the pyramid sounds like a Quetzal bird (is) singing.

[diamondcutter]

When someone does something to make the gods angry, the pyramid sounds like a Quetzal bird singing.

Source: Kid’s Box 6, CUP

I’d like to know whether the verb “is” is omitted before the word “singing”. That is to say, the original sentence should be like this:
...the pyramid sounds like a Quetzal bird is singing.

 

[5jj]

There is no 'original sentence'. The one you quote is fine as it is.

 

[diamondcutter]

Thanks, 5jj.

Let me ask my question in another way.

Could I rewrite the sentence like this?

...the pyramid sounds like a Quetzal bird is singing.

 

[jutfrank]

Not really, I don't think.

In the original, the comparison is to the Quetzal bird. In other words, the complement of like is Quetzal bird. The word singing is modifying the noun phrase Quetzal bird. Another way to express this meaning is: ... sounds like a Quetzal bird (that is) singing.

With your rewrite, the comparison is to the impression that a Quetzal bird is singing. In other words, the comparison is to the content of the whole clause a Quetzal bird is singing rather than just the noun phrase a Quetzal bird. Although it's true that the speaker here does want to express an idea of the impression that a Quetzal bird is singing, I don't think it makes good sense to compare a thing (the pyramid) with a situation. You can compare things with things, or situations with situations, but not things with situations.

I hope I've made clear what I mean.