ABOVE: A young Green Imperial Pigeon Ducula aenea preparing to feed on ripe Ficus drupacea figs on Signal Hill, Kota Kinabalu. Of all the local birds only 3 species are able to swallow the the large ripe fig fruits whole, Green Imperial pigeons, Pied Hornbills Anthracoceros albirostris and Koels Eudynamys scolopacea
These 3 large birds therefore get first choice of the crop.
Other birds with smaller beaks, Pink-necked Green Pigeons, Glossy Starlings and bulbuls have to wait until the figs become soft and over ripe when they can peck chunks out of the figs.
This is an excellent example of how the size of the fig fruit determines the primary disperser. Green Imperial pigeons, Koels and hornbills are relative rare compared to the much smaller bulbuls and starlings but they fly much larger distances and therefore disperse the fig seeds further .
However the main evolutionary reason that Ficus drupacea figs are too large for Pink-necked Green Pigeons Treron vernans to swallow is that Treron Green Pigeons are fig seed predators not seed dispersers. All Treron green pigeons have grit filled gizzards which are used to grind up small fig seeds. In contrast Ducula Imperial Pigeons defecate fig seeds intact.