Incubator birds: biogeographical origins and evolution of underground nesting in megapodes (Galliformes: Megapodiidae)
Unique amongst birds, megapodes (family Megapodiidae) have exchanged the strategy of incubating eggs with the warmth of their bodies for incubation behaviours that rely entirely on environmental heat sources. Typically, mound-builders capture heat released from the decomposition of organic materials, while burrow-nesters lay their eggs in geothermal or solar-heated soils. The evolutionary path towards novel incubation behaviours has led to ecological and physiological adaptations unique to megapodes. Here, we present a species tree for all extant megapodes that settles long-standing debates about megapode evolution: namely, their biogeographical origins and ancestral nesting behaviour.
A time-calibrated multilocus species tree for all extant megapodes was constructed using *beast. We estimated and compared divergence dates for megapodes obtained from molecular rates, fossils, and a combination of fossils and rates. Using this tree, Bayesian estimation of ancestral nesting behaviour was conducted in BayesTraits and ancestral ranges were estimated in BioGeoBEARS.
Recent dispersal has led to the recolonization of mainland Australia and New Guinea by Megapodius. Bayesian estimation of ancestral states indicates that mound building is the most probable ancestral nesting behaviour in megapodes (posterior probability = 0.75). Burrow nesting was acquired early in the diversification of the family (at least 14 Ma), followed by a single switch back to mound building.
Divergence dates and biogeographical reconstructions strongly suggest that dispersal, and not vicariance, led to the isolation of megapodes in Australasia. We propose that flight-mediated dispersal to environmentally variable islands is responsible for the behavioural lability in nesting behaviours observed in some Megapodius species today.