Bluebirds are primarily insect eaters in the spring and summer. When their babies hatch bluebirds know instinctively that mealworms are too big for newly hatched babies. They start them out with tiny bugs for their young. While the parent bird may appreciate your easy mealworm breakfast during a stressful time, they will not feed regular sized mealworms to their chicks until the babies are about six days old.
So when the chicks are new and tiny, they feed them whole, fresh, soft, small, larvae and spiders. They don't regurgitate food. As the chicks grow, they gradually increase the size and toughness of the insects they offer.
The rule of thumb is to offer about 15 mealworms per bluebird once or twice a day, as a supplemental food, unless severe weather conditions limit natural sources. A hundred or so worms offered morning and evening would be adequate for a pair with a box of nestlings a week old. Offering an unlimited supply of mealworms is not recommended, as the nestlings need a varied diet.
It's not necessary to feed birds but it's fun to watch nature up close, and you'll be amused at how quickly a relationship develops between you and the bluebirds!