El pico y el ala parecen pequeños.

Because together they make a compound subject: el pico y el ala = the beak and the wing.

In Spanish, when two singular nouns are joined by y (and), the verb normally goes in the plural.

• El pico parece pequeño = The beak seems small.
• El ala parece pequeña = The wing seems small.
• El pico y el ala parecen pequeños = The beak and the wing seem small.

So parecen is the third-person plural present of parecer.

Because the adjective is describing both nouns together, and Spanish uses these agreement rules:

• If the group is plural, the adjective must be plural.
• If the group contains at least one masculine noun, the adjective usually becomes masculine plural.

Here:

• pico is masculine
• ala is feminine

Since the two nouns are mixed gender, Spanish uses the masculine plural form: pequeños.

That is why you get:

• el pico pequeño
• el ala pequeña
• but el pico y el ala pequeños

In this sentence, because the adjective comes after parecen, it is still agreeing with the whole subject: pequeños.

Because ala is a feminine noun that begins with a stressed a sound. In Spanish, singular feminine nouns like that often use el instead of la for pronunciation reasons.

So you say:

• el ala
• el agua
• el águila

This avoids the awkward repetition of two a sounds together.

But the noun is still feminine. The article changes in the singular, not the gender of the noun.

You can tell because other words around it still behave as feminine when appropriate.

For example:

• el ala pequeña = the small wing
• las alas pequeñas = the small wings

So even though the singular article is el, the adjective in the singular is still feminine: pequeña, not pequeño.

In your sentence, pequeños is not masculine because of ala. It is masculine because the adjective agrees with the mixed plural group: el pico y el ala.

In Spanish, it is very normal to repeat the article before each noun when talking about two specific things.

So el pico y el ala sounds natural and clear.

If you said el pico y ala, that would usually sound incomplete or unnatural in standard Spanish unless there were a special reason.

Repeating the article helps show that you are talking about:

• the beak
• and the wing

as two distinct nouns.

Parecen comes from parecer, which means to seem or to appear.

So:

• parecen pequeños = they seem small
• son pequeños = they are small

This is an important difference:

• parecen pequeños gives an impression or appearance
• son pequeños states it as a fact

So the sentence is slightly softer or less definite than one with ser.

Because here pequeños is not being used as a noun modifier inside the subject. It is being used as a predicate adjective after the verb parecer.

Structure:

• El pico y el ala = subject
• parecen = verb
• pequeños = adjective describing the subject

This is similar to English:

• The beak and the wing seem small

You would not normally say the equivalent of the small-seeming beak and wing here, because that would express something slightly different.

Yes, grammatically you could. It would just mean something different.

• parecen pequeños = they seem small
• son pequeños = they are small

So both are correct Spanish, but they are not interchangeable in meaning.

If you want to talk about appearance or first impression, use parecen. If you want to describe them as actually small, use son.

In standard peninsular Spanish, parecen is pronounced roughly like pa-REH-then, because c before e is usually pronounced like the th in think.

So in Spain:

• parecen ≈ pa-REH-then

In much of Latin America, it would sound more like pa-REH-sen.

Also, the y in y el is usually pronounced like ee, so that part sounds like ee el.

So the whole phrase in Spain sounds approximately like:

el PEE-ko ee el A-la pa-REH-then peh-KEH-nyos

It describes both items.

The idea is that:

• the beak seems small
• and the wing seems small

Spanish uses one plural adjective to cover both nouns together.

So the sentence does not mean that the pair, taken as one object, seems small. It means each of the two things in the subject is included in that description.