Every year there is, a natural phenomenon occurs in a park in the German city of Wiesbaden.

When talking about phenomenons, we usually use the verb "occur".

Three thousand starlings arrive from several parts ofall across the country, rest for a few days in the chestnut trees of the park, then gather and set off for the south where they spend the German winter.

The original sentence was good, but I think rephrasing it this way makes it feel more natural.

Three thousand starlings arrive from several parts of the country, rest for a few days in the chestnut trees of the park, gather and set off for the south where they spend the German winter.

'set off for the South' sounds like a destination.

The city has to clean the children playground every day, which is expensive.

That's why the city tried to scare away the starlings by playing sound recordings of birds of prey, hanging balloons with badscary faces in the trees, and other ways, but thatose methods didn't work.

Now there is a city employee in the park who sets off firecrackers whenever the starlings preparetry to sit in the tops of the chestnut trees.

I don't understand it.

Not necessary, just a personal preference to sound more natural

Three thousand starlings arrive from several parts of the country, rest for a few days in the chestnut trees of the park, gatherand then gather again and set off for the south, where they spend the German winter.

Your grammar was fine, but the last phrase is slightly confusing. There are several ways to change it. Likewise, the comma is not required, but really helps to separate the ideas and give the reader more breathing room.

It is an interesting showevent, but with a small disadvantagenegative aspect.

"Show" is typically used for something like a play or theater. "Disadvantage" implies some sort of competition here, as if the people are disadvantaged in contrast to others.

It contradicts withruins the sense of cleanliness of the people who live close to the park.

The city has to clean the children playground every day, which is expensive.

That's why the city tried to scare away the starlings by playing sound recordings of birds of prey, hanging balloons with badscary faces in the trees, and other ways, but that didn'none of it worked.