We compared House Wren (Troglodytes aedon) nests in nest boxes with either small (28 mm) or large (38 mm) entrance holes. Spring clutch size did not differ between wrens using boxes with large and small holes. Nests in boxes with large holes had a significantly taller ‘berm’ of sticks between the box entrance and the nest cup than did nests in boxes with small holes. This was true even when the same wren pairs were given boxes with different-size holes for spring and summer nests. We compared the difficulty of touching the eggs when we removed the frontplate of the nest box as an alternative measure of nest vulnerability to predators and brood parasites. There was a significant association between hole size and accessibility: boxes without the protection of a small entrance hole were far more likely to be categorized as ‘inaccessible.’ We found a similar relationship between berm height and nest architecture: nests with tall berms were generally less accessible than were nests with shorter berms. Contrary to earlier claims, House Wrens alter nest architecture to compensate for vulnerability of cavities with large entrance holes.