Büyük bir kuş sürüsü geçti.

In Turkish, when bir means a/an, it commonly comes after an adjective:

• büyük bir ev = a big house
• uzun bir yol = a long road

So Büyük bir kuş sürüsü is the normal way to say a big flock of birds.

If you said bir büyük kuş sürüsü, it would sound more like one big flock of birds, with extra emphasis on one.

Here, bir mainly means a/an.

So Büyük bir kuş sürüsü means a big flock of birds, not especially one big flock of birds.

Turkish bir can mean both a/an and one, so context decides. In this sentence, it is functioning like the English indefinite article.

Kuş sürüsü means flock of birds.

This is a very common Turkish structure called an indefinite noun compound. It is built like this:

• kuş = bird
• sürü = herd, flock, group of animals
• kuş sürüsü = bird flock / flock of birds

So the first noun tells you what kind of flock it is.

Compare:

• koyun sürüsü = flock/herd of sheep
• balık sürüsü = school of fish
• kuş sürüsü = flock of birds

Because in this compound, the second noun takes a special ending.

• base noun: sürü
• in the compound: sürüsü

This -sı / -si / -su / -sü ending is typical in this kind of noun phrase:

• kuş sürüsü
• araba kapısı = car door
• ev kapısı = house door

The exact form depends on vowel harmony. Since sürü has front rounded vowels, the ending becomes -sü.

Because sürü already ends in a vowel, Turkish inserts -s- before the ending:

• sürü + sü -> sürüsü

Because Turkish usually puts the verb last.

So the sentence structure is:

• Büyük bir kuş sürüsü = subject
• geçti = verb

This is very normal Turkish word order. A very literal English-like breakdown would be:

• A big flock of birds passed.

Turkish often follows Subject + Object + Verb, and with an intransitive verb like this, it is simply Subject + Verb.

Geçti is the simple past of geçmek.

• geçmek = to pass, to go by, to pass through
• geçti = passed / went by

This past tense is often called the definite past in Turkish, because it is commonly used for events the speaker presents as a clear, completed fact.

Here, geçmek means something like:

• to pass
• to go by
• to pass overhead / pass through the area

So Büyük bir kuş sürüsü geçti suggests that a big flock of birds passed by, probably through the sky or across the speaker’s field of view.

The verb geçmek is very broad in Turkish, and its exact English translation depends on context.

Because the grammatical subject is kuş sürüsü — a flock of birds — and that whole phrase is singular.

So Turkish treats it as one flock, not as many individual birds.

That is why the verb is singular:

• Büyük bir kuş sürüsü geçti. = A big flock of birds passed.

If the subject were simply kuşlar (the birds), then the sentence could be different.

Yes, that is also possible, and it means nearly the same thing: A big flock of birds passed.

There is a small structural difference:

• kuş sürüsü = a flock of birds using a noun compound
• bir sürü kuş = literally a flock/group/lot of birds

Both are natural, but kuş sürüsü feels a bit more like the fixed expression flock of birds.

Yes. The dictionary form is büyük with a lowercase b.

It is capitalized here only because it is the first word of the sentence:

• büyük = big
• Büyük bir kuş sürüsü geçti.

A rough pronunciation guide:

• Büyük ≈ by-EEK
• bir ≈ beer
• kuş ≈ koosh
• sürüsü ≈ syu-RYU-syu
• geçti ≈ GECH-tee

A few useful sound notes:

• ü is a front rounded vowel, like the u sound in French tu or German über
• ş = sh
• ç = ch

So the whole sentence sounds roughly like:

by-EEK beer koosh syu-RYU-syu GECH-tee

Turkish often allows subjects to be omitted when they are understood, especially pronouns. But in this sentence, the subject is the important new information:

• Büyük bir kuş sürüsü geçti.

So it is naturally stated.

You could not really leave it out here unless the flock had already been mentioned and the context made it obvious.