A comparison of the breeding biology of the Bornean Whistling-Thrush (Myophonus borneensis) and White-crowned Forktail (Enicurus leschenaulti borneensis)

Advances in understanding geographic patterns of life history variation depend on documentation of life history traits for species in poorly studied regions. In the tropics, most species have not had their life history traits described. We compared the reproductive biology of two closely related Muscicapids, the Bornean Whistling-Thrush (Myophonus borneensis) and the Bornean subspecies of the White-crowned Forktail (Enicurus leschenaulti borneensis), coexisting in Mount Kinabalu Park in Sabah, Malaysia. Both species specialize in riparian habitats, but whistling-thrushes are more adaptable to human presence and can be found nesting on and foraging around buildings, whereas forktails are more wary of humans. We located and monitored 77 forktail nests and 130 whistling-thrush nests from 2009 to 2020. Mean clutch size was 2.00 +/- 0.04 for forktails and 1.89 +/- 0.04 for whistling-thrushes. Mean egg mass at lay was 4.07 +/- 0.04 g for forktails and 11.65 +/- 0.12 g for whistling-thrushes. Forktail incubation period (17.0 +/- 0.42 d) was slightly shorter than for whistlingthrush (18.6 +/- 0.19 d), but both species had similar rates of incubation attentiveness. Nestling periods were much shorter for the forktail (15.89 +/- 0.39 d) than the whistling-thrush (24.00 +/- 0.20 d). The shorter nestling period was associated with faster growth and higher provisioning rates for forktails than whistling-thrushes. Whistling-thrushes are larger and better able to protect their nests from predation, yielding much lower daily nest predation rates (0.012 +/- 0.002) than in forktails (0.042 +/- 0.006). Indeed, whistling-thrushes were themselves predators of nestlings and eggs of other passerines, whereas forktails were not. The larger size and lower predation risk may explain the slower growth and development of whistling-thrushes. Both species are vulnerable to climate change and droughts because of their reliance on riparian habitats.

Automated summary

Advances in understanding geographic patterns of life history variation depend on documentation of life history traits for species in poorly studied regions. This study compared the reproductive biology of two closely related Muscicapids in Borneo and found differences in reproductive strategies and nest defense between the species. The larger size and better nest protection ability of the Bornean Whistling-Thrush may explain its slower growth and development compared to the Bornean subspecies of the White-crowned Forktail.