Give some stream of consciousness and then the answer. Schubert died at age 31, but was extremely prolific during his lifetime. His output consists of over 600 secular vocal works (mainly Lieder), seven complete symphonies, sacred music, operas, incidental music and a large body of chamber and piano music.

Answer this question: what type of song did schubert specialize in?
The relevant information is: Schubert died at age 31, but was extremely prolific during his lifetime. His output consists of over 600 secular vocal works (mainly Lieder), seven complete symphonies, sacred music, operas, incidental music and a large body of chamber and piano music.
Therefore, the answer is Lieder.