Favorite phrase of men, thrown after a woman: "You are a stupid chicken!" not fundamentally correct. Based on personal observations, chickens are very intelligent birds, even at the level of their instincts. By the way, a stereotype has long been formed that of all animals, the smartest are dogs and monkeys. But no, it turns out. Scientists have determined that it is the pig that is closest to the human in terms of its intelligence. Communicating with animals and birds, I came to a very interesting conclusion: animals understand even our words. not just emotions. And we are not always their "language". There are no stupid animals. They also need to be taught, trained, if you want.
Of course not. The fact that a certain species quite successfully occupies a certain ecological niche for many thousands of years makes this species automatically "smarter" than an extinct one, so that all living animals are "smart". Now, if we talk about different representatives of the same species, then yes, the division is possible both into smart / stupid, and into Thick / thin and beautiful / ugly ... In short, this will only be a characteristic of an individual.
I am not a specialist, but, in my opinion, it is quite applicable. Animals can demonstrate different results in many disciplines: counting, learning ability, solving non-standard problems with the help of available tools, and even learning languages.