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
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.