Learn Portuguese (Portugal): Ambos os pássaros são bastante dóceis.

In Portuguese, the quantifier ambos (both) must always be followed by a definite article (o, a, os, as) when it comes directly before a noun. You cannot say ambos pássaros; it must be ambos os pássaros (literally, "both the birds").

Why didn't bastante change to bastantes to match the plural pássaros?

In this sentence, bastante is acting as an adverb meaning "quite" or "fairly" because it modifies the adjective dóceis. Adverbs never change for gender or number. If it were acting as a quantifier for a noun (like "many birds"), it would change to the plural: bastantes pássaros.

Why use são instead of estão when describing how the birds act?

We use são (from the verb ser) because being docile is described here as an inherent, permanent trait of these birds' temperaments. If you used estão (from estar), it would imply they are just acting calm temporarily right now.

What is the singular form of dóceis, and how does this spelling change work?