Утром на кормушку прилетели две маленькие птицы, и одна долго сидела там.

Утром is the common adverbial form meaning in the morning. It comes from утро (morning), but here it is in a special case form that Russian often uses for time expressions.

So:

• утро = morning
• утром = in the morning

You should learn утром, днём, вечером as very common time expressions.

Because the sentence describes movement toward the feeder.

Russian often uses:

• на + accusative = motion onto/to
• на + prepositional = location on/at

So:

• на кормушку прилетели = they flew to/onto the feeder
• на кормушке сидели = they were sitting on/at the feeder

In this sentence, the birds first arrived there, so на кормушку is correct.

A кормушка is a bird feeder, and Russian usually treats it as a surface or spot that birds come onto, not something they go inside.

So Russian naturally says:

• на кормушку = onto/to the feeder
• на кормушке = on/at the feeder

Using в would sound like going inside something enclosed, which is usually not the idea with a bird feeder.

Russian word order is much freer than English word order.

English strongly prefers:

• Two small birds flew to the feeder

Russian can also say that, but it is very natural to put the verb first:

• Прилетели две маленькие птицы

This often presents the event first: there arrived/flew in two small birds.

So the word order here sounds natural and slightly narrative: first the action happens, then we learn who did it.

Because the subject is две маленькие птицы — two small birds.

In the past tense, Russian verbs agree with the subject in number, and in the singular they also agree in gender.

So:

• прилетел = he flew/arrived by flying
• прилетела = she flew/arrived by flying
• прилетело = it flew/arrived by flying
• прилетели = they flew/arrived by flying

Since there are two birds, Russian uses the plural: прилетели.

Because птица is a feminine noun.

For two, Russian uses:

• два with masculine and neuter nouns
• две with feminine nouns

Examples:

• два дома = two houses
• два окна = two windows
• две птицы = two birds

So две маленькие птицы is correct because птица is feminine.

After два / две / три / четыре in the nominative, the noun usually appears in a form that is historically the genitive singular.

So in:

• две птицы

the noun птицы is the form used after две.

This is one of the patterns Russian learners simply have to get used to:

• одна птица
• две птицы
• пять птиц

The forms change depending on the number.

Numeral phrases in Russian are a bit tricky. After две, the noun is in the special form used with 2–4, but the adjective with a feminine noun is often in the plural form:

• две маленькие птицы

That is the normal, natural phrase here.

So even though number expressions in Russian do not work like English, you should learn the whole pattern as a chunk:

• две маленькие птицы = two small birds

Here одна means one (of them).

Since птица is feminine, Russian uses the feminine form:

• один = masculine
• одна = feminine
• одно = neuter

So одна here really means one bird or one of the two birds.

Because the subject of the second clause is одна — one [bird] — and that implied noun is feminine singular.

Past tense in Russian agrees with the subject:

• сидел = masculine singular
• сидела = feminine singular
• сидело = neuter singular
• сидели = plural

Since one bird is feminine singular, the sentence uses сидела.

Yes, this is a very typical aspect choice.

• прилетели is perfective: it presents the birds’ arrival as a completed event.
• сидела is imperfective: it describes an ongoing state or duration.

So the sentence means roughly:

• first, two birds arrived
• then, one of them sat there for a long time

This is exactly the kind of contrast Russian often makes:

• perfective for a completed event
• imperfective for an ongoing action or state

Долго means for a long time or a long time.

So:

• одна долго сидела там = one sat there for a long time

Russian often uses a simple adverb where English might use a longer phrase.

Compare:

• долго = for a long time
• недолго = not for long

Там means there.

Here it refers back to the feeder area already mentioned:

• на кормушку прилетели...
• ...и одна долго сидела там

So там means there, at/on the feeder.

Russian often uses там just like English uses there when the place is already understood from context.

Because и is joining two full clauses, each with its own predicate:

1. Утром на кормушку прилетели две маленькие птицы
2. одна долго сидела там

In Russian, when и connects two independent clauses, a comma is normally used.

So the comma is standard punctuation here.

Russian has no articles like a or the.

So Russian simply says:

• на кормушку = to the feeder / to a feeder
• две маленькие птицы = two small birds
• одна = one

English has to choose a or the, but Russian leaves that to context. In this sentence, the meaning makes it clear enough without articles.