Reproductive biology of the Whitehead's Broadbill (Calyptomena whiteheadi)

Whitehead's Broadbill, Calyptomena whiteheadi, an endemic species on the island of Borneo, has a declining population, raising the question of whether reproduction might be a factor limiting population growth. We studied breeding biology based on 52 nests in Kinabalu Park, Sabah, Malaysian Borneo in 2009-2020. Length of the egg-laying season was 99 days with a median of 22 March. Clutch size was typical of tropical songbirds at 2.0 +/- 0.13 eggs (SE). Nest success was relatively high at 52%, and daily nest predation rate was relatively low at 0.013 +/- 0.004 (SE). Concordantly, length of incubation and nestling periods were relatively long at 19.0 +/- 0.26 d and 26.8 +/- 0.3 d (SE), respectively. Similarly, the growth rate constants (K) for mass, wing length and tarsus length were slow at 0.252, 0.161 and 0.188, respectively. Slow growth might result in part from their frugivorous diet, as we show using seed traps, but they are reported to include insects in their diets and food provided to their young. Feeding effort, as reflected by the number of fruit seeds dropped below the nest, increased through the nestling period. Brooding effort of parents for nestlings was high on hatch day and decreased until Day 10-15 when it stopped. Food and parental care efforts do not seem to cause their slow growth. Ultimately, our study population has reproductive success that suggests it is not limiting population growth, but studies are needed in other locations to determine if it is a problem elsewhere.