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.