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.
Spin: If a star is spinning (and they all seem to be) then, as it spins, one side of it is moving toward us while the other side is moving away from us. This will cause the width of spectral lines from its surface to be Broadened due to the doppler shift.