The Ticks look like the Tickers in Gears of War 2. Use the Right Trigger (RT) to jump (or pounce) towards a human. You must pounce and completely take control of a human for this to count. It is easiest to sneak up to people who are alone as you can still be killed while on the face of a human and it will not count. Accumulate 15 kills and this will unlock on the last kill.
Note: Does not have to be done in a single match!