Weather Forecast for the Birds
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 whereas the air moves counter-clockwise around a low pressure system. 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) it. 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, 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 force 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.
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.
Over the course of this spring, I will be pointing out areas where there might be lots of migrants landing as a result of the weather. I'll be reporting what I and other people have been seeing. So what does this week look like?
Well, a cold front has stalled along the Gulf coast, bringing rain and clouds to much of the southern US. Any migrants coming up from the tropics will encounter poor flying weather when they reach the coast and will be forced to land. I wouldn't look for large numbers of birds just yet. It is still pretty early in the season, but numbers should pick up in a couple of weeks. If this same situation occurs in late March, April, or May, people along the Gulf coast could expect to see many migrants around. Here in Tennessee, people have started seeing a few of the early migrants, including Purple Martins and Blue-headed Vireos, so I will be keeping my eyes open and I will let you know if I see anything.
Dr. David Aborn
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