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We use a vertically pointing radar to detect and track animals in flight. The radar detects birds, bat, and insects. This is a field of night sky with those animals moving through it.
Every animated dot is a real bird or bat. The application shows how high it was flying, how fast it was moving over the ground, and which way it was heading. However, the exact locations of animals in the animation do not correspond to exact locations in the real world. In addition, the application simplifies flight trajectories to a straight line, when in reality they may not be perfectly straight.
The radar follows each animal typically for less than 30 seconds. The numbers beside each mote are the measured values: height above ground, and speed over the ground. The radar also detects insects, but they are not shown in the animation because they are very numerous.
The headline figure is a migration traffic rate: how many birds and bats crossed a one-kilometer line each hour. The application also shows how many animals were detected in the last 30 minutes, and how many are currently in the radar beam. The radar detects many more insects than birds and bats, so the application also shows how many insects were detected in the last 5 minutes.
The sky holds far more than you see here. The beam is a narrow column, so the radar only detects animals that happen to fly through it. This represents a small sample of animals crossing overhead. Every dot is something that was really detected, but on a good night of migration the airspace above the site holds many times the number detected.
Data from a Swiss BirdRadar MR2. Times are shown in local time at the radar, with UTC alongside. Heights are above ground level. A night is named for the evening it begins — "15–16 May" runs from midday on the 15th to midday on the 16th, so the whole night of flying stays together instead of being split at midnight.