Question: when did the warblers first appear on glee
[{'answer': 'April 19 , 2011', 'provenance': [{'wikipedia_id': '31273376', 'title': 'Glee: The Music Presents the Warblers', 'start_paragraph_id': 13, 'start_character': 508, 'end_paragraph_id': 13, 'end_character': 523, 'bleu_score': 1.0, 'section': 'Section::::Promotion.\n'}]}, {'provenance': [{'wikipedia_id': '31273376', 'title': 'Glee: The Music Presents the Warblers', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 840, 'bleu_score': 0.9669266300094901, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 17038214381427647140}, 'section': 'Section::::Abstract.'}]}]