1 Many people find that boxes with a 1.25 or 1 1/8 hole restrictor on them before an egg is laid are not utilized, even by smaller birds like titmice and chickadees. 2  If the entrance hole is enlarged (e.g., by woodpeckers or chewing rodents), put a hole reducer on it.
The Doppler effect widens the spectral lines of rotating objects because the wavelengths changes from blue or red. Blue-shifted, red-shifted, and non-shifted photons mix together to get how its rotating.
Doppler effect. The Doppler effect is defined as a noticeable change in the frequency of sound, light or water waves as the source and the observer move. An example of the Doppler effect is that the frequency of the sound increases as the source moves closer to the observer.