aki ni wataridori ga kuru.

It is read:

秋に渡り鳥が来る。
あき に わたりどり が くる。

Word by word:

• 秋（あき） = autumn, fall
• に = time marker particle
• 渡り鳥（わたりどり） = migratory bird(s)
• が = subject marker particle
• 来る（くる） = to come

Here, に marks a point or time period when something happens.

So:

• 秋に = in autumn / in the fall

This is very common in Japanese:

• 春に花が咲く。 = Flowers bloom in spring.
• 朝に来る。 = come in the morning

With seasons, に is often used to show when something happens.

が marks 渡り鳥 as the grammatical subject of 来る.

So the structure is basically:

• 渡り鳥が来る = migratory birds come

A learner often wonders about が vs は here:

• が focuses on who/what does the action
• は marks the topic and can give more of a contrastive or general-topic feeling

So:

• 秋に渡り鳥が来る。 = Migratory birds come in autumn.
• 秋には渡り鳥が来る。 = As for autumn, migratory birds come. / In autumn, at least, migratory birds come.

The version with が is a straightforward neutral statement.

Because Japanese normally places the verb at the end of the sentence.

English order:

• Migratory birds come in autumn.

Japanese order:

• Autumn in / migratory birds / come
• 秋に 渡り鳥が 来る。

This is one of the most important word-order differences between English and Japanese.

来る is the plain form (dictionary form).
来ます is the polite form.

So:

• 秋に渡り鳥が来る。 = plain style
• 秋に渡り鳥が来ます。 = polite style

The plain form is very common in:

• dictionary entries
• textbooks
• casual speech
• written explanations
• general statements

Since this sentence looks like a simple example sentence, 来る is perfectly natural.

It can mean either singular or plural, depending on context.

Japanese nouns usually do not change form for singular vs plural.

So 渡り鳥 can mean:

• a migratory bird
• migratory birds

In this sentence, the natural English translation is usually migratory birds, because it sounds like a general statement about the season.

If Japanese wants to make plurality extra clear, it can do so in other ways, but often it simply leaves it to context.

Most naturally, it sounds like a general fact or habitual occurrence:

• Migratory birds come in autumn.

The plain non-past form 来る often expresses:

• habitual actions
• repeated events
• general truths
• scheduled/future events

So here it is not necessarily They are coming right now.
It is more like This is what happens in autumn.

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

So 渡り鳥が来る does not explicitly say:

• a migratory bird comes
• the migratory birds come
• migratory birds come

Japanese leaves that kind of distinction to context.

That is why translation into English requires a choice, but the Japanese sentence itself does not.

Yes, but it changes the nuance.

Compare:

• 秋に渡り鳥が来る。
  = Migratory birds come in autumn.
  A simple statement about when it happens.

• 秋は渡り鳥が来る。
  = Autumn is when migratory birds come. / In autumn, migratory birds come.
  This makes autumn the topic and can sound contrastive, as if comparing seasons.

• 秋には渡り鳥が来る。
  = In autumn, migratory birds do come.
  This can sound even more contrastive, depending on context.

So 秋に is the most neutral choice if you just want to state the time.

渡り鳥（わたりどり） is a compound noun.

It is made from:

• 渡り（わたり） from the verb 渡る（わたる） = to cross, to migrate across
• 鳥（とり / どり） = bird

In compounds, 鳥 often changes pronunciation from とり to どり.
This is a common sound change in Japanese called rendaku.

So:

• 鳥 by itself = とり
• 渡り鳥 = わたりどり

The whole word means migratory bird.

Japanese word order is more flexible than English, as long as the particles stay attached to the right words.

So these can all be understood:

• 秋に渡り鳥が来る。
• 渡り鳥が秋に来る。

Both are grammatical.
However, the most natural order often depends on what you want to emphasize and on style.

In textbook-style basic sentences, time expression + subject + verb is very common, so 秋に渡り鳥が来る sounds very natural.

Good question. In Japanese, 来る means to come, and it is used for something coming to a place that is relevant in the context.

That place does not always have to be the speaker’s exact location in a strict physical sense. It can mean arrival into the area or scene being talked about.

So in this sentence, 渡り鳥が来る means migratory birds arrive or come in autumn.

In English, depending on context, you might also translate it as:

• Migratory birds arrive in autumn.
• Migratory birds come in the fall.

Yes, Japanese often omits information that is clear from context.

So 秋に来る could be a complete sentence in the right situation, meaning something like:

• (They) come in autumn.
• (It) comes in autumn.

But if you are introducing the idea clearly for the first time, saying 渡り鳥が makes the sentence much easier to understand.

So:

• 秋に渡り鳥が来る。 = clear, explicit
• 秋に来る。 = natural only if the listener already knows who/what is coming