El mirlo se posó en la rama.

Because the verb here is posarse, which means to perch, to land, or to settle oneself somewhere.

In this sentence, se is not a separate idea like himself in English. It is simply part of the verb posarse.

• posarse = to perch / to land
• posar without se usually means something else, such as to place something or to pose for a photo or painting

So:

• El mirlo se posó en la rama = The blackbird perched on the branch
• Posó para una foto = He/She posed for a photo

Posó is the preterite tense, third person singular.

That means it refers to a completed action in the past:

• yo me posé
• tú te posaste
• él/ella se posó
• nosotros nos posamos
• vosotros os posasteis
• ellos/ellas se posaron

In this sentence, the bird performed one finished action: it perched on the branch.

The preterite is used because the sentence presents the action as a single completed event.

• El mirlo se posó en la rama = The blackbird perched on the branch / landed on the branch

If you used the imperfect, se posaba, it would suggest something habitual, ongoing, or descriptive:

• El mirlo se posaba en la rama cada mañana = The blackbird would perch on the branch every morning

So in your sentence, the preterite fits because it describes one specific action that happened and finished.

The accent mark shows the correct stress:

• posó is stressed on the last syllable: po-SÓ

It also helps distinguish it from other forms:

• poso = I pose / I place, or dregs/sediment as a noun in some contexts
• posó = he/she/it posed or perched

So the accent is both a pronunciation guide and a way to distinguish forms.

With posarse, Spanish very commonly uses en.

• posarse en una rama
• posarse en una pared
• posarse en una flor

Even though English often says on the branch, Spanish usually prefers en here.

Sobre is possible in some contexts, but en la rama is the most natural and idiomatic choice in a sentence like this.

Spanish uses definite articles more often than English does.

Here:

• el mirlo = the blackbird
• la rama = the branch

This can mean a specific bird and branch in the situation being described, or simply present them as concrete, identifiable things in the scene.

English sometimes drops articles where Spanish keeps them, but in this sentence the articles sound completely natural and expected.

In Spain, mirlo normally refers to the common blackbird, especially the species very familiar in Europe.

So for a learner, it is best to understand mirlo as blackbird, but keep in mind that bird names do not always match perfectly across languages and regions.

In ordinary Spanish from Spain, mirlo is the standard everyday word for that bird.

No, Spanish word order is more flexible than English word order, but El mirlo se posó en la rama is the most neutral and natural version.

You could also hear other orders in context, for example:

• Se posó el mirlo en la rama
• En la rama se posó el mirlo

These alternatives change the emphasis or sound more literary or contextual. For a learner, the original order is the safest default:

• subject + verb + place

A broad pronunciation guide would be:

el MIR-lo se po-SÓ en la RA-ma

More roughly in IPA:

[el ˈmiɾlo se poˈso en la ˈrama]

A few useful points:

• r in mirlo and rama is a single tap, not a strong rolled rr
• z does not appear here, so there is nothing specifically Spain-only in the spelling
• the stress falls on mir-, -só, and ra-

Not if you want to mean perched on the branch.

Without se, posó would normally come from posar, not posarse, and that changes the meaning. It might suggest posed or placed something, depending on context.

So for a bird landing or perching, you need:

• El mirlo se posó en la rama

In this sentence, yes, rama means branch.

It can also appear in other meanings in different contexts, such as a branch of knowledge or activity, but with a bird and a tree-related image, branch is clearly the intended meaning.

So here there is no special complication:

• rama = branch