These are found in or around the red zones dominated by Infected.
They can be spotted on top of buildings nearby flocks of crows circling them. Target the water tower with LT and use your Whipfist attack (Y) to take them down.
If you choose to attack them up close and suddenly find yourself doing battle with a Hunter, then that means you were too slow and the water tower hatched before you could destroy it - this does not count.
If you have successfully destroyed the tower before it hatches, you will gain a bunch of EP immediately, the tower will explode yielding yellow orbs, and you will see a corpse of a dead Hunter.
edit: also very easily done after you gain access to hijack helicopters (mission 16)