I've just read in a [book](http://knihy.abz.cz/prodej/rehek-domaci-uvaha-o-stavu-vedomi-a-nevedomi) that birds' guts can digest almost all the consumed seeds with the exception of mistletoe and loranthus (which stays stuck on the branches). On the other hand, I know that the co-evolution with birds caused the enlarging of the fruits - for the fruit trees as an advantage in fast spreading.

But how do exactly birds spread the seeds they consume if they're digested in their guts?