For the second year in a row, a record number of migrating birds were killed by flying headfirst into the Big Apple’s glassy and nearly invisible skyline. At least 1,250 warblers, flickers and more were lured into their deaths during their annual fall pilgrimage, smashing last year’s record that logged the bloodiest year yet, The Post has learned. But the true death toll is likely much higher, after an unusually heavy migration season sent thousands more birds through the five boroughs than in