I have both 9s and 11s. The Bo-11 Plus are what i want to slowly convert all of them to. They have have that black inner liner that gives better temp insulation specially from heat. Big and deeper space too for them inside. Their caps too are way easier too to open and close. I have one right now hanging with the rest of all of my 9s.
They didnt occupy it this year. I think it more of its positioning in the rack than what it looks like to the Martins. Ill move it next year to the position of the ones where they nested this year and see if they use it.
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I read that wearing a black golf shirt in hot weather was cooler than wearing a white shirt. The black does absorb heat from sun, but it also absorbs heat from the body. A flip of the shirt whisks away the heat. I tried it while playing golf and I really think I was cooler. So I used black shirts all summer. So, maybe the dark interior not only insulates better, hides the nest better but it may also absorb the body heat of the young martins. A light interior would reflect the heat back to the young birds.