Cría cuervos y te sacarán los ojos
 

Este dicho significa que a veces hacemos bien a quien menos se lo merece, y que luego nos darán la espalda, o nos traicionarán.
ENGLISH?

Translation:
If you raise crows, they'll peck out your eyes.
Breed crows, and they'll take out your eyes.

A possible equivalent, but not an idiomatic expression:
You can take an animal out of the wild, but you can't take the wild out of an animal.

Mira aquí. This is the closest fit I could find, but it only works if you take 'good intentions' out of it. We usually associate this saying with sowing bad acts and reaping the same in the end.

Cría cuervos, is when you do sth in good faith and the other doesn´t thank you, or doesn´t care about it.

Commonly we say: a thankless task and rarely refer to crows or other
creatures.

I suppose a free, colloquial translation would be "Bring your children up badly and you'll suffer for it"; a similar British proverb would be "Spare the rod and ruin the child", but I expect that's unknown the other side of the pond.

Oye Sancho,
The "spare- the- rod" phase is also known on this side, but the philosophy is out of style on both sides of the Atlantic.

I still think "cria cuervos y te sacarán los ojos" is more "You can take an animal out of the wild, but you can't take the wild out of an animal" that really "Bring your children up badly and you'll suffer for it" or "when you do sth in good faith and the other doesn´t thank you"

For me it has an old meaning of "what's natural it's natural", or that the effects of education are only in the surface"
The best example it's the scorpion fable
http://en.wikipedia.org/wiki/The_Scorpion_and_the_Frog

I also love a spanish sentence, refering to a good weapon or a good ham knife (cuchillo jamonero)
"cuidado con éste que no conoce ni a su padre"
Saludos :D

I know this from being passed on through my family and my friends' families (Cadiz, Andaluz, Spain) but, when we hear the expression, "cría cuervos y te sacarán los ojos," we are flooded with connotations of a person behaving badly towards their parents but it being the parents fault. When someone says, "You can take the animal out of the wild but you can't take the wild out of the animal," the implication is that you just can't change some behaviors in people, and eventually they will do what they will do, no matter how much you tried to change them or stop them. But "Cría Cuervos" (Raising Crows) isn't implying that someone is attempting to change a person from being who they are. If you were a crow, what kind of offspring would you produce? That's right - crows. and if you raise crows, then expect them to behave like crows, and expect that to come back and bite you in the ass.

The part, "te sacarán los ojos," is the warning to that crow that if it "continues to be a crow," then they will raise crows and those crows will behave like crows. And pecking is what crows do, and they will peck anything, including their own parents

I would say that the closest thing in English would be a non-formal saying, "Don't let Karma bite you."

violent parents raise violent children; they teach their children to fight and to be aggressive. but let those parents be warned; sometimes the violent children they raise may use those same violent things they learned against the parents. Hence, if you raise crows, expect the possibility that they may act like crows - with you, which includes the possibility of getting your eyes pecked out.

Interesting and good explanation. i never heard the karme expression before.

Karma!