sagi ga tobitatta.

It is read:

さぎ が とびたった。

In normal writing, it would usually appear without spaces:

鷺が飛び立った。

The spaces are just a learning aid. Standard Japanese normally does not use spaces between words in ordinary sentences.

So a textbook or study app may show:

鷺　が　飛び立った。

but natural written Japanese is:

鷺が飛び立った。

が marks 鷺 as the subject of the verb.

So the structure is:

• 鷺 = the heron
• が = subject marker
• 飛び立った = took off / flew away into the air

In this sentence, が tells you that the heron is the thing performing the action.

This is a very common question.

In a sentence like 鷺が飛び立った, が often presents the subject as the important piece of information in the event: a/the heron took off.

If you changed it to 鷺は飛び立った, the nuance would be different. は would make 鷺 the topic, with a feeling more like:

• As for the heron, it took off
• or possibly the heron, at least, took off

So:

• が = focuses on who did it / what happened
• は = sets up a topic and comments on it

Both can be grammatical, but が is very natural for simply reporting an event.

The dictionary form is 飛び立つ.

飛び立った is the plain past form of 飛び立つ.

So:

• 飛び立つ = to take off, to fly up
• 飛び立った = took off / has taken off

飛び立つ is a compound verb made from:

• 飛ぶ = to fly
• 立つ = to stand, rise

In compounds like this, 飛ぶ often changes to its stem form 飛び.

So:

• 飛ぶ → 飛び
• 立つ
• together: 飛び立つ

The overall meaning is not just fly in a general sense, but specifically rise into flight / take off.

This is an important nuance difference.

• 飛んだ = flew
• 飛び立った = took off / flew up into the air

So 飛ぶ describes flying in general, while 飛び立つ emphasizes the moment of leaving the ground, branch, water, etc.

For example:

• 鳥が飛んだ = The bird flew.
• 鳥が飛び立った = The bird took off.

In your sentence, the image is more specifically that the heron launched itself into the air.

飛び立つ is intransitive. That means it does not take a direct object.

The heron itself performs the action of taking off. Nothing is being acted on directly, so there is no を object here.

That is why a simple sentence like:

鷺が飛び立った。

is complete on its own.

The small っ shows a doubled consonant sound.

So たった is pronounced roughly like tat-ta, not ta-ta.

That is why:

• 飛び立った = とびたった
• not とびたた

This small っ often appears in past forms of verbs ending in つ, る, or う in certain conjugations.

For 飛び立つ:

• dictionary form: 飛び立つ
• past plain form: 飛び立った

Japanese nouns usually do not show articles like a or the, and they usually do not mark plural the way English does.

So 鷺が飛び立った could mean:

• A heron took off
• The heron took off

The exact English choice depends on context.

In most cases, if the sentence is presented by itself, English learners are often shown A heron took off or The heron took off, depending on the situation.

It is a real kanji, but it is not one of the most common everyday ones. Many native speakers can understand it, but in ordinary writing you may also see さぎ written in kana instead.

Using 鷺 can feel a bit literary, formal, or nature-related.

So both of these are possible:

• 鷺が飛び立った。
• さぎが飛び立った。

A polite version would be:

鷺が飛び立ちました。

The plain form 飛び立った is perfectly normal in casual speech, stories, narration, and dictionary-style examples.

So:

• plain: 鷺が飛び立った。
• polite: 鷺が飛び立ちました。

The core meaning stays the same.