Barn Swallow populations have been and are continuing to decline due to the removal and deterioration of old rural man-made structures such as barns and sheds, and due to the application of agricultural pesticides that eliminate insects that are the swallow’s diet.
This significant decline has been noted in Ontario, Canada, and elsewhere where barn swallow populations were once more plentiful.
There are designs on the internet for Barn Swallow bird houses that you can build that may help in a small way reduce this decline, at least from a nesting opportunity perspective.
In terms of the pesticide problem you need to deal with the farmers, governments and manufacturers to solve this one…
Barn Swallows are listed as ‘Threatened’ in Ontario!