Stacy,
Not all poisons are toxic, to the same extent, to all organisms.
Two quick (admittedly oversimplified) bird-based thoughts:
1) Such differential toxicity may result in some type of "directed"
dispersal strategy; i.e., perhaps the seeds are only "properly" dispersed by a
certain type of bird (which, of course, isn't affected by the toxin).
Perhaps some type of non-animal seed dispersal strategy has evolved.
2) Perhaps the toxins don't effect birds (in this example) at all but
instead are preventing some type of herbivory.
Rob Alba
UGA Botany
