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Migrating Birds : Transgulf Migration

   The most direct route for many Neotropical and Nearctic migrants wintering in the Caribbean and Central and South America to breeding grounds in eastern and central North America is across the Gulf of Mexico. Birds must cross this ecological barrier in a single flight, often choosing to fly when weather conditions are most favorable for this long journey.
   Birds leave Yucutan, Central America and Caribbean islands in the evening (30-45 minutes after sunset), and they fly through the night. The distance from these departures sites to the Gulf coast of the United States is great (400-600 miles), so the peak of arrival of migrants usually occurs in the late morning or early afternoon. Under the most favorable weather conditions birds occasionally arrive much earlier, some times in the hours just before dawn. However, if birds encounter unfavorable weather conditions en route, such as those associated with a frontal passage, their arrival can be delayed by many hours. Occasionally flights do not arrive until after sunset or even later in the evening.
   The following images from 3:15 CDT 21 April, 1999 LCH Lake Charles, LA show a trans-gulf flight arriving. Notice that reflectivity levels reach 16-20 dBZ in many areas representing a density 123-227 birds per cubic kilometer. In this case, birds are experiencing favorable flying conditions (southerly winds) and are continuing through the radar beam and further inland, resulting in the beginnings of the typical "donut-shaped" ring characteristic of overflight. Also visible are several storms offshore to the SW of the radar station, a squall line or chaff trail near the coast SW of the radar, and highly reflective (the white pixels) ground clutter very close to the radar. The blothcy reflectivity just north of the station is of uncertain identity.

Trans-gulf migration arriving at LCH Lake Charles, LA: Base ReflectivityTrans-gulf migration arriving at LCH Lake Charles, LA: Radial Velocity
dBZBirds
/km3
ND
-28
-24
-20
-16
-12
-8
-4
0
458
865
1281
16123
20227
24489
281148
trans-gulf migration gulf as an ecological barrier
kts.
ND
-64
-50
-36
-26
-20
-10
-1
0
10
20
26
36
50
64
RF
LCH: Lake Charles LA (30.13N 93.22W):
Base Ref 124nm / Elev=0.5 deg / 1.16 km²/pixel
Clear Mode / VCP 32
04/21/99 21:05 UTC - Max: 62 dBZ
LCH: Lake Charles LA (30.13N 93.22W):
Radial Vel 124nm / Elev=0.5 deg / 1.16 km²/pixel
Clear Mode / VCP 32
04/21/99 21:15 UTC - Max: -96 kts +122 kts

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