The Hot Troll Deviation (Season 4, Episode 4).
Howard asks Penny to help him win back Bernadette, and Raj and Sheldon have an office war. But who cares?! It’s all about the budgie. Raj brings a budgie into the office because he knows Sheldon is afraid of birds. The shenanigans escalate until there is an explosion. Luckily, the little blue budgie makes it out alive.