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" by Adam Mickiewicz, Poland's greatest poet. It was written in 1841 and uses rather old-fashioned but evocative language.
Słowiczku mój! a leć! a piej!
Na pożegnanie piej
Wylanym łzom, spełnionym snom,
Skończonej piosnce twej!
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, 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 of the original is 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!