Murray, They do not mate with their brothers and sisters. If it is swallows that are messing up your lawnmower, just open a plastic garbage bag and use it to cover your lawnmower. When the swallows have raised their young and departed, throw the plastic cover away. Problem solved. If it is House Sparrows that are doing it, it is legal to kill that species of sparrow and remove their nest because they are not protected by laws that protect native birds. If you don't like killing, just do what I recommended with the cover on your lawnmower. With regard to swallows and swifts, which eat mosquitoes, flies and other small flying insects, no one will be very happy if those birds go extinct.