About 3 weeks ago, I started feeding a mother cat and her three kittens who have made a home in my neighborhor's backyard as well as my own in the hopes of trapping, spay/neuter. The kittens are around 6 weeks. They tend to stay in my backyard all day playing around. Today, I only saw the mother come for breakfast and dinner. I thought the kittens would join her, but they have not been around all day. I have spoken with my neighbor and they have not seen the kittens either, just the mother. Is this normal behavior?