Thanks for your replies guys.
The problem is growing, and growing.
In fact it's the first time that we are having this problem with the swallows, and I don't want to get radical and pickup the shotgun to solve the issue.
The nests are far away from the apiary, and it's forbidden to destroy their nests or eggs..
I already noticed that they come from far away, to feed their juveniles from the bees.
Today I saw about eight different swallows attacking the bees.
I'm going to try to find the plastic owls or plastic falcons.
The idea of building a roof with birds net it's really nice, the problem being the extension of the structure. With ten hives, the area it's already a bit large.
But we must do something about them.
Thanks for your help.