April 12, 1995
The report below is from David Aborn, a PhD student studying ornithology at the University of Mississippi in Hattiesburg, MS. We called him up and asked if he'd teach a weather lesson for Journey North students. He'll show you how to read today's weather map and come away with a bird's eye view of weather's effect on migration. David begins with an incredible story of an event he witnessed along the Gulf coast in Mississippi a few years ago. We've been told that songbirds make this 600 mile trip in a matter of hours, though it takes ships a number of days to travel as far. The tiny birds may burn up half their body mass to fuel the trip! Can you imagine a hummingbird flying so far?
"Dear Students:
"Songbird migration is well underway! Here in Hattiesburg, Red-eyed and White- eyed Vireos, Yellow-throated Warblers, Summer Tanagers, and a number of other species have started showing up. Weather is an extremely important factor in determining how many migrants may arrive along the Gulf coast following their non-stop flight across the Gulf of Mexico. Here's an example:
"On April 25, 1991, my major professor (Dr. Frank Moore) and I had the opportunity to witness what's known as a "fallout" on one of the Gulf coast barrier islands. A front had just moved through, and the winds were howling from the northeast at 25-30 mph. At about 12:00 noon we looked out over the Gulf and we saw about 20 birds coming in over the water. Then we saw another wave of birds, and then another! Over the next 1/2 hour, we watched literally HUNDREDS of migrants struggle to reach land. Warblers, vireos, tanagers, thrushes, cuckoos, and hummingbirds all flew within a few feet of us, and only 3-4 feet off the ground. Some had the strength to make to the woods, but many dropped down as soon as they reached the dunes. Birds that were flying too high got blown back over the water, and probably did not make it. We walked around the dunes and each saltbush had a dozen different species of birds in it. It was like a living kaleidoscope, and something we will never forget.
"So how can someone tell if there will be a lot of birds, or just a handful?
"If you look at a weather map, either from the newspaper, TV, or computer, you will see a number of features. These include frontal boundaries (a dividing line between warm, moist air and cooler, drier air), and some H's and L's. These letters stand for high and low pressure areas.
"The winds around a high pressure area circulate in a clockwise direction, while those around a low pressure area move counterclockwise. High pressure areas follow cold fronts, and the winds on the right side of the high are from the north (headwinds to a spring migrant) and those on the left (or backside) of the high are from the south (tailwinds). Are you starting to see how we can use this information to help predict fallouts?
"Let's use the weather from yesterday and today as an example. A strong cold front moved across the Gulf coast states yesterday producing lots of clouds, strong winds, and heavy rains. Any birds arriving from over the Gulf would be forced to land because of the poor flying weather. Today, the front has passed and the winds are blowing fairly strong from the north because of the high pressure area. Many birds that landed yesterday are probably still too exhausted or do not have enough fat (energy) to fly against the headwinds.
"Migrants arriving from over the Gulf are also having to fly INTO the wind, making an already long journey even more difficult. Many of these birds will put down at the first land they see in order to rest and feed. Therefore, today and tomorrow are probably good days to see and capture migrants along the Gulf coast. We have captured as many as 250-300 birds when these conditions are present. In a couple of days, the high will have moved east, and the winds will shift to the south. These winds help migrants during flight, so most of them will land farther inland. These are very slow days at the study sites, often only capturing 20- 30 birds. In the fall, the opposite trend is seen. Now you should be able to look at a weather map and be able to make pretty good predictions about bird migration.
"I will check back with you next week to see how migration is progressing, and I will talk a little bit about my own research. "In the mean time, think about this:
CHALLENGE QUESTION:
"Why don't many migrants take advantage of low pressure areas?"
Take care, David Aborn david_aborn@bull.cc.usm.edu