February 25, 2009
Dr. David Aborn

Dear Students:

Forget the groundhog; the birds have been saying spring is on its way! Robins and cardinals have been singing here in Tennessee for several weeks, and the bluebirds have started checking out nest boxes. The most exciting sign, however, was seeing the first Purple Martin of the year on Monday! Martins and swallows are the first migrants to return in the spring, so it’s a sign that spring migration will soon be here!

First: How to Read a Weather Map
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. Look for these features on the map below:

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) it. The red lines are warm fronts, with colder air in front of it and warm air behind.

Today's Map: No high pressure systems.

Bird Migration Depends on Right Weather
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, wind 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. So why don't birds use low pressure systems? The reason is that low pressure systems often bring bad weather with them, so even though the winds may be right, flying conditions are not good.

Now Let's Practice!
While it is too early for a lot of migrants to be showing up, let's practice reading this week's weather map:

  • A high pressure area is off the east coast. It is far enough east that much of the eastern US is experiencing southerly winds. That means any migrants around would have good flying weather and would be able to head north.
  • A cold front is in the center of the country. It is bringing some rain and strong north winds to those areas. Any migrants would be forced to land for a day or two, and bird watchers and researchers would be able to see a lot of migrants in their area. As the cold front moves east, it will draw up a lot of moisture from the Gulf of Mexico and bring rain to the eastern US by Friday and Saturday. (We are expecting 1-3 inches in my area). That means this weekend will be good for birding.
  • A low pressure center is in the western US, but no rain is with it. The southerly winds would allow migrants arriving from Mexico to keep flying north until they got to northern California. That is where that cold front is starting to bring rain to the Pacific Northwest, so birds would be forced to land.

Dust off your binoculars and start practicing; it will only be a couple of weeks before migration starts to pick up. I will be keeping my eyes open this week for more early migrants, like that martin, and let you know what I see.

Take Care.

David Aborn
North Chickamauga Creek Conservancy
Chattanooga, TN