What does "trostle" mean?

In the poem Pi, by Wislawa Szymborska, there is this line:

in which we find how blithe the trostle sings!

A Google search for trostle turns up a few hits, mostly as people's last names. Urban Dictionary [nsfw] has two definitions, both of which were voted down. And Webster's Online Dictionary says it's a misspelling of throstle.

I understand that poets are licensed to make up words, but this poem is otherwise very plain. In fact, this whole line seems out of place.

Can anyone explain the author's meaning?

Solution 1:

Blithe = cheerful, carefree; and trostle = misspelling of throstle = a type of thrush = a songbird. So, given just this line, I think it's meant to be interpreted pretty literally:

in which we find [out] how blithe[ly] the [thrush] sings!

Edit: Found a different translation which clearly uses "bird" to translate the Polish word:

[...] a charade, a code,
in which we find "hail to thee, blithe spirit, bird thou never wert"¹
alongside "ladies and gentlemen, no cause for alarm"

_{1 This is a quotation from Shelley's Ode to a Skylark. I don't know if this reference exists in the original poem, or whether it's something added by the translator, but in any case, it's clear that this part of the poem has something about a bird.}

Solution 2:

A native or expert Polish speaker is clearly what we need here! I’m not one, but my best attempt: the original has

w którym słowiczku mój a leć

and the word corresponding to trostle seems to be słowiczku, a diminutive form of słowik, which online dictionaries tell me is nightingale. The nightingale is a species of thrush (roughly — there are some ornithological hairs that could be split here).

Diminutives in Polish (and other Slavic languages) are notoriously hard to translate. They sometimes just indicate familiarity or smallness; sometimes, they have more specific connotations, or may carry echoes of particular well-known poems or fairy-tales; a few have even evolved specific meanings, more distinct from the base words.

I don’t know which of these słowiczka is — whether it sounds old-fashioned to a Polish ear, or playful, or whether it’s even a reference to some other species similar to the nightingale. (Actually I guess not this latter, since it doesn’t appear on Polish Wikipedia.) But it’s certainly some kind of nightingale, thrush or similar bird, and it’s certainly a moderately unusual word for it (słowiczka gets about 9,000 google hits), so throstle seems like a reasonable translation, and trostle a misspelling or variant spelling of that.