tori no mure ga saezuri nagara tobimawatte iru.

鳥の群れ literally means a flock of birds.

Here, の links the two nouns:

• 鳥 = bird(s)
• 群れ = flock, herd, group, swarm

So 鳥の群れ is literally the flock of birds or a flock of birds.

This use of の is very common in Japanese for noun + noun combinations:

• 学生の本 = the student’s book / book of a student
• 日本の文化 = Japanese culture / the culture of Japan
• 鳥の群れ = a flock of birds

In English, we often translate it as of here: a flock of birds.

が marks 鳥の群れ as the grammatical subject of the sentence.

So the basic structure is:

• 鳥の群れが = a flock of birds
• さえずりながら飛び回っている = is flying around while chirping

A natural way to understand it is:

A flock of birds is flying around while chirping.

Japanese often uses が when introducing or describing what is doing the action. In this sentence, the speaker is presenting the flock of birds as the thing performing the action.

Because the subject is being treated as the flock as a unit, not the individual birds one by one.

• 鳥が would mean the birds are the subject directly.
• 鳥の群れが means the flock of birds is the subject.

This is similar to English:

• Birds are flying around
• A flock of birds is flying around

Both are possible, but 鳥の群れが specifically focuses on the birds as a collective group.

It is grammatically a single group, even though it contains multiple birds.

So:

• 群れ itself is one flock
• but that flock consists of many birds

This is similar to English a flock of birds. The noun flock is singular, but it refers to multiple animals.

Japanese does not mark plural the way English usually does, so context matters. Here, the idea of plurality comes from 群れ.

さえずり is the stem (also called the ます-stem) of the verb さえずる, which means to chirp, to twitter, or to sing for birds.

• dictionary form: さえずる
• stem: さえずり

This stem is used before ながら:

• さえずりながら = while chirping

Many verbs use the stem before ながら:

• 食べる → 食べながら = while eating
• 読む → 読みながら = while reading
• 話す → 話しながら = while speaking
• さえずる → さえずりながら = while chirping

So さえずり is not a separate random form; it is the normal connected form used with ながら.

ながら means while doing or as ... happens at the same time.

So:

• さえずりながら = while chirping
• 飛び回っている = flying around

Together: さえずりながら飛び回っている = flying around while chirping

A key point is that ながら connects two actions done by the same subject.

Here, the same subject, 鳥の群れ, is:

1. chirping
2. flying around

So ながら expresses simultaneous actions.

Usually, yes: it strongly suggests overlapping actions.

In this sentence, the image is that the birds are chirping as they fly around.

However, in real usage, ながら does not always mean every tiny moment overlaps perfectly in a mechanical way. It simply presents one action as accompanying another.

So here the natural understanding is:

• the main action: flying around
• accompanying action: chirping

Because 飛び回る is a more specific verb than 飛ぶ.

• 飛ぶ = to fly
• 飛び回る = to fly around, fly about, dart around here and there

So:

• 飛んでいる = is flying
• 飛び回っている = is flying around

回る adds the nuance of movement around an area, often actively or repeatedly. It creates a more vivid image of the birds moving about rather than simply being in flight.

It comes from the compound verb 飛び回る.

Breakdown:

• 飛ぶ = to fly
• 回る = to go around, turn around
• 飛び回る = to fly around

Then:

• te-form of 飛び回る → 飛び回って
• plus いる
• 飛び回っている

So the full verb phrase means is flying around or is in the process of flying around.

Here, ～ている shows an ongoing action, like the English progressive is ...-ing.

So:

• 飛び回る = to fly around
• 飛び回っている = is flying around

This is a standard use of ～ている for actions in progress.

It is worth knowing that ～ている can also sometimes express a resulting state, depending on the verb. But here, with 飛び回る, it is clearly the progressive meaning: the flock is currently flying around.

Both は and が can mark something important in a sentence, but they do different jobs.

Here, が is natural because the sentence is presenting or describing what is doing the action:

• 鳥の群れが = a flock of birds is the one chirping and flying around

If you used は:

• 鳥の群れはさえずりながら飛び回っている it would sound more like as for the flock of birds, it is flying around while chirping, possibly contrasting it with something else.

So:

• が = identifies the subject doing the action
• は = sets the topic

In a plain descriptive sentence like this, が is very natural.

鳴く is a broader verb meaning to make an animal sound.

For birds:

• 鳴く can mean to chirp, to cry, to call, depending on context

さえずる is more specific and usually refers to the pleasant, light, bird-like sound we think of as:

• chirping
• twittering
• birdsong

So in this sentence, さえずる creates a more vivid and natural image of birds making lively chirping sounds while flying around.

Grammatically, it is a verb form plus a grammar word:

• さえずり = stem of さえずる
• ながら = while

So it is better understood as:

• さえずり + ながら

In normal Japanese writing, there are usually no spaces, so you would typically see: さえずりながら

The spaced version in learning materials is often just there to make it easier to read.

No, normal Japanese is usually written without spaces.

A natural written version would be:

鳥の群れがさえずりながら飛び回っている。

Textbooks and learning materials often insert spaces to help beginners see the parts more clearly:

• 鳥
• の
• 群れ
• が
• さえずり
• ながら
• 飛び回って
• いる

That is a teaching aid, not standard spelling.

The main predicate is 飛び回っている.

The sentence is centered on the action: the flock is flying around

Everything before it helps describe that action:

• 鳥の群れが = subject
• さえずりながら = accompanying action, while chirping
• 飛び回っている = main action

So if you want to find the core of the sentence, it is: 鳥の群れが飛び回っている。 Then さえずりながら adds extra detail.

Yes, that is a reasonable natural translation, but it is a bit less exact.

The Japanese specifically says:

• a flock of birds
• while chirping
• is flying around

So birds are chirping and flying around captures the general meaning, but it loses two nuances:

1. 群れ = the birds are presented as a flock
2. ながら = the chirping is framed as happening while they fly around

A closer translation would be:

• A flock of birds is flying around while chirping.
• A flock of birds is fluttering about, chirping as they go.

In a complete standard sentence, いる is normally needed if you want the progressive meaning is flying around.

• 飛び回る = fly around
• 飛び回っている = is flying around

If you remove いる and just say 飛び回って, that is no longer a complete plain statement by itself. It becomes a te-form that expects something after it, unless used in a very casual or fragmentary style.

So in ordinary grammar, いる should stay.

A simple way to see it is:

[subject] + [simultaneous action] + [main action]

So:

• 鳥の群れが = subject
• さえずりながら = while chirping
• 飛び回っている = is flying around

In full: 鳥の群れが さえずりながら 飛び回っている。

This is a very common Japanese pattern:

• X が / は
• Verb-stem + ながら
• Main verb

For example:

• 彼は音楽を聞きながら勉強している。 = He is studying while listening to music.
• 子どもたちが笑いながら走っている。 = The children are running while laughing.

Your sentence follows the same pattern.