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Raptor Movements
Raptors can ride thermals up to 5000 ft, well beyond the range of most binoculars.
At this height, they are detectable by NEXRAD and distinguishable from insects and wind-borne
particles by their rapid motion within and between thermals. Raptors and other soaring birds ride columns
of rising, warm air. At the top of such a column, as the rising air begins to stabilize and sink, birds
begin gliding and losing altitude. This is often called streaming. Birds continue streaming until they
find another thermal to ride, and they repeat the process. This is an energy efficient way for birds with
large body mass and broad wings to migrate.
Streaming raptors are frequently visible on radar. In the following images from BRO
Brownsville, TX, several distinct lines are visible. To the east of the stations, the line represents
higher densities of targets along the coast. Many of these are migrating songbirds flying north along
the coast NE Tamaulipas, Mexico. The lines visible to the south and north of the station, though, are
streams of migrating birds. Notice in the radial velocity image that these lines stand out as brighter
greens and reds against the background of grayish hues representing light northerly winds and slow moving
targets. Spring migration of Broad-winged Hawk is in progress, and although it is impossible to identify
reflectivities to species solely from a radar image, ground-truthing and knowledge of timing of migration
suggest that these streams are almost solely raptors. They are most likely Broad-winged Hawks, Swainson's
Hawks and Turkey Vultures.
| Raptor migration in South Texas | Raptors moving faster than winds | ||||||||||||||||||||||||||||||||||||
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| BRO: Brownsville TX (25.92N 97.42W): Base Ref 124nm / Elev=0.5 deg / 0.5 km²/pixel Clear Mode / VCP 32 04/23/00 19:17 UTC - Max: 42 dBZ | BRO: Brownsville TX (25.92N 97.42W): Radial Vel 124nm / Elev=0.5 deg / 0.5 km²/pixel Clear Mode / VCP 32 04/23/00 19:17 UTC - Max: -55 kts +52 kts | ||||||||||||||||||||||||||||||||||||
In this case of raptor migration, soaring birds are flying into light, northwesterly
winds which push them toward the coast. Birds still move north, though, by "tacking" into the opposing
winds as a sail boat making progress moving against wind; but raptors "pile up" over the coastline because
they do not want to fly across large bodies of water. Water offers little thermal development relative to
land surfaces, requiring much greater energy expenditure by the birds.
In the following magnified animations from 5 April, 1999 at BRO Brownsville, TX,
more migrating raptors are visible. Notice especially the velocity image (below right), in which green
lines are visible in a field of red. These green lines correspond to raptors, most likely Broad-winged
Hawks given the earlier spring date, moving northeast in northerly winds. Even thought they are flying
into a 10-15 knot headwind, the birds are still traveling at 5-15 knots.
| Raptor thermals moving and dissipating | ||||||||||||||||||||||||||||||||||||||
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| BRO: Brownsville TX (25.92N 97.42W): Base Ref 124nm / Elev=0.5 deg 0.5 km²/pixel Precip Mode / VCP 21 04/05/99 17:03 - 21:08 UTC Max: 48 dBZ | BRO: Brownsville TX (25.92N 97.42W): Radial Vel 124nm / Elev=0.5 deg 0.5 km²/pixel Precip Mode / VCP 21 04/05/99 17:03 - 21:08 UTC Max: -58 kts +68 kts | |||||||||||||||||||||||||||||||||||||
Remember that if winds are more favorable, raptor movements tend to be more dispersed in smaller and more numerous streams. As bird densities decrease, identifying raptor migration from radar imagery becomes increasingly more difficult. Also, note that the bird density scale is missing from the above dBZ reflectivity scales. This is because individual raptors probably reflect more energy than nocturnally migrating songbirds as a function of body size.