Second, being small birds, robins are closer to the potential source of earthworm sound. Third, laboratory tests indicate birds ignore certain sounds in much the same way people do. Maybe robins can screen out unwanted sounds and focus on target sounds. Perhaps the robin hears the collateral noise made by an earthworm crawling through dry grass or leaves. Jean Asis, Blue River, Oregon.
Tree swallows naturally nest in cavities, which explains why they so readily use nest boxes.
Choosing the right nest site and the right cavity, or nest box, is always a big chore for any bird. All birds want their nests to have the right exposure, temperature, access to food, security from predators, and so on. Most birds show some range of tolerance for their nesting site, and a few birds demonstrate idiosyncrasies. The tree swallow is one. Tree swallows line their nests with feathers, showing a preference for duck feathers but a willingness to use what they can find.