<P> Stable isotopes of hydrogen, oxygen, carbon, nitrogen, and sulphur can establish avian migratory connectivity between wintering sites and breeding grounds . Stable isotopic methods to establish migratory linkage rely on spatial isotopic differences in bird diet that are incorporated into inert tissues like feathers, or into growing tissues such as claws and muscle or blood . </P> <P> An approach to identify migration intensity makes use of upward pointing microphones to record the nocturnal contact calls of flocks flying overhead . These are then analyzed in a laboratory to measure time, frequency and species . </P> <P> An older technique developed by George Lowery and others to quantify migration involves observing the face of the full moon with a telescope and counting the silhouettes of flocks of birds as they fly at night . </P> <P> Orientation behaviour studies have been traditionally carried out using variants of a setup known as the Emlen funnel, which consists of a circular cage with the top covered by glass or wire - screen so that either the sky is visible or the setup is placed in a planetarium or with other controls on environmental cues . The orientation behaviour of the bird inside the cage is studied quantitatively using the distribution of marks that the bird leaves on the walls of the cage . Other approaches used in pigeon homing studies make use of the direction in which the bird vanishes on the horizon . </P>

When do birds fly south for the winter