A Buddhist ceremony. it is mentioned in Masuji Ibuse's Hiroshima novel Black Rain:

[...] The Mass for Dead Insects was a rite performed on the day after the [harvest] festival, when farmers would make rice dumplings as an offering to the souls of the deceased insects they had inadvertently trodden on as they worked in the fields. [...]

Masuji Ibuse (1898-1993) was a Japanese novelist noted for his psychologically sharp but sympathetic short stories of ordinary people. He gained world fame with KUROI AME (1965, Black Rain), which drew its material from the atomic bombing of Hiroshima. The novel emerged as a significant work of art distinct from the many social or political accounts of the bombing.the lines in mausoleum deal with the horrors of the bombing of the cities of hiroshima and nagasaki on august 6th and 9th , 1945 by the americans, to break japan's resistance and collaboration with nazi-germany. even today the (remains of the) city of hiroshima is totally quiet and desolate (no birds, the sky is swollen black) and therefore this line taken from a novel about hiroshima is used in this song.