Bird house hole sizes: Optimum Hole Sizes (Inches) for Common Birdhouse Nesting Species: American Kestrel 3, Ash-Throated Flycatcher 1 1/2, Barn Owl 6, Black-Capped Chickadee 1 1/8, Carolina Wren 1 1/2, Downy Woodpecker 1 1/4, Eastern Bluebird 1 1/2, House Finch 2, House Wren 1 1/4, Northern Flicker 2 1/2, Prothonotary Warbler 1 1/8, Purple Martin 2 1/2, Tufted Titmouse 1 1/4, Violet-Green Swallow 1 1/2, White-Breasted Nuthatch 1 1/4.