KEEPING WARM 
Shivering Finches 
For some small birds that don’t fly south for the winter, extra 
fat and shivering may boost their chances for survival 
by Cynthia Carey and Richard L. Marsh 
Chilled to the bone by the cold wind 
as we skiied through the woods one 
snowy day in Michigan, we were looking 
forward to a cup of hot chocolate and a 
warm fireplace. Rounding a bend in the 
trail, we were surprised to see a flock of 
small birds noisily foraging in bushes 
blown free of snow. We knew that most 
avian species that breed in arctic and 
temperate climates migrate to warmer 
areas for the winter and that only a few 
hardy species remain all year as perma- 
nent residents. Migratory birds possess 
orientational, behavioral, and physio- 
logical traits that enable them to mi- 
grate successfully, and we wondered 
what kinds of adaptations enabled resi- 
dent birds to survive cold winters. 
In many respects, the problems asso- 
ciated with staying warm in cold cli- 
mates are similar for birds and mam- 
mals, such as humans. Both birds and 
humans maintain high body tempera- 
tures (104° and 98.6°F, respectively) by 
internal production of heat. Such heat 
production is sufficiently great that 
body temperatures are raised above ex- 
ternal air temperatures, somewhat as a 
furnace heats a house above the outside 
temperature in winter. The temperature 
inside the house is maintained at a com- 
fortable and constant level by a thermo- 
stat, which turns the furnace on and off. 
Colder air temperatures increase the 
rate of heat loss from the house and 
require the furnace to produce heat for 
longer periods of time than milder tem- 
peratures do. 
Animals, however, produce heat, and 
regulate heat production, by methods 
different from that of a furnace. All 
living things produce heat by releasing 
energy stored in the chemical bonds in 
their food, specifically in carbohydrates, 
lipids, and proteins. Some of this energy 
is temporarily stored in the high-energy 
bonds of adenosine triphosphate (ATP) 
molecules, \vhich can then be used as 
the energy source for all the cellular and 
physiological functions necessary for 
life, such as ion transport across mem- 
branes, synthesis of cellular compo- 
nents, and contraction of heart and skel- 
etal muscle. The transfer of energy from 
the chemical bonds in the foodstuffs to 
ATP is not very efficient: about 60 per- 
cent of the energy is lost immediately as 
heat. Furthermore, as ATP is used for 
various metabolic functions, all the en- 
ergy in the original foodstuff ultimately 
becomes heat energy. Since the produc- 
tion of ATP requires oxygen and pro- 
duces carbon dioxide, physiologists 
usually estimate heat production by 
measuring the amount of oxygen con- 
sumed or carbon dioxide released by an 
organism. 
Animals, unlike furnaces, do not peri- 
odically turn off their heat-producing 
mechanisms. Maintenance of living tis- 
sue requires continual production of 
ATP, which constantly generates heat. 
The minimal level of metabolic pro- 
cesses needed to maintain life, called the 
basal metabolic rate, produces enough 
heat to maintain a 98.6° body tempera- 
ture in a nude human at air tempera- 
tures down to about 73.4°. If air tem- 
peratures fall below that level, the 
maintenance of constant body tempera- 
ture requires additional heat, which 
both humans and birds produce by shiv- 
ering. During shivering, antagonistic 
muscles contract simultaneously, so no 
external work is done. Thus, all of the 
energy used in contraction appears as 
heat inside the body. The principal com- 
pounds supplying the energy in the mus- 
cles are glycogen (the storage form of 
glucose) and triglycerides (lipids com- 
posed of fatty acids and glycerol). In 
severe cold, shivering can elevate the 
metabolic rate as much as five times 
above the basal level. 
Cold-climate birds, the majority of 
which are diurnal, have a large require- 
ment for food to fuel the costs of shiver- 
ing. The short periods of daylight in 
northern latitudes, however, reduce the 
amount of time for foraging, and in- 
clement weather, snow cover, or ice may 
further restrict food availability. The 
problems of staying warm must be par- 
ticularly intense for small birds weigh- 
ing less than thirty grams. Because 
small birds have a large surface area 
relative to their volume, their problems 
of heat loss are relatively more severe 
than those of larger birds. Smaller birds 
are also relatively less insulated than 
larger ones since their size limits the 
thickness of the insulative layer. Finally, 
calculations by William A. Calder of 
the University of Arizona suggest that 
larger birds can endure fasting under 
cold stress for longer periods of time 
than smaller ones. 
The problems encountered by small 
birds in cold climates have stimulated 
considerable research during the past 
thirty years, most notably by the late J. 
Stanford Hart of the National Research 
Council of Canada, George C. West of 
the University of Alaska, and William 
R. Dawson of the University of Michi- 
gan. Their efforts and the work of others 
have established that wild birds use a 
combination of two general means to 
cope with cold: mechanisms that reduce 
the amount of heat required to maintain 
a high body temperature and mecha- 
nisms that allow high levels of heat 
production for long periods of time. 
One of the ways birds can reduce the 
amount of heat they must generate in 
the cold is by increasing their insulation, 
an advantage that should be obvious to 
anyone who has put on a down jacket on 
a cold winter day. Extra feathers reduce 
the rate of heat loss and the requirement 
for additional heat production. Since the 
thickness of plumage is limited in small 
birds, larger birds can utilize seasonal 
variation in plumage insulation more 
extensively. The plumage of a 550- to 
650-gram willow ptarmigan ( Lagopus 
lagopus) in Alaska, for example, varies 
seasonally not only in color — from 
brown in summer to white in winter — 
but also in insulative value. Because of 
the difference in insulation, the heat 
To maintain normal body temperatures 
in the winter, goldfinches and 
other small birds produce heat by 
shivering. Some birds also adapt to 
cold by growing additional feathers 
or seeking protected roosts. 
Larry Wfest; Bruce Coleman 
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