Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 427

in the spring (usually about the time the snow is beginning to melt), and occupy tem-
porary roosts until appropriate nest building areas are bare (Pattie and Verbeek 1966).
In the short alpine summer of mid- and high-latitude mountains, only one nesting at-
tempt can be made. Because of the overwhelming urgency to complete the necessary
functions within the brief time allowed, the baby birds mature exceedingly quickly.
A wide variety of behaviors are displayed by different species of birds in relation to
local environmental conditions. Often, either the male or the female does not stay and
care for the young, but one or the other leaves as soon as the young are hatched. This is
apparently an attempt to reduce competition for food, thereby increasing the chances of
their offspring's survival (Hoffmann 1974). The timing of brood arrival in many species
is closely synchronized with the seasonal peak in insect supply. This provides abundant
and accessible food for the young at a time when they are poorly equipped to forage
for themselves. From the insect's point of view, this approach also has survival value,
since their avian predators are so swamped with insects that the survival of adequate
numbers is assured (Maclean and Pitelka 1971; Hoffmann 1974).
Another carefully timed function is the molt (replacing feathers). It is usually co-
ordinated with the breeding cycle and migration, so that these energy-expensive func-
tions do not overlap. In some cases, molting may be compressed and take place before
hatching occurs (Holmes 1966; Verbeek 1970), but more commonly it is delayed until
after the breeding cycle has been completed (French 1959; Miller 1961). Long-distance
migratory birds may leave the high country relatively early in the season and molt after
arriving at their winter grounds, whereas short-distance migrants usually molt after the
young arrive, and remain as long as possible before abandoning the alpine tundra (Hoff-
mann 1974).
Mammals show similar adjustments in their breeding cycles. The reproductive or-
gans of many small animals enlarge and mature while the snow still covers the ground,
so that breeding can take place during or immediately after snowmelt (Vaughan 1969).
This is analogous to the tendency among alpine plants to begin growth under the snow
in order to complete their life cycle in the short growing season. Another character-
istic among mammals, as among birds, is that birth usually takes place when food is
most readily available. Thus, animals living at higher altitudes breed later in the season
than do those in the lowlands. This is partly because of the delayed snowmelt, but it is
also because there would not be adequate food if they gave birth earlier (Pearson 1948,
1951; Geist 1971; Sweeney and Steinhoff 1976). The mule deer ( Odocoileus hemionus )
living at high altitudes in the Sierra Nevada of California fawn about the middle of July,
while those at lower elevations fawn by the middle of May or earlier (Hoffmann 1974).
Animals in tropical mountains do not have the problem of timing their breeding cycles,
since food is available throughout the year. Consequently, reproduction can and does
take place at any time (Coe 1967, 1969). It should be pointed out, however, that the
tendency of mid- and high-latitude animals to reproduce in the favorable season of the
year greatly reduces the environmental stress on them during this most vulnerable of
periods. This is not true in tropical mountains, where climatic extremes occur on a daily
basis; there, even newborn animals have to be capable of withstanding the entire range
of environmental conditions present during the day and the night (Salt 1954).
Like birds, most mammals in mid- and high-latitude mountains are restricted to a
single breeding attempt each year. Thus, while deer mice at lower elevations in the