While it's been implied and suggested before, this is the first time I've heard its on-demand nature explicitly confirmed:
Did You Know: Each Guardian can summon their Shrike on-demand to quickly cover a lot of ground. pic.twitter.com/fZqEYrSkJw— Destiny The Game (@DestinyTheGame) March 16, 2014
You may have heard the Shrike called Sparrow in the past. It's your personal speeder, if you've forgotten!