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House Sparrow vs. House Wren vs. Entrance Reducer
A friend gave Mary a small-bird birdhouse, which immediately attracted the attention of a pair of House Wrens:

Bird House entrance reducer – wren exiting The vertical black bar is a DIY Birdsaver cord.
The entrance hole was 1-½ inch ⌀, a bit larger than the 1 inch ⌀ preferred by wrens and entirely suitable for the pair of House Sparrrows who also took an interest in the property:

Bird House – sparrow inside This led to considerable discussion and displays of outright hostility:

Bird House entrance reducer – wren vs sparrow Sparrows and wrens disagree on nestbuilding materials, with the wrens hauling twigs into the box and sparrows hauling them back out again.
Because wrens have better PR agents than sparrows, I intervened by taking the box apart:

Bird House – nest base sticks Although I realize that’s a lot of work for a small bird, I dumped the contents off the patio and set about reducing the entrance hole:

Bird House – interior cleared Because birds aren’t too fussy about looks, I sawed off half an inch of 1 inch (ID) CPVC pipe and glued it in the hole:

Bird House entrance reducer – interior glue The outside looks marginally better:

Bird House entrance reducer – exterior glue The sparrows continued to approach the hole at full throttle, deploying landing gear and speed brakes at the last possible moment:

Bird House entrance reducer – sparrow approach But they no longer fit through the hole and eventually gave up trying. The wrens resumed hauling twigs, although we’re not certain they’ll finish the project, as birds tend to build several partial nests before selecting the final one.
We hope this will end on a happier note than last year’s Wreath Robins.