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 frequency increases as the source moves toward the observer and decreases as it moves away, as the rising pitch of the whistle of an approaching train and the falling pitch of a departing train. The Doppler effect is also observed in electromagnetic radiation, such as light and radio waves.