October 25th, 2022 by georgann
For most birds, egg incubation is a relatively quiet time between the hurried activity of nest building and the frenzied period of caring for the newly hatched young. Once they`ve hatched, the nestlings will require warmth, protection from predators and the weather, a clean nest, and a great deal of food.
Each species` parents vary these activities according to the degree of maturity the nestlings exhibit upon hatching. Alexander Skutch, in Parent Birds and Their Young, classifies nestlings into four basic types: altricial, semialtricial, subprecocial, and precocial.
Altricial birds hatch from comparatively smaller eggs and in less time than precocial eggs. Where altricial young have already hatched and are being fed by their parents, precocial young, at the same age, are still in the egg using the yolk for their food source. When they leave the nest, the altricial fledglings most resemble their parents in size and appearance, while the precocial young are usually downy, small chicks.
Altrical species include all songbirds, woodpeckers, hummingbirds, swifts, kingfishers, pigeons, doves and parrots. Most people would consider altricial hatchlings to be ugly. Their skin is wrinkled, bulging eyes are closed, large mouth is gaping and behavior is weak and unsteady. The wrinkled skin will slowly be covered by tufts of downy feathers, usually emerging first at the top of the head.
Herons, gulls, terns, hawks, goatsuckers, and owls are considered to be semialtricial since they all hatch well covered with downy feathers. Semialtricials usually hatch on the ground or in poorly constructed flat nests, have open eyes (except the owls), and are able to walk or hop within hours. They can wander away from the nest within a day or two, but are still fed and brooded by their parents away from the nest.
Subprecocial chicks hatch with open eyes, are well covered in downy feathers, and are able to leave. nest as soon as they hatch. They follow their foraging parents and are fed from the parents` bills. Grebe, coot, and rail nestlings climb upon the back of either parent while cruising around the marsh or pond.
Generally precocial young are ground nesting species where predators are likely to disturb them. Approaching and departing from the nest would undoubtedly lead predators to it; better to have chicks that are mobile at hatching.
Domestic chickens, quail, pheasants and ducks are the best known precocial chicks. They hatch with downy feathers, eyes open, and leave the nest using muscles and limbs that are more developed for their size than altricial nestlings. Gallinaceous birds follow the hen, who scratches on the ground, finds food, lays it down for her chick, and points to it. She will brood her chicks for awhile away from the nest. Ducklings, on the other hand, are not “shown” food like the domestic hen and her chicks. Young ducklings can swim, dive and capture food as soon as they hatch. Most waterfowl can live without parental care by foraging and carefully hiding, although their parents help ensure their survival by guarding them from predators.
Parents brood their newly hatched young to keep them warm, to dry their skin and natal down, to keep hot harmful rays of the sun off the nest, and to cover and protect them. Factors such as weather, site of the nest, number of predators and age of the nestling determine how much the young are brooded. Altricial nestlings are poikilothermal (cold-blooded) and totally rely upon the body heat of their parents for several days. In general, if both parents shared in the incubation of the eggs, then both parents take turns brooding. If the female alone incubated, then she will continue brooding behavior while her zealous mate attends to other nest behaviors; protection, sanitation and bringing food to the female and the young.
Altricial young are brooded only in the nest where they hatch. The other types of nestling/fledglings are brooded away from the nest. Chickens will simply settle down and their chicks crawl under them. Marsh birds, such as coots and gallinules, will guild a flat brooding platform away from the original nest. Doves and pigeons brood fledglings by tucking them under their wings while sitting upon a perch.
TYPES OF FOOD
Precocious young find food for themselves although their parents lead them to good foraging areas. For parents of altrical young, however, the nestling period is the most strenuous of the reproductive cycle. Within a few days of hatching, altricial young are fed similar to their parents` diet. Seed eaters feed their nestlings insects at first, but then seeds, frugivorous birds feed mostly fruit, fish eaters feed fish, etc. Food brought back to the nest will be in small proportions or a small size, perhaps crushed, mashed or torn apart. Very young insectivore nestlings are fed insects that have had their legs and wings torn off by the parents. This processing of the meal undoubtedly makes it easier for the nestling to digest large amounts of food.
It is most efficient to bring as much food as possible in one trip, thereby avoiding attention to the nest and alerting predators of its presence. Atlantic puffins capture up to twenty fish at one time carrying them back to the burrow where they are poked down the nestlings` gaping mouths until their appetite is satisfied. Any surplus that the nestlings cannot swallow will be eaten by the parent.
Pigeons and doves feed their newly hatched young “pigeon milk”, a fatty secretion from the crop`s epithelial cells which are stimulated by the hormone prolactin. This milk is regurgitated for the young until they are old enough to receive seeds and other solid foods. In a similar fashion, albatrosses, petrels, and shearwaters feed a greenish oil that is secreted from their proventriculus, the glandular part of the stomach.
