ano kuroi mure ha karasu ni tigai nai.

群れ is read むれ.

It means a group, flock, herd, or swarm, depending on what is being grouped. In this sentence, since it is about crows, 群れ is best understood as a flock.

So 黒い群れ is something like a black flock or that black mass/group.

Because 黒い directly modifies the noun that follows it, which is 群れ.

So the structure is:

• 黒い = black
• 群れ = flock/group

Together: 黒い群れ = black flock / black group

A learner might expect the sentence to mean those are black crows, but grammatically 黒い is describing 群れ, not カラス.

If you wanted to say That flock must be black crows, Japanese would need a different structure, such as:

• あの群れは黒いカラスに違いない。
• or more explicitly: あの群れは黒いカラスの群れに違いない。

These are part of the ko-so-a-do system:

• この = this ... (near the speaker)
• その = that ... (near the listener or just mentioned)
• あの = that ... over there / away from both

So あの黒い群れ means that black flock over there.

It is used when the speaker is pointing out or referring to something distant.

は marks the topic.

So:

• あの黒い群れは = as for that black flock...

Then the rest of the sentence says what the speaker concludes about it:

• カラスに違いない = it must be crows / there is no doubt it’s crows

A natural way to understand the full structure is:

• As for that black flock, it must be crows.

Even though English often uses the subject more directly, Japanese very often uses は to set up the topic first.

に違いない is a grammar pattern meaning:

• must be
• there is no doubt that ...
• I’m sure that ...

It expresses strong confidence in the conclusion.

Here:

• カラスに違いない = it must be crows

This is stronger than just saying probably. It sounds like the speaker is making a firm judgment based on what they see or know.

Because に違いない is a fixed expression.

You should learn it as one grammar pattern:

• noun + に違いない
• plain form + に違いない

In this sentence:

• カラスに違いない

The に is not something you should try to translate word-for-word here. It is just part of the established pattern.

So it is better to think:

• X に違いない = X must be / there is no doubt that X

rather than trying to interpret に separately.

With nouns, に違いない attaches directly to the noun without だ.

So:

• カラスに違いない = correct
• カラスだに違いない = not standard

This is similar to other patterns where a noun is followed directly by a set phrase.

So when you use に違いない after a noun, just use:

• noun + に違いない

Japanese nouns usually do not show singular/plural the way English does.

So カラス can mean:

• a crow
• crows

The context tells you which one is intended.

Because the sentence already mentions 群れ (a flock/group), we naturally understand カラス here as crows, not a single crow.

So even though the Japanese word itself does not change, the meaning in context is plural.

に違いない is quite strong.

Very roughly:

• でしょう / だろう = probably / I suppose
• はずだ = should be / is expected to be
• に違いない = must be / I’m certain

So カラスに違いない sounds more confident than:

• カラスでしょう = it’s probably crows

And it is a little different from:

• カラスのはずだ = it should be crows / I expect it to be crows

に違いない often sounds like the speaker has looked at the evidence and feels sure.

Grammatically, it means the flock/group looks black, because 黒い modifies 群れ.

That makes good sense in context: from a distance, you might see a black mass in the sky and conclude that it must be crows.

So the sentence is not mainly describing the individual crows one by one. It is describing what the speaker sees: that black flock.

No. Normal Japanese writing would usually be:

あの黒い群れはカラスに違いない。

Spaces are often added in learning materials to make the structure easier to see.

So for real Japanese text, you should expect the sentence without spaces.

Yes, it is natural.

It sounds like someone is looking at something in the distance and making a strong guess:

• That black flock must be crows.

The phrasing is a little literary or deliberate because of に違いない, which is stronger and more formal than casual conversation forms like だろう or かも. But it is perfectly normal Japanese.