Growth and Development

Hatching is a momentous event because it is switching gas exchange from the surface of the egg, to now using its lungs. Birds have an egg tooth, which is a small point on its upper bill that is used to excavate a ring in the egg. The air space in the egg is used by the bird to drill a hole in the egg. The birds have a very powerful hatching muscle on the back of the neck.

Embryonated chick eggs: Eggs at hatching are usually about 18 days old (ish). Some people then use the eggs when they are 16 days old. Yet, there can often be communication between kids in the nest, and parents to the eggs. There are some calls that the kid in the egg can make to tell the parents what is going on. Thus, it seems to be that the young can feel when inside the egg!

There is a large change in morphology that occurs in the nestling period. The nestling period is from the time the first kid hatches to when the last kid fledges. Thus the nestling period leads to the fledging period. The act that goes from one to the other is the act of ‘fledging’. The fledging periods is from when the last kid fledges to when all the young are independent form the parents.

Altricial birds:
In these types of birds, the chicks are almost useless when they hatch. They are mostly featherless and their eyes are half closed. This is the form of full altricial. There are of course stages of altricial. Hummingbirds are extremely altricial.

Semialtricial kids are unique as well. These birds often have some down feathers, and large feet. Its also dependent on the parents but hatches at an advanced state.

A semi-precocial bird has its eyes wide-open and is covered in feathers.

Precocial chicks, are completely down-covered. It can walk. Its wings are not big enough to fly but it can walk. It can follow its parents around and its eyes and brains are well developed.

Superprecocial: mound builders
Precocial: duck, shorebirds, quails, grouse, murrlets
Altricial: songbirds, woodpeckers, parrots

Character Altricial Precocial
Eyes at hatching Closed Open
Down Absent or sparse Present
Mobility Immobile Mobile

Transition from incubating to taking care of the birds when they hatch.

Delivering food is an important aspect of parental care in altricial birds. The relationship between kids and adults must be very closely evolved when you have sharp bills (ie. Anhinga parent, and kids.

Most birds have a sigmoidal growth curve (s-shaped). The biggest difference between the different sized birds is not that they start growing faster, but that as the other stops growing, the other just keeps on growing more and more.

Endothermy is birds is unique. An altricial bird has no endothermy when it is born. It slowly gains endothermy as they begin to grow.

The growth curves vary significantly amount species. For example, a curve-billed thrasher has a slow growth curve. They fledge when they weigh less than their parents. Cliff swallows, however, actually grow very fast and looses weight after it leaves the parents. This is most certainly an insurance solution that makes sure that it doesn’t starve.

Tissue allocation hypothesis: Bob Brickleff:
If you take a leg tissue, you can grow it or mature it, but you can’t do both at the same time. Thus, tissue allocation, refers to a decision to increase mass or increase maturation of the tissue. Quails, which are precocial, put a lot into the maintenance of their leg muscles. Thus, the growth of the leg muscles is much longer. This is because for a given amount of growth there is a given amount of maturation. Other species like starlings, don’t really need to walk (its altricial), so it can add mass quickly and worry later about maturation.

The strategy of an altricial bird is to start with nothing and grow like mad! Te primitive state is the precocial state and the altricial state. Its believed that the altricial state has evolved to minimize the time for nest predators.

Some birds secrete food for their kids: pidgeons produce crop milk. But flamingos and penguins also produce crop milk.

By Rob Nelson

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