Yesterday I cleaned out two nest boxes that were used by a pair of House Wrens last summer. (I have a Birdcam image of a wren entering one with nesting materials.) House Wrens are cavity nesters and will take advantage of artificial structures such as nest boxes or openings in buildings. Male wrens may build multiple nests, with only some being used for breeding. In this case there were at least two nests, a dummy nest and a real nest. You can see the remains of both nests above, the dummy nest in the foreground and the real nest in the background.
Above is the dummy nest in its box. House Wren nests have a base of twigs and other materials under and around the nest cup. A foundation of sticks may keep the nest cup dry even if water leaks into the cavity and collects on its floor. A male wren starts building as soon as it claims a nest site, and a female will complete the nest after mating. In the case of this dummy nest, there is only the twig foundation that a male placed in the box.
I was not able to extract the entire real nest intact because the opening in the box was too small. However, the nest cup did survive and is pictured above. A nest cup is built by a female wren and is lined with soft materials like grass and feathers. There appears to be some sort of foam and cotton-like materials in this nest. Some of the soft white materials look like spider egg sacs, which may be
placed in nests to eat parasitic mites. I found feathers that I think were from a Blue Jay, a Northern Cardinal, and a Northern Mockingbird among the nesting materials.
Last summer I heard nestlings chirping in the box, but I never saw any fledglings. I found no signs of dead chicks when I extracted the contents, so I assume they all fledged safely. House Wrens should return fairly soon. They may nest here again, as House Wrens tend to return to the same sites year after year.