The
young of most egg-laying reptiles hatch long after the
parents have abandoned the eggs; a few lizards and snakes
guard them, and pythons incubate their eggs for a while. The
young of those female snakes that carry their eggs inside
the body until they hatch also receive no parental care.
Among reptiles only crocodiles and their relatives tend both
eggs and hatchlings. In contrast, nearly all birds provide
extended care for their offspring. The exceptions are brood
parasites, which foist their responsibility onto other
species, and some megapodes, turkey-like birds of the
southwest Pacific. Most megapodes scratch
together mounds (sometimes astonishingly large) of
vegetation or sand and lay their eggs inside. The heat for
incubation is provided by decay of the vegetation, the sun,
or (occasionally) volcanic activity. Some megapodes tend the
mound, opening and closing it to regulate the incubation
temperature; others desert the mound. A few megapodes do not
build mounds, but simply lay their eggs in warm spots on
sand or between rocks and cover them with leaves. Patterns of care in
precocial birds (those with young ready to leave the nest
almost immediately after hatching) vary a great deal. The
major parental duties for most are to keep the young safe
from predators and to watch over them as they feed. In many,
however, the adults also help instruct the chicks in what's
good to eat, how to find it, and how to handle it.
Oystercatchers first present food to their young and then
train them to find food for themselves. The latter is a long
process; oystercatchers specialize in opening mussels and
other bivalve mollusks, a difficult task that can be
accomplished in less than a minute by an experienced
individual, but one that requires many months to
learn. The young of passerines, and
thus of most birds, are altricial (born naked, blind, and
helpless) and require much more care and feeding than
precocial young. One or both parents must bring food to
altricial young until they are ready to leave the nest, and
in most species the offspring are fed by the parents for a
while after fledging. Most passerines are monogamous, and
usually both parents help in rearing the young. Often the
male does more of the food gathering and the female more of
the brooding -- covering the young to keep them warm (or to
shield them from sun or rain) and protecting them from
predators. Frequently, the male also feeds the female, and
she in turn may pass food on to her helpless chicks. In some
cases, however, those caretaking roles are reversed. Female
Red-eyed Vireos, for example, gather about three-quarters of
the food their young receive. In cooperative breeders, such
as Acorn Woodpeckers, nonbreeding adults or juveniles may
help care for the young. In polygynous species (where
one male mates with more than one female), the male's
parental role is reduced in both precocial and altricial
birds. Polyandrous species (one female with more than one
male) are all precocial, and the burden of caring for the
offspring either falls exclusively on the males or is
shared. Generally parent birds feed
their offspring a diet similar to their own, but during the
breeding season the diet of the adults (and thus of the
young) shifts toward higher-protein foods. Many passerine
birds that during the winter subsist mainly on vegetable
foods eat insects and feed them to their young during the
breeding season. There is a tendency for the birds to
consume the smaller insects themselves and, for the sake of
efficiency, to carry larger ones back to the
nest. Other parents swallow the
food as they forage and then regurgitate it for the young
when they return to the nest. As the young mature, the
proportion of solid food in the regurgitant increases --
perhaps an avian analogue to weaning. Some birds, such as
pigeons, produce a special "crop milk," which is also
regurgitated for the young. Petrels regurgitate for their
young an oil along with half-digested food from which the
oil is derived. Raptors usually carry their prey back to the
nest and tear it into bite-sized chunks for their
chicks. The feeding instinct in
parental birds is very strong, and feeding behavior is
usually elicited by feeding calls and gaping on the part of
the chicks. When a bird's own brood is destroyed, it may
transfer its attention to the young of others; observations
of birds feeding the young of other parents of the same
species, and even of other species, are quite common. One
Northern Cardinal was even observed to have adopted a school
of gaping goldfish at a pond where the fish were accustomed
to begging from people! Presumably, the length of
time that adults will care for their young is determined by
several factors. In most cases, the longer the care, the
better the chances that the young will survive to maturity.
Counterbalancing prolonged dependence in the "calculations"
of evolution, however, are the possibility of the parents
rearing a second brood and the physical cost of extended
care. These factors affect the probability of the adults
being able to survive migration or wintering to breed again
the next season. As far as possible, evolution will favor
the strategy that maximizes the reproductive output of an
individual over its entire lifetime; this may limit the
amount of care given to any one set of offspring. There is,
in fact, often a conflict between the evolutionary interests
of parents and young, it being best for the parents to cease
care before it is best for the young to be on their own.
This conflict is not restricted to birds (as some of us well
know) and is one of the more interesting topics in
sociobiology. SEE: Precocial
and Altricial Young;
Incubation:
Heating Eggs;
Monogamy;
Polygyny;
Cooperative
Breeding;
Creches;
Bird
Milk. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.