Swallows are in decline, in 2014 they were declared the SEO bird of the year, rather than being cause for celebration this means that things are going badly for the species.
The reasons for the decline are many, the change in agricultural practices and the increased use of pesticides that affect the insects on which the swallows depend as a food source. The migratory journey that swallows make each year is difficult, and they reproduce a lot to compensate for losses during migration. The destruction and lack of availability of nesting sites affects reproduction.
You can help by creating an artificial nest and providing a protected place where the swallows can nest. Swallows often return to the same nest year after year. They prefer protected sites in semi-rural places, if you can offer a place that is protected from rain and wind, in the shelter of a barn or a yard perhaps you could encourage the swallows to nest.