Migration Notes and Activities
According to Ornithologist Paul Kerlinger:
“The availability of food is the driving force in the evolution of migratory patterns…“Migration evolved as a way for birds to exploit resources that are seasonally abundant and avoid times… or places where resources are scarce or weather is very harsh. Many species can tolerate cold temperatures if food is plentiful; when food is not available, they must migrate.”
Why do birds migrate?
Before a bird migrates, two elements must be present: a genetic predisposition, and one or more environment triggers. The most important trigger is the changing length of daylight – the photoperiod. The photoreceptors are not located in the birds’ eyes, but deep inside the brain in ventromedial-hypothalmuas, these receptor cells react to the low levels of light penetrating the bird’s skull.
Evolution of migration
Most ornithologists agree that the evolution of migration was a three-step process starting with a non-migratory population. Some environmental pressure affecting food supply forced part of the population to move for part of the year. The same population moved back to their ancestral grounds for breeding. If the pressure continued or intensified, the original sedentary population may be eliminated completely, leaving only migrants.
Activity: Modeling the Seasons
Create a simple model of the earth, and observe what happens as the Earth rotates and revolves around a bright light bulb - the sun. explore the astronomy of seasonal cycles, and the cause of the changing day lengths in higher latitudes
Earth globe pattern
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Activity: Solar Motion Demonstrator
Build a simple paper model to explore the seasons and changing daylight hours in the Northern Hemisphere.
How do birds navigate?
Most research into bird navigation has centered on celestial and magnetic orientation.
There is increasing evidence that birds navigate using a system integrating a number of redundant mechanisms:
• Genetic “automatic pilot” time-distance programs “tell” birds to fly certain a direction for a certain number of days, and then fly another direction for a certain number of days, etc. Passage of time triggers next direction change. Young birds make these migrations before they have figured out how to correct for headwinds, tailwinds, etc., and these birds become our “vagrant” species. Over time, older birds learn to correct for headwinds etc., and they also learn landmarks to help guide them.
• Some birds use a “solar compass” – comparing the position of the sun with their internal biological clock, and compensating for the sun’s apparent movement across the sky.
• Many songbirds migrate at night (safety from predators, as well as cooler conditions), and navigate by the stars.
• Polarization of the sky provides orientation information for some species. Gas and water molecules in the atmosphere scatter light in all directions. Sunlight traveling directly toward you is on polarized. Light is no more polarized and directions perpendicular to the suns rays–at noon polarization would be most apparent along the horizon, and at sunset forms a band across the sky pointing north and south. Some but not all birds use polarized light as a cue for orientation.
• Some birds have magnetite in the nasal cavities, which allows them to use the Earth’s magnetic field to navigate. Birds distinguish the strength of the magnetic field, as well as the dip angle. They do not distinguish North and South, but the distance between the nearest pole and equator. The inclination, or dip angle, helps birds gauge their latitude.
• Other orientation techniques that birds use include: detecting extremely low frequency sound waves – like those generated by wind, oceans, earthquakes, and thunderstorms.
• For waterfowl, migration may be a learned behavior. Family groups migrate together.
Earth’s Magnetic Field
Explore the Earth's magnetism and how birds orient themselves.
Activity: Where’s North?
Activity: Magnetic Globe
Activity: Magnetic Prints
Activity: Telling Someone Where To Go
Where do birds migrate?
How do we know what we know? Much of our knowledge about migration comes from bird banding. Scientists have been banding birds for about 100 years, and since 1920 this has been coordinated by the US federal government. The federal Bird Banding Laboratory’s licensed banders ring about 1.2 million birds per year. Band recovery rates are about 1-2% for most birds, and 10-15% for waterfowl (higher rates due to hunters recovering banded birds).
Central America is a vitally important wintering ground, and provides resting areas, for many species, including hawks. North America forms a giant funnel, with Central America located at its narrows. This sets up a tremendous disparity between the amount of land available for breeding and that available for wintering. For some birds, the ratio of breeding range to wintering range is 6-1. This is one of the reasons the deforestation in Central America is of great concern.
