Some birds prefer insects, some fruit, some grains and some are flesh eaters Takes all kinds. Birds have thier preference and prefer food that tastes good and does not fight back too much. Fuzzy caterpillars are not on the list of prefered foods but there are somelarge birds like chickens which will eat snails and slugs. Many thgings which are toxic to humans are not toxic to to ther organisms. Even the most toxic plant will have something which uses it as food.
Note that a weed is simply a plant which you do not want. Dandelions and purslane are edible for humans. Under suitable conditions they outgrow any insect damage, but if you look closely you will find insect damage. Large domestic birds like chickens , turkey , geese will eat them. The old folks use to geese to weed their strawberry patches. They don't like strawberry plants but will eat most of the grasses and broadleaf weeds that infest the patch.

The most toxic substance, oxalic acid, isn't toxic to some organisms. It's found in all parts of the plant, including stalks, but it's especially high in the leaves.

While oxalic acid is to poisonous to some caterpillars, some just have it pass through their system and poop them out without it effecting them. Snails aren't bothered by it.

A very famous quote, here from Wikipedia: Paracelsus, sometimes called the father of toxicology, wrote:
Alle Ding' sind Gift, und nichts ohn' Gift; allein die Dosis macht, daÃÂ ein Ding kein Gift ist.

All things are poison, and nothing is without poison; only the dose permits something not to be poisonous.

Or, more commonly

The dose makes the poison.

Also, to be poisoned by humans are often the reaction of our system. Other warm blooded animals can eat what is poisonous to us and show no reaction. It is similar to allergic reaction.
We eat hot peppers and get the heat and burning sensation. But the birds don't feel it and apparently does not cause them any harm.
I often get a kick of out this that some gardeners make a solution with hot peppers, garlic and what not, believing that it will deter or kill some insects, without realizing that what might be offensive to us might not be so to those insects. Unless, it has been scientifically established WHY and HOW.

Yeah it really is that simple. Just because something is poisonous to humans does not mean it is poisonous to everything else in nature.

Hundreds of examples of that fact in nature.

Dave

And for another example, yews are deadly poisonous to us in all their parts. Deer love em.

My wife puts hot peppers and garlic on everything she feeds me. Is she trying to poison the dear [sic] ? ed - 5/4/14

This post was edited by albert_135 on Thu, May 8, 14 at 17:47