Though translation is out of scope, I thought that I as a Polish speaker might help. Marthaª and PLL are both correct as far as they go.

The original poem by Szymborska has (including the lines immediately preceding and following): 

> obwód w biodrach dwa palce szarada i szyfr,  
> **w którym słowiczku mój a leć, a piej**  
> oraz uprasza się zachować spokój,

Literally translated, this would be 

> hip measurement two fingers charade and cipher,  
> **in which my dear-little-nightingale fly and sing**  
> and are advised to remain peaceful

(Where "dear-little-nightingale" represents my crude attempt at the Polish *słowiczku*, a diminutive of *słowik*, nightingale.)

A person well-read in Polish would know immediately that this a direct quotation from the poem "Do [Bohdana Zaleskiego][1]" by [Adam Mickiewicz][2], Poland's greatest poet. It was written in 1841 and uses rather old-fashioned but evocative language. 

> **[Słowiczku mój! a leć! a piej!][3]**  
> Na pożegnanie piej  
> Wylanym łzom,  spełnionym snom,  
>  Skończonej piosnce twej!  

or 

> **My dear nightingale! Fly! Sing!**   
> In farewell sing  
> Outpoured tears, fulfilled dreams,   
> Your finished song! 

I do not know the exact intent of the poet, but it was written during the [Great Polish Emigration to the west of Europe][4], a time of tremendous artistic and cultural growth for Poles (think Chopin and Curie) despite their lack of an independent nation. Therefore, its import is somewhat 'patriotic', though in a uniquely Polish sense. The phrase is therefore a classic line by a very famous poet about a joyous bird. The translation mentioned in Marthaª's answer used an equivalent classic English poetic line by Shelley and really did a good job in my opinion. 

(The next line is also a bit of a quotation, but for the common phrase "Please remain calm".)

Regarding "blithe" and "trostle":
-

I can only suspect that the translator who used "trostle" was looking for an antique-feeling equivalent to "nightingale" since the Polish language was similarly old-fashioned. He may have used "blithe" to evoke the Shelly quote. The original Polish does not specifically refer to "blithe", but the mood of the Polish poem is definitely joyous!


  [1]: http://en.wikipedia.org/wiki/J%C3%B3zef_Bohdan_Zaleski
  [2]: http://en.wikipedia.org/wiki/Adam_Mickiewicz
  [3]: http://pl.wikisource.org/wiki/Do_Bohdana_Zaleskiego
  [4]: http://en.wikipedia.org/wiki/Great_Emigration
  [5]: http://en.wikipedia.org/wiki/Partitions_of_Poland#.22Fourth_Partition.22\