the tiniest of all warm-blooded animals. Your entire body weighs less than
two dimes. Some insects and worms even weigh more than you! When the air
is cool, you must shiver or fly about to keep your body temperature up,
and shivering or flying requires energy—lots of it! From the time
you awake in the morning until you sleep at night, you are almost constantly
doing one of three things: Eating high-energy food, quietly perching while
you digest your food, or searching for new sources of food.
Photo: Ed Robertson
As long as you can find enough food to stay active, your body can generate
enough heat to stay alive. But you can only eat when it's light. What happens
when you go to sleep at night? How can such a tiny warm-blooded animal keep
up its body temperature without running out of energy? This experiment can
help you figure it out:
will need a small piece of modeling clay, a balance, a thermometer, and
your balance, get two chunks of clay, one weighing about 3 grams and
one weighing about 90 grams. If you measure right, the small piece of
clay will have about the same mass as a Ruby-throated Hummingbird, and
the large one will have about the same mass as a Blue Jay.
- Poke your
thermometer into the large piece of clay and hold it warmly in your
hand or place it on a warm (NOT hot!) surface until the thermometer
reads 80 degrees F (or about 26 degrees C).
- Now place
the clay and thermometer in the refrigerator. Every minute, check the
temperature and record it in your journal. How many minutes does it
take for the clay's temperature to drop to 50 degrees F (10 degrees
- Poke your
thermometer into the tiny piece of clay, and hold this in your hand
or put on a warm surface until it reaches 80 degrees F.
- Put the
clay in the refrigerator and check the temperature every minute, recording
it in your notebook. How many minutes does it take for this piece of
clay's temperature to drop to 50 degrees F (10 degrees C)?
Laure W. Neish
Your experiment should have shown that hummingbirds lose their body heat
a lot faster than Blue Jays and other large birds. If they needed to keep
their nighttime body temperature at even 90 degrees, over 10 degrees cooler
than it usually is, they'd have to waste a lot of food energy and could
If you needed to save energy in your house, you could turn the thermostat
down at nighttime when people were sleeping. And hummingbirds do the same
thing, thanks to a special adaptation. They don't have a thermostat, but
they can allow their body temperature to drop sometimes more than 35 degrees
F (20 degrees C) at night, from a daytime temperature of over 100 degrees
F (over 37 degrees C)all the way down to 60 or 65 degrees F, so they won't
need to burn nearly as many food calories to stay alive during the many
hours they can't eat. This temporary drop in body temperature is called
torpor. Humans normally die if our body temperature drops
that low, because we are not adapted for torpor.
Dropping its temperature slows down a hummingbird's breathing and heart
rate. One Blue-throated Hummingbird whose daytime, active heart beat 1260
times per minute had a heart rate when torpid all the way down at 36 beats
The trickiest part of torpor isn't letting the body temperature fall at
night, but getting it back up in the morning. A hummingbird has to warm
up its body the moment it awakes. To do this, it makes its muscles contract
and release very fast; it shivers! The problem is this: the hummer's stomach
is empty and it must use up stored energy. When a hummingbird's body temperature
is about 20 degrees C, it takes about an hour for it to warm up enough for
normal activity. During this time, the bird is sluggish, groggy, and very
hungry! As soon as it can, it flies off to eat breakfast.
How can the hummingbird keep from falling off the branch during torpor? The feet get a good grip! Most perching birds have four toes on each foot with a claw at the end of each toe. When the hummer settles and bends its legs to perch, the stretched tendons of the lower leg flex the toes around the branch and clamp Hummer in place. This "perching reflex" is automatic.
This! Journal Questions
written your explanations, see how they compare with Journey North Science
Writer Laura Erickson's explanations.
- Why is
a 3-gram chunk of clay so tiny compared to a hummingbird that weighs
the exact same?
- Birds that undergo nighttime torpor are just about
all very small. They include the tiniest birds of all, hummers, but
also some chickadees and swifts. Why do you think no large birds undergo
nighttime torpor? HINT: Think about the experiment with the clay.