Income is not taxed when you are paid, an amount is withheld toward your eventual tax bill. It isn't until you file your tax return for that year that the amount of tax is determined and whether it was regular income or a bonus is not a consideration.

Taxes are withheld on bonuses at a 25% rate but the additional tax that you will eventually be charged for that money will be at your marginal tax rate which could be anywhere from 0% to the maximum marginal tax rate, depending on your situation. If your eventual marginal rate is less than 25% then the difference is added to your refund (or subtracted from what you owe).