I photographed this Thick-billed Flowerpecker Dicaeum agile at Tissa, a few weeks ago. In it, a close examination of the base of the beak—lower mandible to be precise—shows a few seeds sticking to it. Here's a close crop.
Flowerpeckers mainly feed on fruit, most notably those of mistletoes—hemi-parasites that grow on top of other trees, known in Sinhala as pilila. The formal name for flowerpecker in Sinhala is Pilalichcha, which reveals the close ties between the two. Another example that stridently brings this two to the fore is the famous Mistletoebird Dicaeum hirundinaceum of Australia.
Mistletoes depend on birds for dispersing their seeds. For that, these plants have come up with a smart plan. That is to pack their juicy fruits with sticky coated seeds; therefore, when the birds feed on the fruits, the gluey seeds get attached to their beaks—often leaving them rather messy. And when the birds wipe their beaks clean on branches, or pass out the droppings, the seeds achieve dispersal—a payback from the birds to the plants. The stickiness of the seeds is caused by a material called viscin, and it hardens and attaches the seed firmly to its future host.
Long time ago, I saw a documentary by Sir David Attenborough that featured the Mistletoebird, and saw that their droppings, unlike most birds, do not drop like they usually do due to the stickiness caused by viscin. So the birds have to wipe their bottoms on the branches to get rid of the droppings effectively. This in turn makes sure that the droppings packed with mistletoe seeds won't fall to the forest's floor and go wasted, but get firmly established on a branch instead, where they have a better chance of commencing their life, tapping into the nutrients and water of the host plant.