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February 27, 2007
Dr. David Aborn

Dear Students:
As I am sure you are aware, weather plays a very important role in bird migration. This spring, I will teach you how to read a weather map to try to predict areas of the country that might see large numbers of migrants landing.
Let's start by looking at the general features of a weather map.

First: How to Read a Weather Map
Let's start by looking at the general features of a weather map. The H's and L's represent high and low pressure centers, areas of swirling air. The air around a high pressure center circulates clockwise. The air around a low pressure system moves counter-clockwise. The colored lines represent fronts: dividing lines between cold and warm air. Blue lines represent cold fronts, with warm air in front (to the right of) the line and cold air behind (to the left of) the line. The red line is a warm front, with colder air in front of it and warm air behind.

So what does this all mean for birds? Watch the High Pressure Systems
Birds want to fly with a tailwind to help them travel farther. In the spring, this means winds moving south to north. Headwinds, or winds moving north to south (in spring), make it too difficult for birds to fly, so they are forced to land. Bad weather, such as heavy rain, also forces birds to land. Since highs follow cold fronts, birds will be forced to land immediately following the passage of a cold front. After the high has moved east, usually a day or two later, the birds have tailwinds and take off again.

Sometimes birds can land in huge numbers (in the hundreds or even thousands) in what is known as a fallout. Fallouts can be quite impressive to see, and large fallouts are common along the Gulf Coast because birds have been flying non-stop for 18 hours, and are already exhausted when they arrive. Headwinds or rain make the situation even worse. Since highs (H)follow cold fronts, birds will be forced to land immediately following the passage of a cold front. After the high has moved east, usually a day or two later, the birds have tailwinds and take off again.

So why don't birds use low pressure systems (L)? Low pressure systems often bring bad weather with them. Even though the winds may be right, flying conditions are not good.

Test Your Skills With Today's Weather Map
While it is too early to see any major migratory flights, let's practice reading the weather map and seeing how it might affect migration.

The cold front that moved across the country and brought those big storms would have force many migrants to land. There is now a low pressure area over the eastern US, and high pressure over the Midwest. The low pressure area doesn't have much rain with it, so that means migrants forced to land by the storms would now be able to take off again because they would have tail winds, but no rain. Migrants farther west, however, would still be grounded until the high moves east, and bird watchers could expect to see lots of birds. What's in store?

  • Another cold front is moving into the West Coast.
  • Migrants in the West will be forced to land because of the bad weather.
  • In a couple of days, the front will be in the Midwest and western Gulf area, and people there could expect to see a fallout.
  • By the end of the week, it will be the eastern US that would see migrants landing.

Migrants Starting to Arrive!
I have been getting reports of swallows and martins across Tennessee. These
are some of the first migrants to arrive in the spring, so it won't be long before migration really gets going. Dust off your binoculars and start looking! Take care.

David Aborn
North Chickamauga Creek Conservancy
Chattanooga, TN