El mirlo y el petirrojo se posaron juntos.

Because posarse is the usual verb for to perch / to land / to settle oneself when talking about birds.

• posar by itself can mean things like to place something, or to pose.
• posarse means the subject itself comes to rest somewhere.

So in El mirlo y el petirrojo se posaron juntos, the se is part of the verb posarse. It is not really being used like English themselves here; it is just how this verb normally appears for this meaning.

Posaron is the preterite of posarse, used for a completed action in the past.

Here it presents the action as a single event:

• they perched
• they landed
• they settled

That is why se posaron fits well.

If you used the imperfect, se posaban, it would suggest something habitual, repeated, or backgrounded, such as:

• Cada mañana se posaban juntos en la verja.
  Every morning they would perch together on the fence.

So the preterite here tells you that this happened as one completed action.

Because the subject is masculine plural.

The two nouns are:

• el mirlo
• el petirrojo

Both are masculine nouns, so any adjective referring to them together must also be masculine plural:

• juntos

If the subject were two feminine nouns, you would use juntas.

Also, Spanish uses the masculine plural for mixed or masculine groups, so juntos is the default plural form in many cases.

That is the normal place for it in this sentence.

Juntos is describing how they perched: together. In Spanish, adverbs and adjective-like expressions of this kind often come after the verb:

• se posaron juntos
• vivían juntos
• salieron juntos

You could move things around for emphasis in some contexts, but se posaron juntos is the most natural neutral order.

In Spanish, it is very common to repeat the article with each noun in a pair:

• el mirlo y el petirrojo
• la mesa y la silla
• el padre y la madre

This is often more natural than using just one article before both nouns.

In English, we often say the blackbird and robin only using the once in our minds, but Spanish strongly prefers making each noun clear with its own article.

Yes, in ordinary Spain Spanish:

• mirlo usually means blackbird
• petirrojo usually means robin

A learner may notice that bird names do not always match perfectly across languages or regions, but in standard peninsular usage those are the normal everyday equivalents.

Also, both are grammatically masculine here because the nouns are masculine:

• el mirlo
• el petirrojo

That grammatical gender does not necessarily tell you the biological sex of the birds.

Not really.

Although se is often taught as a reflexive pronoun, in many verbs it is simply part of the standard form of the verb with a certain meaning.

Compare:

• lavarse = to wash oneself
• posarse = to perch / land / settle oneself

With posarse, Spanish normally uses the pronominal form for birds landing or settling somewhere. So in this sentence, se does not need to be translated as themselves in a literal way.

Usually no, not if you mean they perched together.

Without se, posar more often means:

• to pose for a picture or artist
• to place something somewhere

So posaron juntos could sound more like they posed together, depending on context.

For birds landing on a branch, se posaron juntos is the correct and natural choice.

Yes, but it changes the feel a little.

• Se posaron juntos = the most neutral, natural order
• Juntos se posaron = more marked, perhaps emphasizing that they did it together

Spanish word order is flexible, but the original sentence is the straightforward default.

It behaves like an adjective used adverbially.

Normally junto/junta/juntos/juntas is an adjective:

• los pájaros juntos = the birds together / the birds who are together

But after verbs, Spanish often uses these adjective forms to describe the state of the subject during the action:

• vivimos juntos
• fueron juntos
• se posaron juntos

So even though English uses the adverb together, Spanish uses a form that agrees in gender and number with the subject:

• junto
• junta
• juntos
• juntas

Because Spanish does not need to include every detail if the sentence is already complete without it.

El mirlo y el petirrojo se posaron juntos is a full sentence meaning that the two birds perched together. The place can be added if needed:

• ... en una rama = on a branch
• ... en el tejado = on the roof
• ... sobre la valla = on the fence

But leaving it out is completely natural if the important information is simply that they perched together.

It is a compound word:

• pecho / peti- relates to the chest or breast area
• rojo means red

So petirrojo literally suggests something like red-breasted. That helps explain why it is the word for robin.

Spanish has quite a few animal names of this sort, where the form gives a clue about appearance.

A rough Spain-Spanish pronunciation guide would be:

• El mirlo y el petirrojo se posaron juntos
• approximately: el MEER-lo ee el peh-tee-RRO-ho seh poh-sah-RON HOON-tos

A few useful points:

• rr in petirrojo is a rolled r
• j in petirrojo and juntos is the strong Spanish throat sound
• y here sounds like ee because it means and

The stress falls on:

• MIR-lo
• peti-RRO-jo
• posa-RON
• JUN-tos