Hawks and Owls pose a huge danger to your Yorkie, especially if he or she is under 5 lbs. I have heard numerous stories of small toy dogs being swooped up right in their own yards by these predators. Never let your Yorkie roam free outside, always keep him on a harness.
Yorkies often have sensitive Trachea's. It is important that you use a harness as opposed to a collar on for this reason as well as others. Collars on toy dogs pose other threats besides the pressure on the trachea. A Yorkie wearing a collar, left unattended in your home, can get hung on a bedpost or gate while trying to jump. It would be tragic to come home to find your baby dead from this.
Use caution when using a retractable leash on a Yorkie. Yorkies have no fear. If you were walking your Yorkie and larger dog came along and acted viscous, your Yorkie might be inclined to go toward the larger dog, showing no fear. Or if the dog came toward you, it would make it difficult to swoop your baby up into your arms. With a solid leash, and a good harness, you can safely pull your Yorkie up to safety in an emergency.