My wife and I were discussing our good fortune in having bluebirds occupy a
couple houses located in our large yard and saying we had two successful
hatches fledge from each of the houses last summer when an urban friend told
us his single bluebird house successfully fledged four hatches last summer.
Of course, we asked how he managed that, and he said his secret was that he
fed the parents a mixture of ingredients which allowed them to feed all
captured insects to their young while they ate his offering. To make a long
story short we got his recipe and tried it late last summer. Here is the
recipe if you're interested:

4 - 18 oz. jars of creamy peanut butter (generic brands are fine)
3 cups vegetable oil
1 cup light corn syrup

Cook over medium heat stirring constant until a candy thermometer registers
165-170 degrees. Take off burner and immediately add one 42 oz. box of quick
oatmeal. Stir mixture together and allow to stand for twenty-four hours. Add
whatever kind of flour you have to the mixture (no cornmeal) until it is the
consistency you want for molding, stiffer in the summer and softer in the
winter. We shape it into squares that fit inside the wire suet holders.
After molding we place between sheets of wax paper. In warm weather we
freeze in plastic zip lock bags and take out as needed, but in the winter we
keep it in a cool garage.

This makes a lot, but we're already on the second batch since last August.
Unfortunately we were too late for the bluebirds to benefit, but other seed
and suet eaters are constant customers. It takes a while for the birds to
realize this is food, but once they get the idea, they consume it quickly.
We now provide between two and three fills a week between two feeders. Our
best patrons at the moment are downy and hairy woodpeckers, chickadees,
nuthatches, and chickadees. We even had a crow hanging on the side of the
metal pole from which the feeder was hanging trying to get at it!

John