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.

A large Ficus drupacea located in the commercial centre of Kota Kinabalu at the base of Signal Hill. In the photo above the Ficus drupacea is directly in front of the HSBC bank HQ building. This F. drupacea fig tree fruits twice  a year for a two week period attracting all the local wildlife.
Including this Plantain Squirrel Calliosciurus notatus.
Flocks of Glossy Starlings Aplonis panayensis
and Pink-necked Green Pigeons Treron vernans
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Male Pink-necked Green Pigeon Treron vernans.  The beak  width (gape) of T. vernans is not large enough  to swallow Ficus drupacea figs whole so they have to wait until the figs become ripe and soft when they can peck  out smaller chunks to swallow. This means that Green Imperial Pigeons get first choice of the ripe crop. When figs are scarce Imperial Pigeons may be the only consumers.
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The beaks of Glossy Starlings Aplonis panayensis are also too small to swallow whole F. drupacea figs.
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Yellow-vented Bulbuls  Pycnonotus goiaiver with relatively small beaks also have to wait until the Green Imperial Pigeons have had their fill.
Because their beaks are large enough to swallow F. drupacea figs whole Ducula Green Imperial Pigeons  get first choice of the crop.

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Of all the bird visitors to this fruiting F. drupacea only the Green Imperial pigeons had a gape large enough to swallow Ficus drupacea  figs whole. So the Imperial Green Pigeons arrived first and ate the figs immediately they ripened whilst the smaller birds  had to take second choice of  the over ripe figs when they had softened up a couple of days later.

Kota Kinabalu