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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 TexasRaptors moving faster than winds
dBZ
ND
-28
-24
-20
-16
-12
-8
-4
0
4
8
12
16
20
24
28
image of raptor migration base velocity image of raptors
kts.
ND
-64
-50
-36
-26
-20
-10
-1
0
10
20
26
36
50
64
RF
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
dBZ 
ND40
0545
1050
1555
2060
2565
3070
3575
raptors migration windrows raptors tacking into wind
kts.
ND0
-6410
-5020
-3626
-2636
-2050
-1064
-1RF
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.

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