There is a book called Birds and Berries by Barbara and David Snow (1988) that has useful details on the different wild birds that that eat the fruit and seeds of rowans, whitebeams and wild service trees as well as those of other of other plants. See here:
The Snows made their observations between 1980 and 1985 in Buckinghamshire and the adjacent parts of Hertfordshire in central southern England.
Not surprisingly the commoner British fruit- and seed-eating birds are the main culprits and, while they undoubtedly reduce the overall burden of seed available for germination, they also help to spread them.
In the case of the wild service (Sorbus torminalis) bullfinches (Pyrrhula pyrrhula) were particularly active seed eaters. Redwings (Turdus iliacus) and song thrushes (Turdus philomelos) swallowed the whole fruit. There is, however, no mention of mistle thrushes (Turdus viscivorus), that are reported as being very fond of the berries to the extent that a pair will 'guard' a tree and drive off any new arrivals.
The Snows suggest that a poor dispersal of the wild service could be a consequence of "unsuccessful competition with other plants" (meaning that birds are more attracted to various other fruits available when the services were ripe) combined with the high level of seed predation and that this might account for the species' relative scarcity. However, they also say that the fruit normally has only one seed, making seed predation more of a problem than for species with more than one seed. In my experience, though, the wild service often has three or even four seeds per fruit.
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