Yes, a player can voluntarily choose not to use a bird's power when it is activated, even if the effect is beneficial.

This is supported by the rulebook, which states:

Bird powers are always optional. For example, if you do not want to spend a card by tucking it under a flocking bird, you do not have to do so.

(page 10)

This applies to all types of bird powers, including those that are beneficial.

Source: page 10

Source: Core Rulebook (pp. 1, 10, 7, 8, 9)