I was watching an Asian Glossy Starling (Aplonis panayensis) perched on the ripened fruiting bunch of my Alexandra palm (Archontophoenix alexandrae) one morning when I noticed it regurgitating a seed. This it did a few times before picking out one to swallow (left). The nearly rounded 10×12 mm fruit has a covering of pulp that is less than1 mm thick. This coral-red covering encloses a single seed.
Intrigued by this behaviour, I asked around but even seasoned birdwatchers could not give me an explanation. Then I stumbled upon a paper by Richard Corlett and his student on the Asian Koel (Eudynamys scolopacea).
Because frugivores generally have a reduced protein requirement, they can easily subsist on a fruit diet. Such birds also exhibit a high ingestion rate and a short gut retention time, as well as reduced loss of nitrogen through the faeces and urine. Some of these birds supplement their protein requirement with insects or the seeds in the fruits they ingest.
The study shows that Asian Koels swallow large fruits like those of Chinese fan palm (Livistona chinensis), Syzygium chumini and Arenga engleri whole. But they rapidly regurgitate the cleaned seeds, dropping them under the tree, rather than defecating them. This appears to be a common adaptation of specialist frugivores, presumably serving to reduce the weight and volume of material that must pass through the gut. On the other hand other birds peck the fruits and leave the seeds. When eating fruits like figs with small seeds, koels swallow them whole and defecate the seeds below the tree.
The intake of indigestible seeds results in extra load in the gut. The energy expenditure necessary for flight is thus increased. Additional energy may also be required to manipulate the seeds in the gut, separating them from the pulp and transporting them through the gut. Again, the presence of the indigestible seeds in the gut limits further food ingestion. This in turn reduces the rate at which food can be processed and nutrients assimilated.