In the past, scientists thought that birds were unintelligent because they have smaller brains that lack some of the same parts that human brains have such as the cerebral cortex. Recently, scientists discovered that birds actually use the part of their brain called the pallium to do higher level thinking. Also, birds have shorter connections between the different parts of their brains than humans do, allowing to make quicker decision. Some birds are extremely smart such as New Caledonian Crows which have particularly larger brains compared to other birds.

https://www.theguardian.com/environment/radical-conservation/2016/nov/05/birds-intelligence-tools-crows-parrots-conservation-ethics-chickens