Remember that "amen" is just an agreement to what was said previously - it's dependent on there being something already there in the first place in order to attain its meaning. So, if you're someone who doesn't have anything to say (to God, presumably), it's literally meaningless. I suspect this is supposed to reflect the helplessness of a baby bird - they can't even leave the nest to feed themselves. Also, this echoes the helplessness of children in general, thus (if my prior interpretation was right) alluding to the couple's own dead child.
Remember that "amen" is just an agreement to what was said previously - it's dependent on there being something already there in the first place in order to attain its meaning. So, if you're someone who doesn't have anything to say (to God, presumably), it's literally meaningless. I suspect this is supposed to reflect the helplessness of a baby bird - they can't even leave the nest to feed themselves. Also, this echoes the helplessness of children in general, thus (if my prior interpretation was right) alluding to the couple's own dead child.