Gus makes an excellent point that you can sometimes get valuable information
on landing radar that you would not have predicted when making a report the
night before (see Stephane Perrault's report from Hempstead Lake yesterday).
But I have still found significant discrepancies.  I can't tell you how many
times I have headed for the final spot of high landing density on radar only to

David, there is no doubt that large birds reflect more than small birds.  So in
that case you would argue that the reason there are days when the reflectivity
is high but the ground is quiet is due to the fact that that night mostly large
birds migrated.  This could be a factor, but I have not noted it on the ground.
I will try to be more aware of the possibility in the future though.  On the
other hand, there is some reason to think that when birds are mostly small,
e.g. a predominantly warbler migration, that any effects of velocity to
spuriously increase reflectivity will be exaggerated rather than reduced.

The analogy again would be cars in the rain. The cars are birds and the rain is
radar. If the windshields are almost vertical, there's little surface to hit
the windshield.  These are warbler windshields. If they are almost horizontal
they are flickers. If you make the horizontal windshields flatter and flatter
corresponding to bigger and bigger birds, eventually it doesn't matter how fast
the car is driving, the rain hitting it will approach that of a flat non-moving
surface and speed will no longer increase reflectivity.

That being said, after thinking about it a bit more and an off-line discussion
with meteorologist John Kent, I have come to the conclusion that my theory is
flawed.  The reason is that it did not take into account the relative speed of
radar and birds.  Using the car analogy again, while doubling speed of a car
from 30 to 60mph would have a have a significant effect on the rate of rain
hitting your windshield over baseline, increasing from 1mph to 2mph would not
likely produce a perceptible increase. Given the fact that radar travels at the
speed of light, the speed of the radar is so much faster than the bird velocity
it would be as if the birds were standing (flying?) still, so that increasing
migration speed from 20 to 60 knots would not likely have an effect.

John did note that some nights with high reflectivity but low turnout could be
due to birds that are not new or notable which could be lead to an
underinterpretation of the true migration.

Theories about the discrepancy from David La Puma include:

1. The radar is sensitive, and even low densities of migrants can show up as
2. Birds concentrated at a particular altitude can also produce a stronger
signal, which appears to be heavier migration when it’s actually fewer birds
concentrated at a particular altitude. I see this a lot with strong upper level
winds, or when conditions are less favorable at upper levels, and birds are
concentrated closer to the ground.
3. Heavy migration but low concentration. Migration is heavy, but conditions
are such that birds are spread out, and therefore not concentrated when they
land. We see this in the spring when we have heavy migration but no westerly
component to the wind, so no forcing of birds to Cape May.
4. It’s not all birds. Bats and insects can contaminate the imagery, and if
there are birds involved, they can mask any lower velocity of insects (although
insects can have significant positive velocities especially when the tailwind
is favorable to the direction of migration)

So ultimately we have a number of theories for why radar is often not reliable
for predicting birds on the ground, but I'm not sure we have the answer.  But
imperfect as it is, though, it's fun to watch the reflectivities bloom,
imagining hordes of birds up there in the sky headed our way.  Because
anticipation of birds is half of the enjoyment (or, almost half), and when we
actually do see what we anticipate, I think we enjoy it more.  So that in
itself may be a reason for you, Steve, to keep on looking at that radar.
Because one day soon, Forest Park will be filled with birds.  And if you are
chomping at the bit when you enter the park , when you hear that birdsong, you
will enjoy it even more.

Good birding to all,

Peter

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