We found our fair share of nests in New Guinea during fieldwork, and noticed that a surprising number of species lay clutches of only a single egg. While tropical forest birds commonly lay two eggs (temperate zone species typically lay much larger clutches), single-egg clutches are rare even in the tropics. Back in the library, I conducted a systematic review of the breeding biology of New Guinean passerines to build a database of clutch sizes. This database confirmed my suspicion that single-egg clutches are prevalent in New Guinean passerines. I then teamed up with fellow graduate student Nick Mason to demonstrate that small clutch sizes are associated with New Guinea and not an intrinsic feature of the lineages found on the island; species that live in Southeast Asian rainforests lay larger clutches than their relatives inhabiting New Guinea. The mechanisms that drive this pattern remain unknown, but could include rates of nest predation or parasitism in New Guinea that are abnormally high.
The paper can be found at http://www.publish.csiro.au/?paper=MU14023. Or email me for a copy.