Ross this is info I found, wondering the same thing.
Eastern Bluebird Box With Hole Guard
A female Eastern Bluebird feeds her youngster through an extended nest hole entrance. Not all extenders are made of wood; some designs are made of metal or plastic. These keep predators from easily reaching into the box.
Article it was in also goes on to say this was a hypothesis of the inventor that the spaced entry would allow distance to keep, let's say a Racoon from sticking an arm in and swiping out a young bird, cheeeping for Momma. Where a flush hole would allow it's arm to enter at least an inch more. The metal was evidently to keep predators who would chew into the wood, and open it larger, with a metal insert chewing would be stopped.
On all of these "nesting" type boxes there are as many naysayers as proponents, so it's a mixed bag, on who is right. I'd say if it sounds like a good idea, if it's easily doable, do it, until you see where a real study shows it doesn't help. On both of these it seems like it makes sense to me. Now if a naysayer has the distance guard, and frequently see's a coon sweeping out baby birds, is the design actually flawed, or maybe just not deep enough? So if I did one, I'd make the entryway a few pieces of glued up 3/4" wood to make a little more tunnel, before it opened to where the babies were. In fact most of the plastic formed "extenders" I have seen are much longer, and I would assume for this reason, many are also curved, I surmise, to make line of sight difficult.
From what I have observed, Other species going into the nest, and breaking holes into unhatched eggs frequently happens. We see Grackles often going into our Bluebird boxes to eat, or at least kill off Bluebird eggs. I think from what I have seen it's their #1 predation. They could go into just about the same hole size as a Bluebird, so stopping that with any of the items I have seen would also keep Mamma away from her young.
