Q: My 8-year-old bonus son and his dad love the Miami Heat. They never miss a game. About a month ago, when trying to figure out what to buy him for his birthday I found a jersey I knew he would like, so I bought it for him. In my attempt to coordinate efforts I called his dad and told him about the jersey. He was very nice and thanked me. Three days later his mom and I see dad and child at the mall and child is wearing the exact jersey I bought him — and it was brand new. I’m trying to support this guy and he stabbed me in the back! How can I cooperate if he doesn’t?

A: What I’m about to tell you I ask you to listen to with an open mind because it hasn’t been explained quite this way in most of the information you read about bio/bonus co-parenting — and it’s a little subliminal, so go with me on this one. One of the keys to successful co-parenting is for the bio and bonus parent to establish their niche with the child and not cross over it. The father’s niche is basketball with his child. That’s what they do together. Without knowing it, you crossed over into dad’s niche with his son. As it was, Dad was polite to you, which was commendable, but he also one upped you because his perception was that you one-upped him by buying the jersey in the first place — you entered his territory.

Here’s the really crazy aspect — if Mom would have called Dad, told him about the jersey and explained that she was going to give it to support Dad and son’s mutual love of The Heat, the response may have been different. You, on the other hand, are a different story. Even if you all get along, you’re direct competition when it comes to his son. This unspoken competition gets less as the kids get older, but when they’re at the age of your bonus son, you’re right in the thick of it.

Of course we all do our best to put our insecurities aside in the best interest of the children, but it certainly isn’t easy — especially if you’re sharing custody with an ex who has remarried and your child actually likes this new guy. So, my advice to you, figure out what you like to do with your bonus son and make that your niche. For now, don’t pick basketball. If you like basketball, too, of course you can’t help that, but keep your eyes open and follow Dad’s lead. And, look for something else that you can share with your bonus son that’s completely different and can’t be perceived as a way to step on Dad’s attempt to stay close to his son.

Dr. Jann Blackstone is the author of “Ex-etiquette for Parents: Good Behavior After Divorce or Separation,” and the founder of Bonus Families, www.bonusfamilies.com. Reach her at drjannblackstonegmail.com