After defeating the Elite Four, one of the three legendary birds from the original Pokemon Red and Blue will start roaming throughout Kalos. Which legendary bird that appears depends on your starter: if you chose Froakie, you'll find Moltres. If you chose Chespin, you'll find Articuno. If you chose Fennekin, you'll find Zapdos. After encountering one of the birds in the wild, check its entry in your Coastal Kalos Pokedex to see the bird's current location. You must walk, not fly, to the location and challenge the bird again. After fleeing several times, the bird will eventually retreat to Sea Spirit's Den, finally offering an opportunity to catch it.