It might be that the chicken is only capable of using its understanding to understand, but a heart can go deeper and actually discriminate.
Nu, either way. The question then is even bigger according to the simple translation. Why is it not enough to thank Hashem that the chicken has understanding? Why does he have to "go deeper" as you say and actually discriminate?
But my point--and this is the answer to Henche--is that "bina l'havin" is no big deal. Imagine a chicken who "understood" the difference between day and night but didn't discriminate between the two. So much more so a heart that can explain the difference between one thing and another but doesn't make choices between the two.
Understanding unto itself is not enough. Understanding must lead to discrimination.