhyticeros undulatus, Oriental Pied Hornbill Anthracoceros albirostris) from a tropical forest in northeast India to examine whether trends in long-term breeding parameters reflect directional climate change or interannual variability. Over the study period, we found no significant directional trends in nest entry timing, occupancy, or breeding success over 26 years, despite modest regional warming. However, breeding parameters showed sensitivity to interannual temperature variation, with pre-breeding temperature (January-February) explaining 36-70% of variance in nest timing. Temperature sensitivity differed among species, with Wreathed Hornbill showing the strongest response (-15.9 days/{degrees}C), followed by Great Hornbill (-9.8 days/{degrees}C), and Oriental Pied Hornbill showing the weakest (-4.8 days/{degrees}C), consistent with predictions that larger, frugivorous species with extended breeding cycles would exhibit greater climatic sensitivity. The underlying mechanism appears related to a body size-phenology-thermal exposure interaction: Oriental Pied Hornbills nest later (mid-April) with shorter breeding cycles ([~]90 days), completing reproduction before peak monsoon heat (June), whereas both larger species nest earlier (mid-February) but endure thermal stress throughout extended breeding cycles ([~]130+ days) into August. Notably, annual occupancy and breeding success remained stable across years despite strong phenological responses to temperature, suggesting that phenological plasticity may help maintain reproductive performance under current climatic variation. These findings indicate that tropical cavity-nesting frugivores exhibit phenological plasticity coupled with demographic stability, though the mechanisms underlying this apparent buffering remain incompletely understood, and body size-dependent differences in thermal exposure suggest heterogeneous vulnerabilities to future warming scenarios.