The early bird incentive works like this: 
If you buyin among the first 10% of all the bitcoin received during the ICO, you get 10% more than if your buyin was received among those in the second half.  If your buyin fell into the second 10% grouping of cumulative funds received, then you would get 8% more than those in the second half.  If your buyin fell in the 3rd 10% group (decile), you'd get 6% more.  4th decile gets 4% more, 5th decile gets 2% more, and buyins, thereafter, being in the last half of funds received, get their pro-rata amount without any early bird bonus.

So, answering your specific question, the early birds get their respective bonus % relative to the non-early birds in the second half.  We won't know in advance when or where those decile cutoffs fall.  We will find out at the end of the ICO, at which point everything can be calculated.  Everyone gets a fair shot, with early birds earning a bonus.

http://fundraiser.breakoutgaming.com/offering/