马 – 乌 – 鸟

For beginners, the characters 马， 乌 and 鸟 look similar. However, in dictionaries they can be found under different radicals. In fact, 马 and 鸟 are radicals themselves. 

马 and 鸟 Radicals 

马 is simplified from 馬 (meaning “horse”). It can appear on the right-hand or left-hand side of a character, as well as at the bottom. See the following examples: 
驴 – lǘ – donkey 
吗 – ma – a particle used to make a general question 
骂 – mà – to scold 

鸟 (niǎo) is simplified from 鳥. It means “bird”. The simplified version is based on the cursive script form. As a radical, 鸟 often appears in the names of birds’ species: 
鸡 – jī – chicken 
鸭 – yā – duck 
鹅 – é – goose 

Other Examples of Usage 

这匹马 – zhè pǐ mǎ – this horse 
骑马 – qí mǎ – to ride a horse 
鸵鸟 – tuó niǎo - ostrich 
布谷鸟 – bù gǔ niǎo – cuckoo 
鸟笼 – niǎo lóng – birdcage 
乌 is not so frequently used. It is a part of the following words: 
乌鸟 – wū niǎo – crow (black bird) 
乌龟 – wū guī - turtle