Andrew Bird’s music is often poetic and melancholy, made light by his soaring violin lines and remarkably deft whistling. But his latest album, “Are You Serious,” takes a more somber direction. On “Puma,” he reflects on the radiation therapy his wife was given after she had been diagnosed with thyroid cancer. She had to be isolated from Bird and their young son for at least seven days while her body was radioactive from the treatment. Atul Gawande, a staff writer for The New Yorker and a practicing surgeon, found the song so beautiful that he plays it in the operating room. They spoke about songwriting, confronting illness, the nature of cancer, and whistling at this year’s New Yorker Festival.