suzume ga saezutte iru.

It is read:

すずめ が さえずって いる。

In romanization:

suzume ga saezutte iru

A more natural no-space writing is:

雀がさえずっている。

The spaces are usually added only in beginner materials to make the sentence easier to read.

雀 means sparrow.

Its reading here is すずめ. This is just the normal Japanese word for a sparrow, but the kanji reading is not very guessable for beginners, so it is one of those words you simply memorize.

So:

• 雀 = sparrow
• すずめ = how it is pronounced

が marks 雀 as the grammatical subject of the verb.

So 雀がさえずっている means something like:

• A sparrow is chirping
• The sparrow is chirping

Why not は?

• が often introduces or points out the thing doing the action
• は would make 雀 the topic and can sound more contrastive or thematic

Compare:

• 雀がさえずっている。 = A sparrow is chirping / The sparrow is chirping
• 雀はさえずっている。 = As for the sparrow, it is chirping

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

The dictionary form is さえずる, which means to chirp, to twitter, or to warble.

The sentence uses:

• さえずる → dictionary form
• さえずって → て-form
• さえずっている → te-form + いる

So the full verb phrase means is chirping.

Because さえずる is a godan verb, even though it ends in る.

For this kind of verb, a final る changes to って in the て-form:

• さえずる → さえずって

This can surprise learners because not all る verbs behave the same way.

For comparison:

• 食べる → 食べて because it is an ichidan verb
• さえずる → さえずって because it is a godan verb

So the small っ is there because of that conjugation pattern.

Here ている shows an action that is in progress.

So さえずっている means:

• is chirping
• is singing in the sense of a bird singing

In this sentence, the most natural interpretation is that the sparrow is chirping right now.

Yes. ている can have different meanings depending on the verb.

Common uses include:

• ongoing action
  
  • 読んでいる = is reading
• resulting state
  
  • 結婚している = is married
• habitual action in some contexts
  
  • 毎朝走っている = runs every morning / is in the habit of running every morning

In 雀がさえずっている, the usual reading is the ongoing-action one: the bird is chirping now.

Yes, there is a kanji: 囀る for さえずる.

So you might theoretically see:

• 雀が囀っている。

But in everyday writing, さえずる is very often written in hiragana because the kanji is uncommon and harder to read for many people.

That is why beginner materials usually use:

• さえずる
• さえずっている

instead of the kanji form.

Yes, in normal Japanese writing, it is usually written without spaces:

雀がさえずっている。

The spaced version:

雀 が さえずって いる。

is just a teaching aid. Japanese normally does not use spaces between words the way English does.

So grammatically, think of さえずっている as one verb phrase.

By itself, 雀 does not explicitly show singular or plural.

Depending on context, it could mean:

• a sparrow
• the sparrow
• sparrows

In this sentence, English often translates it as A sparrow is chirping or The sparrow is chirping, because the sentence sounds like one sparrow is being noticed.

But Japanese does not force that distinction the way English often does.

Yes, it is natural. It sounds like a simple observation of something happening in the environment.

The tone is:

• neutral
• descriptive
• plain style

It feels like someone noticing a sparrow chirping nearby.

If you wanted a polite version, you could say:

雀がさえずっています。

That would be the polite present form.