Use of 'the' in sentence: "Watch the birds in the sky"

Question

During learning English with 'Grammar' app from British Council I have noticed one sentence which confused me and that's why I'd like to ask you: Why do we have to use 'the' before birds in this sentence: "Watch THE birds in the sky" I thought that we use the in cases when we talk about something specific or something that was mentioned earlier?

Answer 1

When you use the before a noun, there is an underlying relative clause that we commonly omit:

Watch THE birds [that are visible/are now flying] in the sky

The emphasis is on those particular birds on that particular occasion. Yet, this is not absolute, there are instances with the that do not necessarily intend to particularise the birds (see example 3 below).

Watch birds in the sky

without the, can also exist, but the emphasis is not on which particular birds you watch, but on the action of watching (often called birdwatching).

Compare:

1. When I watch birds in the sky, moving together in a flock, I’m constantly amazed by their ability to move as one. (source)

with

2. As he watched the birds in the sky, he tried to make few drawings​ (source)

but also with

3. Have you ever stopped to watch the birds in the sky? (source)

This Ngram will show that both exist, but that with birds is presently in decline. People prefer the use with "the".