Plauged by Bird Excrement

If the birds are swifts, then they are legally protected and you cannot interfere with them or their nests while they are using them. The nesting period is from about May until August and once the chicks have fledged and flown, then you can deal with removing the nests and filling in the space where the nest is located. If you don't block up the holes, they'll just return to the nest next year.