There are thousands of different kinds of birds . They come in all sizes and colors . And they each make their own sound . Here is a nightingale . And this is the song of a canary . Canaries are active , cheerful birds . And they are known for their ability to sing . Some animal experts say they are among the best singing birds . So , if someone says that you sing like a canary , you would think that they mean you sing cheerfully all day long ! But that is not what singing like a canary means . Not even a little . In English , if you sing like a canary , you are reporting on someone to high-level officials , like the police . Let's say police officers question someone suspected of a crime . The officers offer the woman a deal -- a lighter prison sentence in exchange for her cooperation . Wanting to avoid jail time , she sings like a canary and provides details about the people who helped her carry out the crime . In fact , even after the police have enough evidence , she keeps on talking , telling them about other crimes . She is like a canary that cannot stop singing ! In this case , you could also say that she snitched or ratted on her criminal friends . If I snitch or rat on someone , I can also simply say that I have told on them . To tell on a person is to tell someone else a bad thing they have done . So , for example , if I tell on you , I share something bad you have done with someone else . Although , to tell on someone sounds a little childish . Brothers and sisters often tell on each other to their parents . Classmates can tell on each other to their teachers . Now , at this point , you might say , "Anna , wait a minute . Why does English use canaries in this expression ? People love canaries and keep them as pets . They are happy , harmless little birds .” Well , yes , they are . But that is not the point . To say someone “sang like a canary ” simply states what the bird does naturally and often . The expression does not mean that the birds are bad . Also it does not liken them to criminals . So , if someone sings like a canary and rats on their friends , we would never call them a “canary .” But , we could call them a rat . Canaries , it seems , are easier to love than rats . There is an important thing to note when using these expressions . When we use “sing like a canary ,” most often the person doing the singing is also guilty themselves . But when you use sing like a canary , you usually have had a hand in the trouble . Unlike the cheerful canary . It just wants to sing .