This discussion thread got me wondering exactly what radar imaging shows us
about objects moving in air and bird migration, so I searched on line and
found many answers to various posters' questions.
Do birds make mass migrations at night? Yes, lots of species. Passerines
constitute more than half of European bird species and almost all make
massive, long-distance, high-altitude, nocturnal migrations in spring and
fall. So do eastern US passerines. No reason to assume west coast US birds
travel differently. We also know they ride the winds aloft just like Cliff
Mass's data showed and they travel in dense layers of mixed species.

Do on-the-ground observations of robins (or other birds) confirm radar
observations? No, because the long-distance migrants don't stop and rest by
day. Depending on the jet stream they're riding, they may change elevation,
but they keep going--and they reach their destinations in hours to days and
disperse. Often they can't stop because they are crossing parts of oceans
and deserts (like the Sahara). Woe to the unlucky birds who get sucked into
a storm gyre while crossing the Gulf of Mexico, but eureka for the birders
who see flock to see the fall-out of rare species in the aftermath.

Does the radar show birds taking flight at dusk and landing at dawn? No,
the radar shows changing bird density in the air at high altitudes at night.
European bird migration studies in temperate latitudes show that some birds
"fly under the radar" by day, i.e., they descend to an altitude at which the
radar doesn't "see" them. They are screened out of the signal with all the
other terrain, objects, backscatter, etc. Around dusk, they gradually
ascend into radar range over a period of about 2 hours and reverse this
pattern at dawn, which indicates that their altitude shifts are likely
driven by changing temperature/pressure/air currents.

How do we know the image is of birds? By eliminating other possibilities,
such as insects, bees, and butterflies (all of which are documented to make
mass high-altitude long-distance migrations across vast areas, including
oceans). That leaves birds and bats. How do we screen out bats? I don't
know. Yet.

Allison Reak