У маленькой птицы жёлтый клюв, а у взрослой — тёмный.

Russian very often expresses possession with the pattern у + genitive + nominative noun.

So:

• У маленькой птицы жёлтый клюв
  literally means something like At the small bird, [there is] a yellow beak.

In natural English, we translate that as The small bird has a yellow beak.

This у + genitive structure is especially common for:

• possession in general
• body parts
• things belonging to someone or something

So here, Russian is not using a direct verb to have. It is using its normal possession pattern.

Because птицы is in the genitive singular, and у requires the genitive.

The dictionary form is:

• птица = bird

After у, it becomes:

• у птицы = of the bird / at the bird

So:

• у маленькой птицы = of the small bird / the small bird has

That is why you do not see nominative птица here.

Because маленькой has to agree with птицы.

Since птицы is:

• feminine
• singular
• genitive

the adjective must also be:

• feminine
• singular
• genitive

So:

• маленькая птица = a small bird
• у маленькой птицы = of the small bird / the small bird has

This is standard adjective-noun agreement in Russian.

Взрослой is feminine because it refers to птице / птицы, meaning bird.

The full version would be:

• а у взрослой птицы — тёмный

But Russian often omits a noun if it is already clear from context. So птицы is left out after взрослой.

That means:

• у взрослой = for the adult one / for the adult bird

This kind of omission is very common and natural in Russian.

Because клюв is the thing being described as existing or belonging.

In this possession pattern:

• у + genitive = possessor
• the possessed thing usually stays in the nominative

So:

• у маленькой птицы жёлтый клюв

breaks down as:

• у маленькой птицы = the small bird has
• жёлтый клюв = a yellow beak

That is why клюв is nominative singular, not genitive or accusative.

Because тёмный refers to клюв, not to птица.

• клюв is masculine
• so the adjective describing it must also be masculine

In the second part, the noun клюв is omitted, but it is still understood:

• а у взрослой — тёмный
• literally: and the adult one’s [beak is] dark

So тёмный is masculine because the hidden noun is клюв.

The dash marks an omission of words that are understood from the first part.

The full idea is something like:

• а у взрослой птицы клюв тёмный
• or а у взрослой птицы — тёмный клюв

Russian often avoids repeating obvious words. The dash helps show that something has been left out.

So here the dash is a neat, natural way to avoid repeating клюв or the whole repeated structure.

Because а often shows contrast or comparison.

Here the sentence compares:

• the small bird
• the adult bird

and also contrasts:

• yellow
• dark

So а is the natural choice.

Very roughly:

• и = and
• а = and/but, with a contrast

In this sentence, а sounds like:

• while
• whereas
• and as for the adult one...

Yes, from an English point of view, it can feel like something is missing.

Russian normally does not use a present-tense verb to be in sentences like this. So there is no word for is in the present tense here.

Also, Russian often expresses has with the у + genitive construction instead of a real have verb.

So English might say:

• The small bird has a yellow beak, and the adult one has a dark one.

But Russian naturally says:

• У маленькой птицы жёлтый клюв, а у взрослой — тёмный.

Nothing is actually wrong or incomplete; this is just standard Russian grammar.

Yes. Russian word order is flexible, and you could say similar things in other ways.

For example:

• У маленькой птицы клюв жёлтый, а у взрослой — тёмный.

This puts a little more focus on клюв first, then the color.

The original version:

• У маленькой птицы жёлтый клюв, а у взрослой — тёмный

sounds very natural and smooth, especially in descriptive writing. It presents each beak as a unit:

• yellow beak
• dark one

So the chosen word order is normal and idiomatic, not the only possible order.