No espantes al petirrojo.

Because this is a negative command addressed to tú.

In Spanish, the negative tú imperative uses the present subjunctive form:

• espantar → espantes

So:

• Espanta al petirrojo. = Scare the robin.
• No espantes al petirrojo. = Don’t scare the robin.

That change is very common in Spanish:

• Habla → No hables
• Come → No comas
• Escribe → No escribas

It is addressed to one person informally: tú.

So No espantes al petirrojo means Don’t scare the robin when speaking casually to one person.

Other possible versions are:

• No espante al petirrojo. = formal singular (usted)
• No espantéis al petirrojo. = informal plural in Spain (vosotros)
• No espanten al petirrojo. = formal plural (ustedes)

Since you asked for Spanish from Spain, vosotros forms are especially relevant.

Because Spanish often uses the personal a before a specific animate direct object, including animals when they are treated as individual living beings.

Here, petirrojo is a specific bird, so Spanish says:

• No espantes al petirrojo.

not:

• No espantes el petirrojo.

Also, al is just a contraction of:

• a + el = al

So literally it is:

• No espantes a el petirrojo → No espantes al petirrojo

You see the same thing in sentences like:

• Veo al perro. = I see the dog.
• No molestes al caballo. = Don’t bother the horse.

No. The word is grammatically masculine, but grammatical gender does not necessarily mean biological sex.

So el petirrojo is the normal dictionary form for robin, regardless of whether you know the bird is male or female.

If you specifically wanted to talk about a female robin, you would usually make that clear in some other way, for example:

• la hembra del petirrojo = the female robin

For normal use, though, el petirrojo just means the robin.

Espantar means something like to scare, frighten, startle, or even scare away, depending on context.

With an animal or bird, it often suggests causing it to become frightened and possibly run or fly off. So in this sentence, espantar fits very naturally.

A close alternative would be:

• No asustes al petirrojo.

That also means Don’t scare the robin, but espantar can sound especially suitable when talking about animals being frightened away.

Normally, no.

Here petirrojo is a common noun meaning robin, so it needs the article:

• al petirrojo

Without the article, it sounds incomplete or wrong unless Petirrojo were being used as a name or title.

So the natural form is:

• No espantes al petirrojo.

Yes. In Spain, petirrojo is the standard word for the European robin.

It is built from:

• pecho / peti- related to chest/breast
• rojo = red

So the word is roughly connected to the idea of red-breast.

That said, learners do not need to analyse it every time; you can simply learn petirrojo as the normal noun for robin.

A simple pronunciation guide would be:

• no es-PAN-tes al pe-ti-RRO-ho

A few useful points:

• espantes is stressed on PAN
• petirrojo is stressed on RRO
• the rr in petirrojo is a rolled or trilled r
• the j in rojo sounds like the Spanish j, a throaty sound, not like English j

So in Peninsular Spanish it sounds quite different from how an English speaker might first guess from the spelling.