Sri Garuda carrying Lord Vishnu

Once upon a time, Garuda, the carrier of the Lord, snatched away a nectar pot from the hands of the demigods in heaven in order to liberate his mother, Vinata, from the clutches of his stepmother, Kadru, the mother of the serpents.

On learning of this, Indra, the King of heaven, hurled his thunderbolt against Garuda. With a view to respect the infallibility of Indra's weapon, Garuda, though otherwise invincible, being the Lord's own mount, dropped one of his wings, which was shattered to pieces by the thunderbolt.

The inhabitants of the higher planets are so sensible that even in the process of fighting they observe the preliminary rules and regulations of gentleness.

In this case, Garuda wanted to show respect for Indra; since he knew that Indra's weapon must destroy something, he offered his wing.

– SB 3.19.14 Purport