Translation:Duo is a smart owl.
I think the main reason is to make it sound affable. Take the names of animals as examples: 鸟儿 (birds, fledglings), 猪儿 (pigs, piglets), 牛儿 (cattles, calves), 马儿 (horses, ponies).
This is one of the common methods to make pet names — by suffixing the last character of the first name with 儿. For example, the pet name for 李明月 can be 月儿 which is pronounced “yuè ér”, not with the neutral tone “yuè er”.
Grammatically, these forms are called 'diminutives', and most languages have them.