Зимой у окна стоит кормушка, и на ней часто сидят маленькие птицы.

Зимой is the instrumental case of зима. In Russian, names of seasons are very often put in the instrumental to mean in / during that season.

So:

• зимой = in winter
• летом = in summer
• весной = in spring
• осенью = in autumn

This is a very common time expression. Russian can also say в зимнее время, but зимой is the normal, simple way.

У окна means by the window / near the window.

The preposition у normally takes the genitive case, so:

• окно = window
• у окна = by the window

A learner often expects something more like около окна or возле окна, and those are also possible. But у окна is very common and natural.

So the pattern is:

• у дома = by the house
• у двери = by the door
• у окна = by the window

Russian often uses verbs like стоять (to stand), лежать (to lie), and висеть (to hang) when talking about where something is located.

So:

• книга лежит на столе = the book is lying on the table
• лампа висит над столом = the lamp is hanging above the table
• кормушка стоит у окна = the feeder is standing by the window

In English, we often just say is, but Russian prefers a more physical description of position.

Also, Russian usually does not use a present-tense verb meaning is the way English does. So instead of a simple there is, Russian often expresses existence/location through word order or verbs like стоит.

Yes, depending on the kind of feeder and where it is.

• стоит suggests the feeder is standing somewhere, perhaps on a windowsill or support.
• висит would suggest it is hanging.

So this sentence gives you a mental image: the feeder is placed by the window in an upright position, not hanging.

Кормушка means feeder, especially a bird feeder in this context.

It comes from the idea of feeding:

• корм = feed, food for animals
• кормить = to feed
• кормушка = a feeding container / feeder

It is a feminine noun, which matters later in the sentence when you get на ней.

After many prepositions, Russian third-person pronouns add an initial н-.

So:

• она = she / it
• на ней = on her / on it
• not на ей

Other examples:

• у него = by him / he has
• с ней = with her
• для них = for them

Here ней refers back to кормушка, which is feminine, so на ней means on it.

Here ней is in the prepositional case, because на is being used to mean on in a location sense.

Compare:

• на ней сидят птицы = the birds are sitting on it
  (location → prepositional)

But if there were movement onto it, Russian would use the accusative:

• птицы сели на кормушку = the birds sat down onto the feeder

So:

• where? → на ней / на кормушке
• onto what? → на неё / на кормушку

Часто means often, and its placement here is very natural in Russian.

• на ней часто сидят маленькие птицы

This puts the emphasis on frequency: birds often sit there.

Russian word order is more flexible than English, so you may also hear:

• маленькие птицы часто сидят на ней
• часто на ней сидят маленькие птицы

But the sentence as given sounds natural and neutral.

In Russian, birds can сидеть when they are sitting or perched somewhere.

So:

• птицы сидят на ветке = the birds are sitting/perching on the branch
• птицы сидят на кормушке = the birds are sitting/perching on the feeder

In English, sit may sound a little human in some contexts, but in Russian сидеть is completely normal for birds resting on something.

Because the subject is маленькие птицы = small birds, which is plural.

The infinitive is сидеть. Present tense forms include:

• я сижу = I sit
• ты сидишь = you sit
• он/она сидит = he/she/it sits
• они сидят = they sit

So сидят agrees with птицы.

Because the adjective must agree with the noun in gender, number, and case.

Here:

• птицы is plural nominative
• so the adjective must also be plural nominative
• маленький becomes маленькие

Compare:

• маленькая птица = a small bird
• маленькие птицы = small birds

Because птицы is the subject of сидят.

In the second clause:

• маленькие птицы = the subject
• сидят = the verb

So nominative is exactly what you expect here.

Russian, like English, often uses a pronoun to avoid repetition.

So instead of:

• ...и на кормушке часто сидят маленькие птицы

it says:

• ...и на ней часто сидят маленькие птицы

Both are possible. Using ней sounds smoother because кормушка was just mentioned.

The basic meaning would stay the same if you changed the word order a bit, but the emphasis would shift.

The given sentence:

• Зимой у окна стоит кормушка, и на ней часто сидят маленькие птицы.

This sounds natural and descriptive: first the setting, then the feeder, then what happens on it.

Other possible orders:

• Зимой кормушка стоит у окна, и маленькие птицы часто сидят на ней.
• У окна зимой стоит кормушка, и на ней часто сидят маленькие птицы.

Russian word order is flexible, but the original sentence is a very normal, neutral version.

Yes. The comma is there because the sentence joins two clauses:

• Зимой у окна стоит кормушка
• и на ней часто сидят маленькие птицы

Each clause has its own verb:

• стоит
• сидят

So the comma before и is standard Russian punctuation here.