This book is based on a good idea that fits very well within the WTA universe. The author very concisely explains their idea and gives us a solid set of rules.
Both the idea and the outlined rules seem unique and fresh, not a simple copy of an existing Mockery Breed or Shapechanger race.
It should be noted that the history and idea behind the Murder Birds are only briefly outlined. For me, this is more than enough, as it can be expanded upon easily and fits snuggly into the existing lore.
[5 of 5 Stars!]