Development In Young Birds At Hatching

A Nidicolous bird (/naɪˈdɪkələs/ ny-DIK-ə-ləs; from Latin nidus “nest” and -colus “inhabiting”) is the one that stays at its birthplace for a long time because it depends on the parents for food, protection, and the learning of survival skills.

All passerines are altricial. During the life span, the brain of a nidicolous animal expands 8–10 times its initial size; in nidifugous birds, from 1.5 to 2.5 times.

Nidifugous (/naɪˈdɪfjʊɡəs/ ny-DIF-yuu-gəs) organisms are those that leave the nest shortly after hatching or birth. The term is derived from Latin nidus for “nest” and fugere meaning “to flee”. The terminology is most often used to describe birds and was introduced by Lorenz Oken in 1816.The chicks of birds in many families such as the waders, waterfowl and gamebirds are usually nidifugous.

The term nidifugous is sometimes used synonymously with precocial, as all nidifugous species are precocial – that is, born with open eyes and capable of independent locomotion. However, not all precocial birds leave the nest; some may stay at the nest, and are thus considered nidicolous rather than nidifugous.

“Precocial” and “altricial,” two words describing the degree of development in young birds at hatching, are good examples of useful scientific jargon. They save ornithologists from repeatedly using phrases when single words will do. A precocial bird is “capable of moving around on its own soon after hatching.” The word comes from the same Latin root as “precocious.”

Altricial means “incapable of moving around on its own soon after hatchling.” It comes from a Latin root meaning “to nourish” a reference to the need for extensive parental care required before fledging in altricial species. The term “nidifugous” used to describe precocial young that leave the nest immediately, and “nidicolous” to describe young that remain in the nest. All nidifugous birds are precocial, but some nidicolous birds are precocial, too-they remain in the nest even though capable of locomotion. These terms are less widely used than precocial and altricial, and we will not employ them outside of this essay.

Instead of a sharp dividing line between hatchlings that are precocial and those that are altricial, there is a gradient of precociality. In this guide, we recognize the following categories of young:

Precocial Hatched with eyes open, covered with down, and leave the nest within two days. There are four levels of precociality.

Level 1 of development (Precocial 1) is the pattern found in the chicks of megapodes (Australian Malee fowl, Brush Turkeys, etc.), which are totally independent of their parents. The megapode young are incubated in huge piles of decaying vegetation, and upon hatching dig their way out, already well feathered and able to fly.

Precocial 2 development is found in ducklings and the chicks of shorebirds, which follow their parents but find their own food.

The young of game birds, however, trail after their parents and are shown food; they are classified as precocial 3.

Precocial 4 development is represented by the young of birds such as rails and grebes, which follow their parents and are not just shown food but are actually fed by them.

Semi-precocial Hatched with eyes open, covered with down, and capable of leaving the nest soon after hatching (they can walk and often swim), but stay at the nest and are fed by parents. Basically precocial but nidicolus, this developmental pattern is found in the young of gulls and terns.

Semi-altricial: Covered with down, incapable of departing from the nest, and fed by the parents.

In species classified as semi-altricial 1, such as hawks and herons, chicks hatch with their eyes open. Owls, in the category semi-altricial 2, hatch with the eyes closed. If all young were divided into only two categories, altricial and precocial, these all would be considered altricial.

Altricial: Hatched with eyes closed, with little or no down, incapable of departing from the nest, and fed by the parents. All passerines are altricial.

Why have these different modes of development evolved? They are obviously tied into two important aspects of the bird’s environment: food availability and predation pressure.

Precociality puts a premium on the ability of females to obtain abundant resources before laying. They must produce energy-rich eggs to support the greater in-egg development of the chicks (eggs of precocial birds may contain almost twice the calories per unit weight as those of altricial birds).

Females of altricial species do not have such large nutritional demands before egg laying, but must be able (with their mates) to find sufficient food to rush their helpless young through to fledging. While the young are in the nest, the entire brood is extremely vulnerable to predation and is dependent on concealment of the nest and parental defence for survival.

In contrast, precocial young, having left the nest, have some ability to avoid predation, and there is a much smaller chance of the entire brood (as opposed to single chicks) being devoured.

Interestingly, there seems to be an evolutionary trade-off in bird brain sizes related to the degree of precocity. Precocial species have relatively large brains at hatching-as one might expect since the young, to one degree or another, must be able to fend for themselves. But precocial species trade for this advantage an adult brain that is small in relation to their body size. Altricial young, in contrast, are born small-brained, but on the Protein-rich diet provided by the adults (and with their highly efficient digestive tracts) postnatal brain growth is great, and the adults have proportionally larger brains than precocial species.

Parrots have evolved their way into the best of both worlds. They are altricial, but the female invests in a nutrient-rich egg just like females of precocial species. Parrots are among the most intelligent of birds; they have adopted the same evolutionary strategy as we have. People (like other primates, elephants, and antelopes, but unlike rodents and marsupials) are precocial-born with hair, open eyes, and large brains. But our brains and those of parrots, both large at birth, also grow a great deal after birth as a result of large parental investments of food energy.

Thus a complex evolutionary problem of balancing the need to provide nourishment to the young and to protect them from predation has been “solved” by each group of birds-and the solutions are the different avian developmental patterns we now observe. Similar problems have been solved, also in diverse ways, in the course of mammalian evolution. But many more groups of mammals than birds have managed to become big-brained as both young and adults.