Traveling
with Neotropical Migratory Birds

The Built-in Barometer
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Background
Scientists have long noticed that birds feed intensely as air pressure falls. They apparently have an inborn barometer that is extraordinarily sensitive. This is a handy adaptation for all birds, even non-migrants, because storms usually are associated with falling pressure, and birds have a hard time getting food during a storm. The sooner they can predict a storm before it hits, the more time they have to prepare.

Scientists also have known for a long time that migrating birds fly at different altitudes than non-migrating birds, and maintain this altitude even on moon-less nights when they can't see the ground at all. How do they maintain a particular altitude? Many scientists suspect that this is also due to their ability to "feel" air pressure. Studies have proven that birds are extremely sensitive to small changes in air pressure, comparable to differences of only 5 to 10 meters in altitude.

Recognizing air pressure is also handy because birds often migrate along frontal systems, and changing air pressure is one of the first signs that a front is coming. High pressure systems often have clear skies, which make using celestial navigation easier, and flying on high pressure days may even help "buoy" birds up a bit.

The Big Question
How do birds judge air pressure? Scientists don't know!! They do have a couple of guesses. One is that birds may be able to detect it through their inner ear. We detect large changes in air pressure in our own inner ear when we make a fast change in altitude--that's when our ears "pop." Another guess is that the birds detect air pressure somehow though the huge air sacs that connect to their lungs and fill much of the space inside their bodies.


Activity
A. Watch how barometric pressure changes with the weather. Over the period of at least 3-4 days, students record the barometric pressure. This should be done as often as 3-4 times during the school day. (Ideally students could also be assigned to keep these records during off-school hours for these 3-4 days.) Tip: Keep your eye on the weather map! Try to time this activity when a storm is approaching.

B. After observing how pressure changes over time, study weather maps. Track high and low pressure systems, associated wind directions, and cold and warm fronts for two weeks. Then discuss: Why would knowing the air pressure be important to birds? What would this information tell birds?

C. Design a barometer--or actually build one. How could you build a tool to help you measure air pressure?

Try This! Discussion or Journaling Questions
1. How might sensing an oncoming storm help birds?

2. How might air pressure help or hinder birds in flight?

3. If you were to fill 2 balloons with helium, one when air pressure is high and one when it's low, predict what will happen if you kept them in your classroom for several days. How many days does each balloon stay buoyant? Is there a difference? Why? (Try it and find out!)

4. Some scientists say that animals can sense an earthquake before it hits. Do you think this is possible? How might animals do this? Which might be more likely to sense an oncoming earthquake--a mammal or a bird?

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