When all dice in the birdfeeder show the same face (including if there is only 1 die), and you are about to gain food from the birdfeeder for any reason, you may first throw all 5 dice back into the birdfeeder. This reroll occurs before you select dice for a power or action that gains food.

This is supported by the following rule:

• "If the dice in the tray all show the same face (including if there is only 1 die) and you are about to gain food from the birdfeeder for any reason, you may first throw all 5 dice back into the birdfeeder."
  (Page 7)

Therefore, the reroll option is available immediately before you select dice to gain food, not after.

Source: page 7

Source: Core Rulebook (pp. 10, 2, 4, 5, 7)