aki ni kita tori ha wataridori ni tigai nai.

Yes. In Japanese, a clause can directly modify a noun by coming before it.

• 秋に来た鳥 = the bird(s) that came in autumn
• literally: autumn-in came bird

This is called a relative clause. Japanese does not need a word like that, which, or who here.

So:

• 来た鳥 = the bird that came
• 秋に来た鳥 = the bird that came in autumn

Because it describes the bird by referring to the event of coming as something already completed.

In a noun-modifying clause, Japanese often uses the past form to mean:

• the one that came
• the one that has come

So 秋に来た鳥 identifies a bird by the fact that it came in autumn.

It does not necessarily mean the whole statement is only about the distant past. It just means the coming happened before the speaker makes the judgment.

Here, に marks a point in time.

• 秋に = in autumn

Japanese often uses に with specific times, seasons, dates, and so on.

Examples:

• 3時に = at 3 o’clock
• 日曜日に = on Sunday
• 秋に = in autumn

So 秋に来た鳥 means the bird that came in autumn.

No. Here 秋 is not a destination. It is a time expression.

So:

• 秋に来た = came in autumn

not

• came to autumn

Japanese に has several uses, and one of them is marking time. In this sentence, that is the correct interpretation.

は marks 鳥 as the topic of the sentence.

So the structure is:

• 秋に来た鳥は = as for the bird(s) that came in autumn...

Then the sentence says something about that topic:

• 渡り鳥に違いない = must be migratory birds

So the full pattern is:

• [topic] は [comment]

This is very common in Japanese.

に違いない is a set expression meaning:

• must be
• there is no doubt that it is
• is surely

So:

• 渡り鳥に違いない = must be migratory birds

The speaker is making a strong judgment or inference.

It sounds stronger than a simple guess like probably.

Because に違いない is the fixed pattern used after nouns and な-adjectives.

With a noun:

• 学生に違いない = must be a student
• 渡り鳥に違いない = must be migratory birds

With an い-adjective:

• 高いに違いない is incorrect
• you say 高いに違いない? No—actually with an い-adjective, you attach it directly:
  • 高いに違いない is wrong
  • 高いに違いない should be 高いに違いない?
    The correct pattern is:
  • 高いに違いない is incorrect
  • 高いに違いない should be 高いに違いない?

To make this completely clear, the real rule is:

• noun / な-adjective stem + に違いない
• verb / い-adjective plain form + に違いない

Examples:

• 先生に違いない = must be a teacher
• 静かに違いない = must be quiet
• 来るに違いない = must come
• 高いに違いない = must be expensive/high

So in your sentence, 渡り鳥 is a noun, so に is required.

に違いない is quite strong.

Rough comparison:

• かもしれない = might / may
  weak possibility
• でしょう = probably / I expect
  moderate confidence
• に違いない = must / there’s no doubt
  strong confidence

So this sentence sounds like the speaker is drawing a fairly confident conclusion:

• The birds that came in autumn must be migratory birds.

It can be either, depending on context.

Japanese nouns usually do not show number clearly unless the context makes it clear.

So 鳥 can mean:

• a bird
• birds

Likewise, 渡り鳥 can mean:

• a migratory bird
• migratory birds

A natural English translation might use singular or plural depending on the situation.

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

Whether the meaning is:

• a bird
• the bird
• birds
• the birds

is understood from context.

So 秋に来た鳥は could mean:

• the bird that came in autumn,
• the birds that came in autumn,
• as for birds that came in autumn,

depending on the context.

Yes, but the grammar changes slightly.

A plain statement would be:

• 秋に来た鳥は渡り鳥だ。 = The bird(s) that came in autumn are migratory birds.

But your sentence uses:

• 渡り鳥に違いない = must be migratory birds

Here, に違いない replaces the simple copula idea of is/are with a stronger inferential meaning: must be.

So it is not just a plain statement of fact; it is a conclusion.

渡り鳥 is read わたりどり.

It is a noun meaning:

• migratory bird

It comes from:

• 渡る（わたる） = to cross / migrate
• 鳥（とり） = bird

So literally it is something like crossing/migrating bird.

Yes.

• 秋に来た鳥 = the bird(s) that came in autumn
• 秋に来る鳥 = the bird(s) that come in autumn

The first one points to a completed arrival. The second one sounds more general or habitual, like birds that come in autumn as a regular pattern.

In many contexts, 秋に来る鳥 could sound more like a general species/category description, while 秋に来た鳥 may refer more to actual birds that arrived.

It breaks down like this:

• 秋に来た鳥 = the bird(s) that came in autumn
• は = topic marker
• 渡り鳥 = migratory bird(s)
• に違いない = must be / there is no doubt that it is

So the full structure is:

• [The bird(s) that came in autumn] + [topic marker] + [must be migratory birds]

That is why the sentence means:

• The bird(s) that came in autumn must be migratory birds.