These two queens emerged in roller cages in the incubator and have been calling back and forward to each other for two days.
The piping sound is made by vibrating the wings and is a territorial declaration.
If two virgin queens meet in a hive a fight to the death will ensue.
Some queens even pipe before emergence from the queen cell and in this case a free virgin may locate the cell and sting into it to kill the occupant.
Piping is also thought to be a signal which allows workers to keep queens confined within the cells to avoid fighting between the virgin queens.