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They’re back! Radar reveals arriving birds
by Karen Benson
Apr 23, 2013 | 1462 views | 0 0 comments | 11 11 recommendations | email to a friend | print
Wikimedia commons photo
A Prothonotary Warbler is a beautifully colored small songbird that is one of many that come through during spring migration.
Wikimedia commons photo A Prothonotary Warbler is a beautifully colored small songbird that is one of many that come through during spring migration.
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National Weather Services photo
Millions of small colorful birds arrive on the coast at Lake Charles, La., in this radar image. Even though any one of these tiny (sparrow-sized) birds would not show up on modern weather radar, the combined reflected signal from hundreds of thousands makes a spectacular feature on the radar display. The large red arch represents birds dropping from the sky to find food and shelter after the long non-stop flight across the Gulf.
National Weather Services photo Millions of small colorful birds arrive on the coast at Lake Charles, La., in this radar image. Even though any one of these tiny (sparrow-sized) birds would not show up on modern weather radar, the combined reflected signal from hundreds of thousands makes a spectacular feature on the radar display. The large red arch represents birds dropping from the sky to find food and shelter after the long non-stop flight across the Gulf.
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If you are a birder, you know that the spring migration of birds is an exciting event. This is the time when all sorts of colorful and delightful birds come back north from their winter homes in the tropics.

The birds start arriving in late March and continue moving through until the end of May. The peak of the migration is late April and early May. That means we should be seeing bright little birds any day now!

However, almost all the warblers are here for only a day or two. Then they keep on going to the deciduous forests of the northeastern part of the continent.

This means that, for most of us, the only way we get to see these travelers is during migration. If you are lucky and vigilant, you may see several species in your own backyard during the migration season. However, what we really want is to see a lot of species all at once! In other words, we want a “fallout.”

A fallout is a birder’s term for a large number of migrating birds being force to land. These migrants are usually being grounded because of weather conditions. Rains and strong northerly winds will bring down birds pretty quickly. The birds seek shelter in trees and underbrush and even on ships. Trans-Gulf migrants can be particularly exhausted if they have been flying for hours against a headwind. They fall out of the sky at the first sign of land.

Hopefully, they find fresh water and food as well as shelter. For a few hours, the tired birds can rest and refuel. And birders are treated to a colorful show!

So how can birders know when to expect a fallout? They watch the weather forecasts, of course! If a frontal system is predicted any time during April or May, birders get excited. A front, especially if it includes rain, is bound to produce a fallout. This is particularly true along the Gulf Coast, where Trans-Gulf migrants reach land after a long, 600-mile nonstop flight from the Yucatan.

When a front with rain and northerly wind is due, birders clear their schedules and head for the nearest “hotspot” to see the fallout. There are well-known fallout hotspots all along the coast. Famous ones include High Island and Sabine Woods on the northeast Texas coast, Paradise Pond in Port Aransas and Blucher Park in Corpus Christi. All these places host birds and birders in abundance during migration.

When Weather Surveillance Radar (WSR) came along in the 1960s, forecasting the weather got easier. And, as so often happens with technology, weather radar proved to have other uses.

A new scientific field was born: Radar Ornithology.

A graduate student at Louisiana State University, Sidney A. Gauthreaux, began supplementing his visual observations of songbird migrants with radar observations from two WSR sites. His research from 1965-1967 revealed that migrants, in large flights, came across the northern Gulf of Mexico, mostly at night.

The next generation in weather surveillance radar was the addition of the Doppler capability. In 1992, a new Doppler WSR site was located just south of Houston. Doppler radar tracks the speed and position of targets (weather systems or birds). A loop of images can tell us not only where the objects are, but also how fast they are moving, and in which direction they are moving, relative to the radar station.

Dr. Gauthreaux discovered Doppler weather radar to be a fine tool to study the patterns of migratory bird flights. His research team found they could determine the number of birds per unit area based on the amount of reflected signal. This density of birds aloft could be contrasted with the speed of movement of the birds. Songbirds fly considerably slower than shorebirds and waterfowl. With the new radar information, the scientists found that not all of the birds in a Trans-Gulf flight were the little songbirds. Typically, a flight contained flocks of shorebirds, gallinules, ducks and even an occasional raptor as well as songbirds!

A surprising discovery also came about from this research. Important migration stopover areas along the Gulf coast could be detected. When night-migrating birds stop over for a day of rest and food, they are ready to go again that night (or the next). The rested birds take off 30 to 45 minutes after dark and rise up into the evening sky. This mass take-off is called an exodus event. As they climb up into the altitudes of the WSR’s beam, they become visible on the radar image. These exodus images pinpoint the locations where huge concentrations of migrants had stopped over.

Subsequently, the type of habitat at the stopover sites was determined by satellite imagery. Early findings indicate that forested floodplains and wetlands, even far inland, are preferred stopover areas. Coastal woodlands are important during bad weather, but these inland sites may be more vital to the populations of songbirds.

Radar ornithology is giving us many insights into the world of migratory birds. With it, we can continue to monitor the passage of birds on their way to their breeding grounds. And since radar can show us the massive flights arriving at the coast, we know to the hour when to grab our binoculars and rush out to witness the marvelous migration. See you there!
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