nagai kutibasi no tori ha sagi ni tigai nai.

It is read:

ながい くちばし の とり は さぎ に ちがい ない。

In normal Japanese writing, there would usually be no spaces:

長いくちばしの鳥は鷺に違いない。

The spaces here are just for learning.

The sentence breaks into two main parts:

• 長いくちばしの鳥は = as for the bird with a long beak
• 鷺に違いない = must be a heron

So the structure is:

[description of the bird] は [speaker’s strong conclusion]

A very literal way to see it is:

• 長い = long
• くちばし = beak
• の = linking marker
• 鳥 = bird
• は = topic marker
• 鷺 = heron
• に違いない = must be / there is no doubt it is

Because 長い is an i-adjective, and i-adjectives directly modify nouns with no particle.

So:

• 長いくちばし = a long beak
• 赤い車 = a red car
• 大きい犬 = a big dog

This is just like how adjectives work directly before nouns in English.

Here, の connects the noun phrase 長いくちばし to 鳥.

So:

• 長いくちばし = long beak
• 長いくちばしの鳥 = bird of a long beak → more naturally, a bird with a long beak

This use of の is very common when one noun phrase describes another noun.

It directly describes くちばし.

So the internal structure is:

• 長いくちばし = long beak
• 長いくちばしの鳥 = bird with a long beak

It does not mean the bird itself is long.

は marks 鳥 as the topic of the sentence.

The speaker is saying something like:

• As for that bird with a long beak...
• A bird with a long beak...

Then the speaker gives a conclusion about it:

• ...it must be a heron.

Using は often gives a more general, descriptive, or topic-setting feel.

Sometimes yes, but the nuance changes.

• 鳥は makes the bird with a long beak the topic
• 鳥が would put more focus on the bird itself as the thing being identified

In this sentence, は sounds natural because the speaker is making a judgment about a topic already in mind.

に違いない means must be, there is no doubt, or I’m sure that ...

It expresses a strong conclusion based on what the speaker sees or knows.

So 鷺に違いない means:

• It must be a heron
• It has to be a heron
• I’m sure it’s a heron

It is stronger than just probably.

Because with a noun, the pattern is simply:

noun + に違いない

So:

• 鷺に違いない
• 先生に違いない
• 犯人に違いない

You do not normally insert だ before に違いない here.

A rough comparison is:

• かもしれない = might / maybe
• でしょう = probably
• はず = should / ought to / expected to be
• に違いない = must be / I’m sure

So に違いない is one of the stronger expressions of inference.

A quick nuance guide:

• かもしれない: weak possibility
• でしょう: reasonable guess
• はず: based on expectation or logic
• に違いない: strong conviction

No. There are other natural ways too, such as:

• くちばしの長い鳥
• くちばしが長い鳥

All of these can mean a bird with a long beak, but the feel is a little different:

• 長いくちばしの鳥 = a bird with a long beak
• くちばしの長い鳥 = a bird whose beak is long
• くちばしが長い鳥 = a bird that has a long beak

For many learners, くちばしが長い鳥 may feel the easiest to understand.
The version in your sentence is compact and perfectly grammatical.

Japanese nouns usually do not show articles or number the way English does.

So 鳥 could mean:

• a bird
• the bird
• birds

The exact meaning depends on context.

In this sentence, English might use:

• A bird with a long beak must be a heron
• That bird with a long beak must be a heron

Both are possible depending on the situation.

鷺 is read さぎ.

It is a real and correct kanji, but it is not one of the most basic everyday kanji for beginners. In some materials, you may also see this word written in kana as さぎ.

So yes, it is worth recognizing, but you do not need to worry if it feels advanced.

A polite version is:

長いくちばしの鳥は、鷺に違いありません。

That keeps the same strong meaning, but makes the sentence more formal.

If you want to sound softer and less forceful, you could also say something like:

長いくちばしの鳥は、鷺だと思います。

That means I think the bird with a long beak is a heron, which sounds less certain than に違いない.