mizudori ga oyoide iru.

It is usually read みずどり.

• 水 = water
• 鳥 = bird

So 水鳥 means waterbird or waterfowl, depending on context. It is a general word, not the name of one specific species.

が marks 水鳥 as the subject of the sentence: the thing doing the action of swimming.

So the structure is:

• 水鳥 = waterbird
• が = subject marker
• 泳いでいる = is swimming

Using が often feels like you are pointing out or describing what is happening: A waterbird is swimming / The waterbird is swimming.

If you used は instead, the nuance would change a little:

• 水鳥が泳いでいる。 = A waterbird is swimming / The waterbird is swimming.
• 水鳥は泳いでいる。 = As for the waterbird, it is swimming.

So が is very natural when simply observing or reporting a scene.

泳いでいる is the て-form + いる pattern.

It comes from the verb 泳ぐ, which means to swim.

Breakdown:

• 泳ぐ = to swim
• 泳いで = て-form of 泳ぐ
• いる = auxiliary here, making an ongoing action

So 泳いでいる means is swimming or is in the state of swimming.

In this sentence, it describes an action in progress.

This is because 泳ぐ is a godan verb ending in -ぐ, and its て-form changes in a regular way.

For verbs ending in -ぐ, the て-form becomes -いで.

So:

• 泳ぐ → 泳いで
• 急ぐ → 急いで

This is just a standard conjugation pattern you need to memorize.

A few common て-form patterns are:

• -う / -つ / -る → -って
• -む / -ぶ / -ぬ → -んで
• -く → -いて
• -ぐ → -いで
• -す → -して

Not exactly. Here, いる is being used as an auxiliary verb, not the standalone verb meaning to exist or to be for living things.

Compare:

• 鳥がいる。 = There is a bird.
• 鳥が泳いでいる。 = A bird is swimming.

In 泳いでいる, the いる combines with the て-form to show an ongoing action or continuing state.

So in this sentence, you should understand 泳いでいる as one grammar unit.

In most contexts, 泳いでいる means is swimming.

It usually describes an action currently in progress, like what someone sees right now.

So if you are looking at a pond and notice a waterbird moving through the water, 水鳥が泳いでいる。 is a very natural thing to say.

Japanese ている can sometimes express a habitual action in other situations, but here the most natural reading is is swimming.

Japanese nouns usually do not show singular vs. plural unless the speaker makes it clear some other way.

So 水鳥 could mean:

• a waterbird
• the waterbird
• waterbirds

In this sentence, the most natural English translation is often singular, but Japanese itself does not force that.

The exact number depends on context.

Japanese does not have articles like English a, an, or the.

That means 水鳥 by itself can mean:

• a waterbird
• the waterbird
• sometimes just waterbirds in a general sense

You choose the best English article based on context, not because Japanese has a separate word for it.

The spaces are usually added for learners.

Normal Japanese writing does not usually use spaces between words, so this would normally be written as:

水鳥が泳いでいる。

Textbooks and beginner materials often add spaces to make the sentence easier to read and analyze.

In a full, normal sentence like this, you usually keep が.

Japanese often omits things that are understood from context, but particles are not just randomly dropped in standard speech. Since 水鳥 is the subject, が is doing important grammatical work here.

So:

• 水鳥が泳いでいる。 = correct and natural
• 水鳥泳いでいる。 = not standard in normal written Japanese

In very casual speech, people sometimes shorten things, but learners should treat が as necessary here.

The difference is mainly aspect.

• 水鳥が泳ぐ。 = The waterbird swims / A waterbird will swim / Waterbirds swim
  This is more general, habitual, or non-progressive.
• 水鳥が泳いでいる。 = The waterbird is swimming
  This shows the action is ongoing.

So if you are describing what you see right now, 泳いでいる is the better choice.

Yes, it is completely natural.

It sounds like someone is observing a scene and commenting on it:

• at a pond
• in a park
• at a lake
• in a nature documentary

It is simple, neutral, and correct Japanese.

If you wanted to sound a little more conversational, you might also hear things like:

• 水鳥が泳いでる。
  This is a casual spoken contraction of 泳いでいる.

But the original sentence is the standard form.