Our Mallard ducklings are 4 weeks old. For the first 2 weeks they all matured at an equal rate. About 2 weeks ago one of them stopped growing at the same rate as the others. He seems completely healthy, but he is half the size of the rest of the flock. They are also maturing more quickly. Their tail feathers have come in and they are also starting to get feathers on their wings. The little guy is still just soft and downy. The strange thing is that he seems healthy. He's eating normally (I watch for this) and keeps up with the rest of the flock just fine. He is also growing a little, but not nearly as fast as the rest. What do you think?