METHODS OF FEEDING
Whether fed at the nest or away from it, the parent has a variety of ways to deliver the food. Food that has been swallowed by the parents needs to be regurgitated into the nestling`s mouth. The parent may place its bill into the gaping mouth, forcing or pumping the food into the nestling`s throat. Pigeon parents grab the nestling`s bill, while the young bird reaches inside for food. Many gull species have a conspicuous spot on the tip of the beak which the young peck, forcing regurgitation. Regurgitated food is assumed to have been partially digested by enzymatic activity of the parent`s digestive system.
If a parent has to travel long distances with food, such as albatrosses which gather fish from hundreds of miles away from the nest, or if the beak cannot hold enough food, as in the case of hummingbirds, then usually the food is swallowed first and regurgitated at the nest.
Goldfinches, siskins and tanagers all regurgitate food. Flickers that eat a large amount of small ants regurgitate, while other woodpeckers carry insects, seed and fruit in their bills.
Feeding can often appear violent. The young of herons, bitterns and especially pelicans grasp their parents` bill, roughly pulling it down so that it can stick it entire head into the parent`s throat. With tiny wings flapping and legs struggling, the young bird comically appears to have been partially swallowed by its parent.
Large items collected close to the nest can be placed directly into the mouth from the parent`s beak. Swallowing this food would be a waste of time and effort.
Young hawks, eagles and owls receive food pieces that the parent brings to the nest, holds down with one foot, and tears out small chunks which are given to the young. As these young mature, they are given whole mice small birds, lizards, etc., which they have learned to tear apart. As a rule, altricial nestlings have large gaping mouths that are enhanced with bright yellow/orange flanges of skin. When the parent arrives at the nest, it is usually greeted by the vertical gapes of the nestling. The bright colors and marks act as targets for the feeding parent. European cuckoos that parasitize other species, gape with a pattern of spots that mimic foster siblings` gaps, thereby eliciting food from foster parent hosts.
The amount of feeding is highly regulated by begging postures and cries of the nestlings. The more twittering and begging by the nestlings, the more the parents increase their efforts to satiate them. Parents that regurgitate food tend to feed less often than those that carry food in the bill. They may feed only once per hour. As the young mature, the number of feedings may not increase significantly. Evidently, the amount of food at each feeding increases rather than the frequency of feedings.
On the other hand, for birds that bring food in their bills, feeding can be amazingly rapid. Although the average number of feeding trips is four to twelve meals per hour, House Wrens were seen to feed 491 meals in one ten hour period to five nestlings. Feeding continues throughout the day, with periods of feeding activity highest in the early morning. Birds with large broods have fairly short, if any, periods of non-feeding activity.
When nestling becomes satiated, its swallowing reflex ceases to function. The parent will hurriedly grab the unswallowed food from the nestling`s mouth and offer it to another gaping mouth. There is a constant shifting and pushing by the young to get in position for food delivery. Stronger, hungrier birds may get fed more often than their weaker or younger siblings. In the case of raptors, the youngest, weakest nestling may be killed or driven from the nest by the older, stronger nest mates.
The last major event in the care of nestlings is nest sanitation. In general, a satiated nestling will turn around to deliver a fecal sac to its parent, who will either swallow it or carefully dispose of it away from the nest. Cavity nesters would obviously benefit the most from this particular. tidiness. In other cases, the nestling will maneuver to the edge of the nest, place its tiny vent over the rim and, with a little squirting motion, deposit its own feces to the ground below. The smaller the nest, the more effective is this squirting behavior. In the large, stick platforms of herons and egrets, the nestlings cannot quire reach the rim when defecating. The edges of the nest become encrusted with excrement giving the rookery an offensive odor.
Nest sanitation appears to be of little importance to pigeons an doves. Both the young and the parents foul the nest. Perhaps the accumulation of sticky feces helps to glue their flimsy nest, stabilizing it for future broods.
Other objects have to be removed from the nest besides feces. Egg shells have already been evacuated away from the nest so as not to attract predators. Carrying out seed pits, uneaten food, insects, etc., keeps the nest clean and free of harmful molds. fungi, and parasites that endanger the nestlings` survival.
DEFENSE OF THE NESTLINGS
Nest concealment is of primary importance in the success of nesting, but occasional predators find a nest. To minimize the commotion of approaching and leaving a nest, the idea is to do so with concealed, cautious movements. Anyone who has ever tried to locate an active nest know how patient they must be while waiting for the adult to approach. Even then, the bird may zigzag its way or change its approach pattern to make it difficult to follow.
Occasionally predators find a nest. The parents have a choice of defense strategies. They may try to divert or lure the predator from the nest (the Killdeer strategy) or attack the intruder (the Northern Mockingbird strategy).
Perhaps the most exciting part of this reproductive cycle is leaving the nest. This happens much quicker, of course, with precocial and semiprecocial chicks than with altricial nestlings. The fledglings must still escape many dangers in order to survive to become totally independent.
Reference: Skutch, Alexander, Parent Birds and Their Young, University of Texas Press,1976 .
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