Landbirds migrating over open water at dawn frequently increase their flight altitudes,
presumably to gain better vantage in search of landfall. The animation below depicts
the dawn ascent of birds over Lake Michigan. About 10 minutes separate each frame. As
dawn approaches, bird-caused radar echoes over water are briefly detected at longer
ranges from the radar as birds over water ascend and become more visible to the radar's
beam. (This brief description of radar imagery
might help in interpreting the radar's view of this phenomenon.) At the same time,
echoes over land decrease in extent and strength as migration ends and birds decend
into the landscape to begin their stopovers.