It would be better if you used requirements than lumber. You could simply have a unit that gets created whenever the unit gets a kill-streak, which fulfills a tech-tree requirement which allows the ability to be used. Once the kill-streak ability is used, the unit would be removed until the next kill-streak is obtained.
You would also have the cool tool-tip of "- 3 kill-streak required.", or however many kills the streak is. Then players would know how many kills they need in a streak to use the ability.
Here I through together a little demo of how this works. The code is pretty preliminary, so don't pay much attention to it.
You start with a Paladin capable of learning all your basic spells. They are all disabled to begin with, labeled with tool-tips for how many kills in a row you have to obtain. You can obtain kills by killing the nearby Footmen.
When you die, your streak is reset. Once you obtain enough kills to use an ability, it will become available. Once you use the ability, the ability will become disabled again until you achieve the streak again. If you want to really test it out you could add some units to the test-map; currently there are only 7 and the 3-kill streak cannot really be tested since Devotion Aura cannot be used.
The only thing it doesn't allow for is stacking of multiple streak bonuses. For example, if you obtain a 5-kill streak twice you will still only be able to use Holy Light once - I was planning on fixing this up but tell me what you think of this concept.
~ Updated ~
Okay so I increased the amount of Footmen on the map and I made it work with a queue so that the game keeps track of abilities that you have not used. The idea here is that (let's use Call of Duty examples) if you get a spy-plane but forget to use it, you will not "lose" the spy-plane upon reaching another kill streak. Instead, you would be able to use 2 in a row.
I believe this is how it works in Call of Duty.