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.
The phase shift effect is similar to the Doppler shift observed with sound waves. With the Doppler shift, the sound pitch of an object moving toward your location is higher due to compression of sound waves. As an object moves away from your location, sound waves are stretched resulting in a lower frequency.