Distinguimos la especie por el plumaje.

Yes. Distinguimos can mean either:

• we distinguish / we identify / we tell apart (present)
• we distinguished (preterite)

For -ir verbs, the nosotros form is often identical in the present and the preterite.

So in isolation, Distinguimos la especie por el plumaje is ambiguous. Context tells you which meaning is intended:

• Normal/general statement: We distinguish the species by the plumage.
• Past event: We distinguished the species by the plumage.

Without any past-time context, learners will usually understand it as a present statement.

Because the verb is distinguir, and in the present tense it is a stem-changing verb: e → i in most forms:

• distingo
• distingues
• distingue
• distinguimos
• distinguís
• distinguen

The important point is that in the present tense, nosotros and vosotros usually do not show the stem change. So you get:

• distingue but distinguimos
• not distinguemos

This is very common with stem-changing verbs.

Spanish often leaves subject pronouns out because the verb ending already tells you who the subject is.

Here, distinguimos already means we distinguish / we distinguished, so nosotros is unnecessary unless you want emphasis or contrast.

Compare:

• Distinguimos la especie por el plumaje. = neutral, natural
• Nosotros distinguimos la especie por el plumaje. = emphasizes we

For example, you might use nosotros if you want to contrast with someone else:

• Ellos se fijan en el canto, pero nosotros distinguimos la especie por el plumaje.

Yes. Especie is a feminine noun, so it takes la:

• la especie
• una especie

In Spanish, nouns ending in -e can be masculine or feminine, so you have to learn the gender with the word. The ending -e does not reliably tell you the gender.

So:

• el animal
• la especie

Because la especie is a thing/category, not a person.

The personal a is normally used before a specific human direct object, and sometimes with pets or personified animals. But here la especie means the species, so no a is needed:

• Distinguimos la especie por el plumaje.

Compare:

• Vemos al profesor. = we see the teacher
• Vemos la especie. = we see/identify the species

So the lack of a is completely normal here.

Here por means something like by, from, or on the basis of. It introduces the criterion or means used to identify the species:

• por el plumaje = by the plumage / from the plumage

That is why por works well.

Para usually expresses purpose, destination, or in order to, so it would not sound right here.

Compare:

• Distinguimos la especie por el plumaje. = We identify the species by its plumage.
• Estudiamos el plumaje para distinguir la especie. = We study the plumage in order to identify the species.

So:

• por = basis/means
• para = purpose

Spanish often uses the definite article where English would use a possessive, especially when the possessor is obvious from context.

Here, el plumaje sounds natural because the sentence is talking about plumage as the identifying feature:

• Distinguimos la especie por el plumaje.

This feels like we identify the species by the plumage / by plumage.

You could also say por su plumaje, but that usually sounds a bit more explicit or specific:

• Distinguimos esa especie por su plumaje.

So both are possible, but el plumaje is very natural when you mean the characteristic in general.

Not exactly.

• plumas = feathers as individual feathers
• plumaje = plumage, the bird’s feathering as a whole, especially its overall appearance, colouring, or pattern

So in a sentence about identifying a species, plumaje is the better word because you usually identify birds by their overall feather pattern, not by individual feathers.

For example:

• El ave tiene plumas largas. = The bird has long feathers.
• Su plumaje es muy llamativo. = Its plumage is very striking.

Distinguimos is pronounced roughly:

• dees-teen-GHEE-mos

In gui, the u is normally silent in Spanish, unless it has a diaeresis: güi.

So:

• gui = hard g
  • i
• gue = hard g
  • e

That is why distinguir and distinguimos keep the hard g sound.

If Spanish wrote gingimos, it would sound very different, with the soft Spanish j-like sound. The spelling gui prevents that.