Weather and Migration Dr. David Aborn March 1, 2005 Dear Students: I know it doesn't seem much like spring with all the snow around, but migration will soon be here. 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. Getting Ready: Tips on Reading Weather Maps Let's start by looking at the general features of a weather map. You'll see H's and L's, which represent high and low pressure centers. These are areas of swirling air. The air around a high pressure center circulates clockwise. The air around a low pressure system moves counter-clockwise. You will also see colored lines, which 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 and warm air behind. What Makes Good Migration Conditions? So what does this all mean for birds? Watch the High Pressure Systems. Birds want to fly with a tailwind (winds pushing them from behind). Tailwinds help birds travel farther with less effort. In the spring, this means winds moving south to north. Headwinds, or (in spring) wind moving north to south 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, the birds have tailwinds and take off again. That's usually a day or two later. So why don't birds use low pressure systems? The reason is that low pressure systems often bring bad weather with them. Even though the winds may be right, flying conditions are not good. Click to enlarge Test Your Skills With Today's Map Is today a good day for migration? Check out the current surface map and test your skills. Find the high pressure systems and look at the wind direction. A major cold front has passed across the country, and most places east of the Rocky Mountains have strong north winds. If migrants were arriving from the tropics, they would encounter headwinds and would be forced to land. This is a phenomenon called a fallout. Why? Because the birds seem to "fall out" of the sky in large numbers. Bird watchers and researchers would be seeing lots of birds around. While it is too early for a fallout, migration is close at hand. Someone I know is currently in Costa Rica and reports seeing lots of Black-and-white Warblers, Chestnut-sided Warblers, Tennessee Warblers, Wilson's Warblers, and Baltimore Orioles. He reports many of the birds have grown in their colorful spring plumage in readiness for migration. I can't wait to see them here! David Aborn Chattanooga, TN The Next Weather and Songbird Migration Update will Be Posted on March 7, 2005. Copyright 1996-2005 Journey North. All Rights Reserved. Please send all questions, comments, and suggestions to our feedback form