Meal worms may be purchased at most pet stores and sometimes at fishing bait places. They are available in different sizes -- small, medium, large, and extra large.
The birds seem to prefer the medium sized ones especially when they are feeding their young. Don't get the extra large ones.
To grow your own meal worms, purchase a hundred or so, more if you want to start feeding some to the birds right away.
Place the tiny critters in a trash can, lard stand, or old washtub, etc. which has been filled with two gallons of bran, the type used to feed farm animals, not from the grocery store, and two pounds of meal (plain, no salt or additives).
Cut two or three medium apples or potatoes or both in half and lay open side down on top of the meal. These supply the necessary moisture for the worms and should be replaced or replenished every week or two.
Fold a piece of worn bath towel and place it on top of everything. Don't add water or dampen the towel because it would probably mildew.
When the worms are ready to be fed to the birds, they will crawl up between the bath towel for picking.
Accommodating little creatures, aren't they?
The worms develop in three stages. When the worms you buy (approximately an inch in length) crawl up into the towel they actually are preparing to turn into pupas and will become immobile. From pupas they turn into black beetles (and no, they don't fly out but they do crawl around like crazy). The beetles then lay eggs which hatch into tiny meal worms starting the cycle all over again.
No need for a lid, though, because they can't climb up the slick surface.