<P> Bird migration routes have been studied by a variety of techniques including the oldest, marking . Swans have been marked with a nick on the beak since about 1560 in England . Scientific ringing was pioneered by Hans Christian Cornelius Mortensen in 1899 . Other techniques include radar and satellite tracking . The rate of bird migration over the Alps (up to a height of 150 m) was found to be highly comparable between fixed - beam radar measurements and visual bird counts, highlighting the potential use of this technique as an objective way of quantifying bird migration . </P> <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 to quantify migration involves observing the face of the moon towards full moon and counting the silhouettes of flocks of birds as they fly at night . </P>

When do birds start flying south for the winter