Leaf galls are caused by very tiny gall-making insects (Family Cecidomyiidae) when the adults lay their eggs in the leaf tissues. Certain chemicals introduced during the process stimulate the tissues to develop abnormally, thus forming the galls seen on the leaf (above left). When the eggs of these gall-making insects hatch, the larvae feed on these nutritive gall tissues. The leaf gall provides an almost perfect place for these larvae to escape predation.
However, there are a few predators that feed on these hidden insect larvae. And bird photographer Chan Yoke Meng documented a Blue-crowned Hanging-parrot (Loriculus galgulus) (above right) feeding on these leaf galls. The parrot has somehow associate these galls with the larvae they host and is benefiting from the insect protein.
Apparently woodpeckers can also locate these hidden insect larvae and regularly prey upon galls. But we have yet to receive any feedback from photographers or even birdwatchers on woodpeckers feeding on leaf galls.
Note: The Blue-crowned Hanging-parrot is seen feeding on the nectar of the golden penda (Xanthostemon chrysanthus) in the image above LINK.
Chan Yoke Meng