Translation:Red birds are singing in the tall trees.
(1) The English sentence with "the" in it is a perfectly normal and idiomatic sentence.
(2) The original Hungarian sentence has 'a' in it, which is almost always translated as 'the'.
(3) It is also possible to write the Hungarian sentence without using 'a', if that was what was intended.
(4) The Hungarian sentence here is an isolated sentence in a learning exercise. There is no further 'context' to which to appeal when translating it.
I hope this answers your question.