KEEPING WARM 
dissolved in water. Triglyceride, in con- 
trast, is hydrophobic and can be stored 
in an energy-dense form that minimizes 
its bulk. 
Existing data on mammals (including 
humans) suggest that carbohydrate, al- 
though quantitatively less important 
than triglyceride, is the limiting fuel in 
conditions requiring a high rate of en- 
ergy expenditure, such as exercise or 
cold stress. Although the exact mecha- 
nism causing muscle fatigue is not 
clearly understood, muscle exhaustion 
correlates with a depletion of glycogen 
reserves and/or a reduction in blood 
glucose, which is either derived from 
liver stores of glycogen or synthesized in 
the liver from other compounds, princi- 
pally amino acids. People who jog or run 
recognize that fatigue sets in long before 
triglyceride reserves become exhausted; 
marathon runners usually attempt to 
prolong their endurance by “carbohy- 
drate loading” before a race. It is also 
interesting to note that in humans sub- 
jected to cold stress, low blood glucose 
often leads to hypothermia. 
Goldfinches captured in winter store 
approximately 11 to 25 milligrams of 
glycogen in the pectoralis muscles and 
liver, an amount representing only about 
one-hundredth the mass of triglyceride 
■ stores. When these birds were tested at 
moderately and severely cold tempera- 
tures, they removed less glucose from 
the blood and used less glycogen stored 
in the pectoralis muscles than summer 
birds tested under similar conditions. 
The endurance of these birds during 
cold stress may depend significantly on 
the ability to utilize the available carbo- 
hydrate at a lower rate in winter than in 
summer. We believe this ability results 
from an enhanced capacity to catabolize 
triglyceride in winter: the activity of the 
enzymes that break down fatty acids is 
50 percent higher in the muscles of 
winter birds than in those of summer 
birds. The use of more triglyceride in 
the muscles may spare the limited car- 
bohydrate for such essential functions as 
energy supply to the brain, which is 
House finches, unlike goldfinches, 
do not show a marked increase in 
ability to produce heat in the 
winter. For them, survival in cold 
climates may depend partly on 
seeds from bird feeders. 
Laura Riley 
principally dependent on blood glucose 
for energy. 
Although the augmented capacity to 
store and use triglyceride and the de- 
creased rate of carbohydrate breakdown 
appear to play important roles in the 
seasonal increase in thermogenic capac- 
ity in goldfinches, other metabolic fac- 
tors may exist that we have not yet 
documented. It is important to remem- 
ber, too, that these metabolic changes 
constitute only some of the methods by 
which goldfinches cope with cold win- 
ters. By slight increases in insulation and 
the use of protected nocturnal roosts, 
they also reduce the amount of heat 
production required to maintain body 
temperatures. William A. Buttemer, an- 
other graduate student working with 
Dawson, found that goldfinches roost 
among the branches of conifers during 
winters in southern Michigan. Measure- 
ments of the heat production necessary 
to maintain a high body temperature in 
these roosts indicate that the birds may 
conserve as much as 30 percent of the 
energy that would be required if they 
roosted in an unprotected spot. The suc- 
cess of goldfinches in cold climates is 
clearly due to a suite of metabolic, 
insulative, and behavioral adaptations. 
As some features of the goldfinches 
seasonal adjustment to cold became ap- 
parent to us, we wondered whether this 
pattern was typical of other cardueline 
species. To address this question, we 
turned to house finches ( Carpodacus 
mexicanus). These 18- to 21-gram birds 
have a more southerly distribution than 
goldfinches and are very common in the 
southwestern United States. Their dis- 
tribution extends northward up the Pa- 
cific Coast and into the Rocky Moun- 
tain states. Before the 1940s, house 
finches did not occur naturally in the 
eastern United States. Pet dealers in 
New York, however, illegally trans- 
ported some house finches from south- 
ern California for sale as cage birds and 
abruptly released them just prior to 
raids by the authorities; the descendants 
of these birds have now successfully 
colonized most of the eastern coastal 
states. 
We were able to detect only a minor 
seasonal difference in the thermogenic 
capacity of house finches subjected to 
—94°. Birds captured in Boulder, Colo- 
rado, during the coldest winter on rec- 
ord tolerated below —76° for an average 
of ninety-eight minutes, whereas birds 
captured in summer averaged only nine 
minutes. These results are interesting 
because of both the low endurance 
capacities of house finches, particularly 
in winter, and the lack of a dramatic 
seasonal increase in their thermogenic 
capacity, compared with the goldfinch 
findings. Other important differences 
between house finches and goldfinches 
emerged when body masses and con- 
stituents were analyzed. House finches 
exhibit no seasonal fluctuation in either 
triglyceride content, which remains con- 
stant at about one gram, or body mass. 
Finally, in winter, house finches do not 
reduce their reliance on blood glucose 
during exposure to cold. 
These findings indicate that house 
finches lack the characteristics that we 
believe contribute importantly to the 
abilities of goldfinches to stay warm in 
cold climates. How, then, do house 
finches survive cold winters? Perhaps 
the large thermogenic capacity in gold- 
finches represents an ability for pro- 
longed heat production that they exploit 
fully only under exceptionally severe 
weather conditions; the lower thermo- 
genic capacity of house finches may 
signify a smaller margin of safety and 
may result in substantial mortality dur- 
ing prolonged, severe winter storms. The 
success and persistence of house finches 
in the northeastern United States, how- 
ever, suggests that alternative methods 
are employed for coping with cold win- 
ters. House finches, more than gold- 
finches, appear to rely on filling their 
crops with seeds just prior to retiring to 
their roosts for the night. Perhaps other 
behavioral and physiological adapta- 
tions will be identified. The survival of 
house finches in cold climates may be 
enhanced, for instance, by their associ- 
ation with humans, in terms of their use 
of buildings and houses as nocturnal 
roosts and their dependence on seeds 
from feeders. 
Cold winters provide substantial chal- 
lenges to small birds, and our research 
indicates that at least some of the birds 
respond in ways that make them physio- 
logically and biochemically distinct 
from their summer counterparts. Fur- 
ther research will undoubtedly uncover 
additional physiological and behavioral 
patterns that foster the survival of birds 
and other organisms in stressful environ- 
ments. Study of the flight muscles of 
birds, with their specialization for high 
oxidative metabolism, can be particu- 
larly helpful in understanding the fac- 
tors that limit the abilities of all animals, 
including humans, to sustain high rates 
of metabolism for long periods of time.D 
63 
