Subject: How do animals avoid eating poisonous yet inconspicuous plants?

How do animals avoid eating toxic plants that do not have external warning signs 
such as distinctive fruit color or odor? For example, why do deer avoid poison 
ivy that is not fruiting or flowering? (or maybe they don't - but there are 
surely other examples). Do they learn to recognize the leaf shape like humans 
do?

For that matter, how do animals know to avoid toxic plants WITH warning 
features? Do they learn by trial and error and remember the shapes/colors/odors, 
or has natural selection resulted in preprogrammed, instictive 'knowledge'? 

Re: How do animals avoid eating poisonous yet inconspicuous plants?