Перед дождём чайки летали ниже, а потом одна чайка села на пирс.

Because перед normally requires the instrumental case when it means before / in front of.

• дождь = rain
• Instrumental singular: дождём

So:

• перед дождём = before the rain

This is a very common pattern:

• перед домом = in front of the house
• перед уроком = before the lesson
• перед поездкой = before the trip

Also note the spelling/pronunciation:

• дождём is pronounced with ё
• In normal Russian writing, ё is sometimes written as е, but the pronunciation is still yo

In this sentence, перед дождём most naturally means before the rain or before the rainfall / before the storm started.

Russian often uses a noun phrase where English might use a clause. So instead of saying:

• before it started raining

Russian can simply say:

• перед дождём

It refers to the time just before the rain came.

Because the sentence is talking about seagulls in general at that moment, not just one bird.

• чайка = seagull (singular)
• чайки = seagulls (plural)

So:

• чайки летали ниже = the seagulls were flying lower

Later the sentence switches to one specific bird:

• одна чайка = one seagull / a single seagull

That is why you see both:

• чайки (plural)
• чайка (singular)

Because its subject, чайки, is plural.

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

• чайки летали = the seagulls flew / were flying
• plural subject → plural past tense verb летали

Compare:

• чайка летала = the seagull was flying
• чайки летали = the seagulls were flying

This is a very common learner question, because Russian has two different verbs related to flying:

• летать → летали
• лететь → летели

The difference is roughly this:

• лететь = to be flying in one direction, as a single movement
• летать = to fly around, to fly repeatedly, to fly in general, to be airborne in a less directional way

Here, чайки летали ниже suggests the seagulls were flying around lower than usual, not making one single directional flight from point A to point B.

So летали fits better than летели.

Ниже means lower.

It is the comparative form of низко (low):

• низко = low
• ниже = lower

Russian often forms comparisons without a separate word for than when the comparison is general or understood from context.

So:

• чайки летали ниже = the seagulls were flying lower

English often wants lower than before, but Russian can leave that unstated if it is obvious.

If Russian wants to say than something explicitly, it often uses чем:

• ниже, чем обычно = lower than usual

But here чем is unnecessary.

Both а and и can sometimes translate as and, but they are not used in exactly the same way.

• и simply adds information
• а often marks a transition, contrast, or shift

In this sentence:

• Перед дождём чайки летали ниже, а потом одна чайка села на пирс.

а потом feels like:

• and then
• but then
• then after that

It signals a shift from the general background scene (seagulls were flying lower) to the next event (one seagull landed on the pier).

Одна literally means one (feminine form), because чайка is feminine.

• masculine: один
• feminine: одна
• neuter: одно

Russian has no articles like a or the, so sometimes один/одна/одно can feel a bit like a single or one for emphasis.

Here одна чайка most naturally means:

• one seagull
• a single seagull

It helps single out one bird from the group.

Because its subject is одна чайка, and чайка is a feminine singular noun.

In the Russian past tense, singular verbs agree with the subject in gender:

• masculine: сел
• feminine: села
• neuter: село
• plural: сели

So:

• одна чайка села = one seagull sat down / landed

The feminine ending -а tells you the subject is feminine singular.

Both are possible, and that is normal.

The verb сесть literally means to sit down, but with birds it often naturally translates as:

• landed
• settled down

So:

• чайка села на пирс

can be understood as:

• the seagull sat down on the pier
• the seagull landed on the pier

In good English, landed on the pier is often the most natural translation here.

Because села на пирс describes movement onto a surface, not a location where something already is.

With на, Russian often distinguishes:

• куда? (where to?) → accusative
• где? (where?) → prepositional

So:

• на пирс = onto the pier
• на пирсе = on the pier

Here the bird is moving and coming to rest there:

• чайка села на пирс = the seagull landed on / onto the pier

Compare:

• Чайка сидела на пирсе. = The seagull was sitting on the pier.

It is in the accusative case.

The basic noun is:

• пирс = pier

In this sentence, the accusative form looks the same as the nominative because пирс is an inanimate masculine noun.

So:

• nominative: пирс
• accusative: пирс

That is why you see:

• на пирс = onto the pier

If it were location instead of motion, you would get:

• на пирсе = on the pier

Because the sentence presents two different kinds of action:

1. летали describes a background situation or ongoing activity
2. села describes one completed event

So:

• чайки летали ниже = the seagulls were flying lower
  
  • ongoing, descriptive, background
• одна чайка села на пирс = one seagull landed / sat down on the pier
  
  • single completed action

This is a very typical Russian aspect contrast:

• imperfective for ongoing/background action
• perfective for a completed event in the sequence

Yes, Russian could say it differently, because Russian word order is more flexible than English word order.

The given sentence is very natural:

• Перед дождём чайки летали ниже, а потом одна чайка села на пирс.

It presents the information in a clear flow:

1. time setting: Перед дождём
2. background action: чайки летали ниже
3. next event: а потом
4. specific subject + action: одна чайка села на пирс

A different order is possible, but it would slightly change the emphasis. For example:

• Одна чайка потом села на пирс.
• На пирс села одна чайка.

These are grammatical, but they highlight different parts of the sentence. The original version is the most neutral and natural for a learner to follow.