Biology Reference
In-Depth Information
As with human athletes training at high elevations to build up their red blood cells,
birds that move to higher elevations or are subjected in laboratories to lower air pressure
producemoreredbloodcells. Birds haveslightly less hemoglobin intheir redbloodcells
than do mammals, but avian hemoglobin is more efficient at picking up oxygen.
Those birds, by the way, are Alpine Choughs.
HEARTBEATS AND BREATHING RATES
Because birds inhale and exhale
relatively large amounts of air in each breath, as
compared with mammals, their breathing rates are much slower than those of mammals
ofcomparable size. The smallest hummingbirds take about 250breaths perminute while
the smallest shrews take about 800. Of course, flying birds breathe more rapidly than
resting ones: a duck at rest takes about 14 breaths per minute while a flying duck takes
about 96.
The rapid rate at which oxygen is taken up in the blood is reflected in a bird's rapid
heart rate. A mammal's average heart rate is about 3 times its respiration rate whereas
a bird's average heart rate is over 7 times its respiration rate. A chickadee's heart beats
about 500 times per minute at rest and doubles that during activity. To pump blood ef-
ficiently, the heart of a bird is larger than that of a mammal of comparable size, and the
heartisrelativelylargerinsmallerbirdsthaninlargerones.It'salsolargerinalpinebirds
than in their relatives who live at lower elevations.
Q Why do chickens and turkeys have white breast meat while ducks and geese have
all dark meat?
A
Birds and mammals have two kinds of muscle fibers: white, or “fast twitch,” fibers
and red, or “slow twitch,” fibers. Red fibers may not twitch as fast as white ones, but the
red fiber cells are richly supplied with mitochondria and red muscle tissues are heavily
oxygenated with a rich blood supply. This enables these fibers to work steadily for long
periods, so birds capable of long bouts of walking or swimming, such as turkeys, chick-
ens, ducks, and geese, have mostly red fibers in their leg muscles. Birds capable of sus-
tained flight, such as ducks, geese, songbirds, hummingbirds, and the like, have mostly
or even all red muscle fibers in their wing and breast muscles. Most birds have more red
than white muscle fibers; some hummingbird and House Sparrow muscles are made of
100 percent red fibers.
Whitemeatcomprisesmostlyfasttwitchwhitefibersbutusuallyhasatleastafewred
fibers, too. Gallinaceous birds in general (including turkeys and grouse) have white meat
intheirpectoralmuscles,whichmeansthemusclefibersaremostlythefasttwitchvariety
allowing the birds to take off in flight with a powerful burst. These muscles are seldom
