When bird-photographers and even birders make observations in the field on the fruits birds take, they could keep an eye on how the fruits are eaten – swallowed whole, pecked piece by piece, squashed the contents out, etc.
And another observation that needs be to noted is how soon and how far away are the seeds excreted or even regurgitated.
Prof Richard T Corlett, formerly of the University of Hong Kong, now back with the National University of Singapore, is keenly interested in such information. One of his fields of studies is seed dispersal and he is hoping that local birders will make such observations.
As Richard noted in his comment on bulbul eating fruits of Tabernaemontana corymbosa: “One of my postgraduate students in Hong Kong, Jacqui Weir, measured how long seeds take to pass through the guts of Chinese (= light-vented) (Pycnonotus sinensis) and Red-whiskered Bulbuls (P. jocosus) and, using radio-telemetry, how far the bird moved, on average, during the gut passage time. It turned out that the bulbuls were moving most seeds less than 100 metres, but the occasional one more than a kilometre.”
These two bulbul species are common in the degraded habitats of Hong Kong. They are fruit eaters and excellent seed dispersers, helping in plant succession of barren areas. The limitation of these seed dispersers is their maximum gape of about 14 mm. This prevents them from swallowing bigger fruits, although they are able to handle larger, multi-seeded soft fruits.
However, these bulbuls are capable of dispersing most of the wild fruit plants.
Weir, J. E. S. & Collett, R.T. (2007). How far do birds disperse seeds in the degraded tropical landscape of Hong Kong, China? Landscape Ecology 22:131-140.