About Hawks:
How hawks migrate:
Hawks migrate by “kettling” - soaring in tight circles to stay within a thermal air current -- and then descending in the direction of migration to the next thermal. Thermals are convective air currents and are a source of potential energy. In relation to the rising air mass in a thermal, the hawks are descending, but they are doing so inside a column of heated air that is ascending faster than the birds are descending – like walking slowly down the steps of a rapidly rising escalator. The interior of the thermal rises much faster than the air along its edges, which is exploited by smaller hawks which have smaller turning radii. Migrating hawks use both thermals and updrafts to gain altitude. The hawks that migrate the longest distances – the flock migrants -- use thermals almost exclusively. This kind of migration is only possible during the day, because convection can only occur after the sun has started heating up the earth’s surface.Hawks gain altitude at a rate between 1 - 5 m/s. A typical altitude rate is 1 m/s.
Activity: Convection Currents
This model demonstrates convection currents in water, which we can generalize to understand the movement of water vapor in our atmosphere as part of the water cycle. Try this adaptation to really drive the convection: Support your plastic box with two coffee mugs – one filled with hot water, and the other one empty. Add the red food coloring to the hot water side of the box.
Activity: Convection Spiral
Simple paper toy demonstrates upward movement of heated air.
Physiology:
Bird’s lungs are 10 times more efficient than those of mammals. Fixed in size, fine tubes (air capillaries) lie parallel to the capillaries of the circulatory system, and air passes through them in the opposite direction from blood flow, permitting a much greater gas exchange while taking up less space.
Diet:
Hawks have simple digestive systems, and their diet of meat places no great demands on the system. The crop, the sac located at the base of the neck, allows substantial amounts of food to be stored. As the stomach empties, the hawk can push food from the crop into its stomach.
Eyes:
Raptors have excellent eyesight. Each eye of the hawk has two foveas. The central fovea detects movement while the side fovea detects detail.
Activity: Peripheral Vision (Human Eye Physiology)
Wings:
Three wing properties affect its function:
1) Camber is the degree of front-aft curvature. High camber wings are very broad and suitable for prolonged slow speed soaring and provide much lift, and are typical of vultures, eagles, and buteos. Low camber wings are narrow and enable the bird to travel at high speeds while providing little lift at slow speeds, typical of falcons.
2) Aspect Ratio is the ratio of the wing length to the wing width, and is calculated by dividing the square of the wingspan by the surface area of the wing. Wide wings have a low aspect ratio, and provide much lift but also generate more drag.
3) Wing Loading is the amount of weight to the unit area the wing must support, and is calculated by dividing bird’s mass (g) by the wing area (cm2). Raptors with relatively low body weight and large wings such as Turkey Vultures have low wing loading, and are well suited for soaring. Birds with higher wing loads (either with higher masses or smaller wing areas) must spend more of their time flapping. Smaller hawks can be identified by their flapping/soaring ratios. The greater the wing load, the faster a bird must fly. Hawks typically have a wing load between .3-.5
Activity: Handy Measuring Tool
Use ratios and proportions along with your hand to make indirect measurements of distant objects. Estimate the altitude of migrating birds using wingspan data.
BirdWingspan
American Kestrel.52 m
Sharp Shinned Hawk.53 m
Merlin.6 m
Broad Winged Hawk.84 m
Cooper’s Hawk.71 m
Harrier1.07 m
Red Shouldered Hawk1.02 m
Red Tailed Hawk1.22 m
Osprey1.59 m
Turkey Vulture1.75 m
Golden Eagle2.0 m
Bald Eagle2.24 m
Flight:
Think about the weight that a set of wings can support. Both Bernoulli and Newton provide qualitative explanations for lift.
The average angle of attack for horizontal flight is 6°.
Glide ratio tells you how many units an object travels horizontally for each unit it drops vertically. The goal is to cover the maximum distance per unit of altitude. Gliding performance depends on a bird’s mass, wingspan, wing area, and aspect ratio (wing area divided by wing width).
Some glide ratios:
American Kestrel9
Paragon Falcon10
Red Tailed Hawk10
Turkey Vulture15.5
Flying Things: Glide Ratio
Make a paper airplane glide like a bird
Unlike most forms of animal locomotion, for which going faster requires more energy, for flight, there is an optimal velocity above and below which requires more energy.
Resources:
Raptors of California, by Hans Peeters and Pam Peeters
University Of California Press, 2005
ISBN: 0-520-24200-9
Living on the Wind: Across the Hemisphere with Migratory Birds, by Scott Weidensaul
North Point Press, 1999
ISBN: 13-978-0-86547-591-5
The Simple Science of Flight From Insects and Jumbo Jets, by Henk Tennekes
MIT Press, 1997
ISBN: 0-262-20105-4
Lori Lambertson
Exploratorium Teacher Institute
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