You’ll need 30+ Ultra Balls. Reset to your save point if you accidentally faint him or run out of Pokeballs. The only attacking move Zapdos knows is Drill Peck. This means he is easier to catch if you bring a Pokémon who resists Flying attacks. Geodude, Graveler or Golem are perfect for this as they’re Rock type, have high defense, and are immune to Thunder Wave. Give them Leftovers and have them use Defense Curl a few times, if you like. Bring Zapdos’ health down to the red and apply a status condition such as sleep or paralyze before attempting to catch him. Sleep and freeze are most effective, but are difficult to use because they’re not permanent. A Thunder Wave user will usually do the trick.