Some birds don't like storms. That could have set him off. It could have been anything. Mine will do it when a thunderstorm comes by, he needs water, doesn't like his food, wants attention, or is just happy and letting us know!
If the screaming gets bad for long periods, you can ignore it, I started to whisper to Teal (he is soooo nosey) and I learned when I started whispering he shut up so he could hear what I was saying. The funny result of that is 1. he stopped screaming, 2. he now whispers to me all the time!
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"Humans are the only animals that are mean on purpose"
Teal- Blue and Gold Macaw
Ellie - Mollucan/umbrella cockatoo hybrid