Swallows migrate north in the spring, and return to the same spot where they nested in the prior year. They build sturdy nests from mud. Nests and birds are protected, so they cannot be destroyed until the birds have migrated south.
The work should be completed well before the birds start to return in the spring.
Be Careful
The birds and nests are protected. They occasionally defeat systems by moving further down the building. The goal should be to exclude them from the shaded areas.