Not only are insects their only food, they have to take /catch them 'on the wing' ie, while actually flying ! No good breeding insects and then trying to feed them to the birds ! It just does not work !
A staggering fact about swallows is that the chicks cannot survive more than 24 hours without food unlike many other species.
When it rains and there are no insects flying around it has been discovered that a swallow will fly from the south of England across the sea as far as Holland to return with a mouthful of insects for them - mindboggling!
That is mind boggling Khandro and shows what wonderful creatures they really are. None in my neck of the woods yet, but I'm hoping they will appear any time now, fingers crossed.