What parts of fleshy fruits contain secondary compounds toxic to birds and why?

Saponins, flavonoids and cyanogenic glycosides were surveyed in pulps and seeds of wild, bird-dispersed fleshy fruits of hawthorn (Crataegus monogyna), ivy (Hedera helix), holly (Ilex aquifolium), and yew (Taxus baccata). Interactions between three of the four species (hawthorn, holly, and yew) and their avian seed dispersers were studied in the field. The results indicate that when different bird species foraged on the same fruit they spend similar periods of time on the tree and eat a similar number of fruits at each feeding bout. Frugivorous birds stayed on all plant species for short periods of time (1.3-5.3 min) and consumed only a few fruits in each feeding bout (4.3-6.5 min). There is a differential occurrence of secondary compounds in fruit parts: in most cases allelochemicals were found in pulps but not seeds. These findings confirm the hypothesis that mild toxicity in the pulp can prevent consumption of too many fruits in one foraging bout and regulate seed retention time. This, combined with the short visits ensures better seed dispersal, as only few seeds will be deposited in one site at one time. Yew presents a special case, since cyanogenic glycosides were identified only in seed-coats, while both the fleshy aril and seed content are free of this toxin. A possible ecological explanation is suggested for this finding. The concentrations of some compounds may change during fruit ripening.