El águila es un ave rapaz.

Because águila is grammatically feminine, but in Spanish, some feminine nouns that begin with a stressed a- or ha- sound use el instead of la in the singular to make pronunciation smoother.

So:

• el águila
• el agua
• el hambre

But the noun is still feminine. You can see that from other words that agree with it:

• el águila blanca
• el águila herida
• esta águila is usually avoided in careful standard Spanish; speakers normally restructure it, for example esta ave or esta águila imperial may be heard, but the article rule is the clearest sign.

In the plural, the normal feminine article returns:

• las águilas

So águila is feminine, even though it takes el in the singular.

For the same reason as el águila.

Ave is a feminine noun, but because it begins with a stressed a- sound, Spanish uses the masculine-looking singular article un instead of una:

• un ave
• un águila

This is only with the singular article. The noun itself is still feminine, so adjectives remain feminine:

• un ave rapaz
• un ave hermosa
• un águila grande

And in the plural:

• unas aves
• las aves

So un here does not mean the noun has become masculine.

Because rapaz is an adjective with the same form for masculine and feminine.

Some Spanish adjectives change:

• blanco → blanca
• herido → herida

But others do not change for gender:

• rapaz
• grande
• fácil

So:

• un ave rapaz
• un pájaro rapaz

Both use rapaz unchanged.

Only the number changes:

• un ave rapaz
• unas aves rapaces

Because es is used for classification, identity, and general characteristics.

In El águila es un ave rapaz, the sentence is saying what an eagle is: it belongs to the category of birds of prey. That calls for ser, not estar.

Use ser for things like:

• Es un ave rapaz.
• Es grande.
• Es un animal salvaje.

Use estar for states, conditions, or location:

• El águila está herida.
• El águila está en el nido.

So here, es is correct because this is a defining fact, not a temporary state.

Ave is a general noun meaning bird, usually in a slightly more formal or scientific style than pájaro.

So in this sentence:

• águila = eagle
• ave rapaz = bird of prey / raptor

A learner may wonder why Spanish uses both águila and ave in the same sentence. The reason is that the sentence is classifying the eagle:

• an eagle is a kind of bird
• more specifically, a bird of prey

This is very natural in Spanish.

The accent mark shows where the stress goes: Á-gui-la.

Without getting too technical, the stressed syllable is the first one:

• Águila

That written accent is important because it helps show pronunciation and also helps explain why the article changes to el: the noun begins with a stressed a- sound.

Also note that in gui, the u is not pronounced separately here, so águila sounds roughly like:

• AH-gee-la

not AH-goo-ee-la

A simple pronunciation guide is:

el AH-gee-la es oon AH-beh ra-PAHTH

In most of Spain, z is pronounced like the th in think, so rapaz sounds like ra-PAHTH.

A few useful details:

• el águila: the l at the end of el links smoothly into águila
• un ave: the n links into the next word
• rapaz: stress on the last syllable, -paz

If you speak Latin American Spanish, the last word would usually sound more like ra-PAS instead.

Because in Spanish, this type of adjective normally comes after the noun.

So the natural order is:

• ave rapaz

not:

• rapaz ave

Also, rapaz is describing the kind of bird it is, so post-noun position is the normal one.

Spanish adjective position can sometimes vary, but with classifications like this, the standard order is:

• noun + adjective

So un ave rapaz is the natural and correct phrase.

Yes, in a normal standalone sentence like this, both articles are natural.

• El águila = the eagle, meaning the species or the eagle as a representative example
• un ave rapaz = a bird of prey

This pattern is very common in Spanish when giving definitions or general truths:

• El perro es un mamífero.
• La ballena es un mamífero.
• El águila es un ave rapaz.

English often does something similar, but article use does not match exactly between the two languages, so it is best to learn the Spanish pattern as its own structure.

Here it most naturally refers to the species or to the eagle as a general type of animal.

Spanish often uses the singular definite article to talk about a whole class:

• El león es un mamífero.
• La jirafa tiene el cuello largo.
• El águila es un ave rapaz.

So this does not necessarily mean one particular eagle you can point at. It often means the eagle as a species / in general.

In the plural, the special article change disappears, and the normal feminine forms are used:

• Las águilas son aves rapaces.

Notice all the agreement:

• las = feminine plural article
• águilas = plural noun
• aves = plural noun
• rapaces = plural adjective

This is a good way to confirm that both águila and ave are feminine nouns, even though in the singular they use el and un.