Tracking interannual temperature variation: phenological plasticity and reproductive stability in tropical hornbills

Abstract

Climate change in the tropics is expected to manifest differently than in temperate regions: rather than directional warming, its ecological signature may be dominated by increased year-to-year variability against a backdrop of comparatively modest average change. This distinction matters for organisms that evolved in historically stable climates and may respond strongly to year-to-year fluctuations even in the absence of long-term trends. Whether tropical birds track this kind of interannual variability, and whether doing so has an influence on nesting success, remains poorly understood, particularly for cavity-nesting frugivores whose breeding is tightly bound to seasonal resource availability. We used 27 years of nest monitoring data on three sympatric hornbill species at Pakke Tiger Reserve, northeast India, to examine long-term changes in nest-entry timing, nest occupancy, nesting success, and breeding duration, and to test whether interannual variation in local climate predicted breeding parameters. Local temperature and rainfall varied substantially among years without clear long-term trends. Nest-entry timing remained stable in Great and Wreathed Hornbills but became later in Oriental Pied Hornbills; Wreathed Hornbill nest occupancy also declined, whereas nesting success showed no directional change. Across the three species, warmer pre-breeding conditions were associated with earlier nest entry, while rainfall added no independent explanatory power once temperature was accounted for. The three species did not differ detectably in their sensitivity to temperature, despite differences in body size and breeding-cycle length. Our results illustrate how tropical cavity-nesting frugivores may track interannual climate variation without immediate breeding costs, although the strength of support for the temperature association differs among species and climate change may manifest primarily as altered variability rather than strong directional trends.