NESTING, MATING & REARING CHICKS
According to researchers, millions of years before man appeared on earth, birds had already learnt to build nests and rear their young in every corner of the earth. As we know all birds lay eggs, but all do not make nests. In the same way they show different kinds of behavior all through their breeding period and in every aspect of it.
After selecting a site for nest, birds claim and defend their territory or nesting ground, which is generally respected by other birds. If any other bird trespasses, it is usually driven away by the established owner. Territories are defended only against others of the same species. Perhaps, the reason is to ensure enough food supply for their own family and joint protection against enemies.
Nesting grounds or territories vary in size, even amongst birds of the same species. Birds that form colonies, e.g. swifts, gulls and cormorants, generally have small to very small territories, in some cases it is limited just to the nest and its immediate vicinity. Some birds form mixed colonies. One of the most important example of this kind of nesting is the Bird Island, a tiny island – just three miles long and half a mile wide — half a mile off the coast of South Georgia, which, in summer becomes home for breeding colonies of thousands upon thousands of sea birds. It is thought to be the largest concentration of bio-mass in the world. The sea birds pack this tiny speck of land in the ocean, with nests on every inch of the available space. Despite this melee and cacophony, birds recognize their mates, nestlings and nests.
Once the territory is occupied, different species of birds advertise their ownership in different ways, e.g. songbirds by singing; raptors with sharp cries, woodpeckers by drumming on a resonant tree with their bills and so on.
Nest-building is an inherited trait and birds need not to be taught, how to do it. Each species builds a particular type of nest, which can generally be easily identified. Some of them build very simple nests while some very complex, some others dig holes in the tree trunk, while some seek out ready-made cavities therein. Usually both partners share the task, but in some species females do it single-handedly and only on rare occasions, the male alone performs this tedious job.
Usually birds build new nests before every breeding season, but in some species, the same nest is used for a number of years, adapting or adding something to it every year. Water birds construct their nests on or near water. There are many water birds, like moorhens, coots, grebes and swans, that build floating nests, hidden among reeds. Newly hatched babies of many waterbirds can soon swim, but some of them like those of grebes and swans usually climb aboard their mother’s back. This strategy protects them from predatory fishes like pike and catfishes that eat them.
As the breeding season begins, courtship is the first thing to happen, for which every species has a characteristic manner. This process can take place anywhere – on the ground, on water, in trees or in the air.
Birds perform courtship either through visual display of their body language or by sound. By comparison, all visual displays have limited reach and can operate only over a short distances, whereas sound can cover long distances. In the case of birds that depend predominantly on visual displays they adopt intensely bright colors by moulting during the breeding season and then station themselves at vantage points from where they can display their vivid colors to their species members and attract mates. This activity has grave risk too, because in the process of attracting mate the bird may also invite danger. Some predators thrive by exploiting the need of animals to announce their sexual presence. In the case of courtship through sound the risk of being killed by the predator can be avoided. Bird can keep hiding in bushes or at any other safe place and can still attract mates by singing without being spotted by the predators.
Males arrive before females
As the breeding season starts, among the migratory birds, it is the males that arrive slightly before the females, set up territories and start singing. The purpose of the song is two-fold, one, to announce to the rival males that the territory has been claimed, hence, they should keep away from it and second, to convey the potential mates the whereabouts of the singer.
Normally in any given place in the wild, a number of different species of birds are found. This co-existence in each location has led to evolution of more and more complex song-patterns as each bird tries to keep itself different from the other, so that it can become easily identifiable by others. For instance, all woodpeckers use their strong bills for drumming after selecting a part of a tree that has good resonance and the sound thus created acts as a means of communication. All species of woodpeckers communicate in the same way but everyone has its own specific beat patterns that keep it different from others.
In case of visual displays, if many species exist in close proximity, the body markings become more complicated and more brightly colored. Apart from courtship displays, in many species of birds, partners, especially males, present various types of wedding gifts to their mates, which may include pebbles, algae, twigs and berries etc.
Why males are brightly-colored?
One question frequently asked is, why nearly always male birds have brightly colored plumage, while females are usually drab. This phenomenon is known as sexual dimorphism. It is believed that males have to attract females, therefore, they don bright colours but it is also risky in the sense that it attracts predators. The females, with heavier breeding duties than the males, cannot afford to take such risks.
Experts are of the opinion that in the case of polygamous species it makes quite a sense, as the male has to attract many females, but why do majority of monogamous species too have this arrangement, where male and females are linked together by a tight pair-bond and both share parental duties? There is no clear answer. Many explanations and clues have been put forward to this puzzle but none of them is universally accepted. One that seems most reasonable is that female always selects robust, healthy and strong male to breed with so that the young ones thus produced should also be strong enough to face the challenges of life.
Now, the question is how would the female know which male has good robust health? In birds one of the surest signs of good health is the sleek condition of the plumage which can be further enhanced by shimmering and shining of the bright colors. Perhaps, this could be one of the answers to the mystery surrounding the colorful appearance of the male birds.
Nature has provided these animals with an instinct due to which generally the females, in choosing a male with which to mate, avoid close genetic relatives and incest, but at the same time also avoid out-breeding with genetically very distant males. This is because they may be adapted to different environmental conditions, so if there were any local population adaptation to different habitats, it would be advantageous for females to mate within their own population. The same is true for local dialects; different dialect groups might be adapted to different habitat conditions.
Many observations have revealed that animal partners, particularly birds, know each other and their offspring by voice and “by face”. As in humans, the head, beak, nose, snout, ears and eyes of animals of different species have different proportions. And with the help of these traits, often imperceptible to human eyes, both males and females distinguish their mates from thousands of their kind of the same coloration.
Birds also have penis!!!!
In majority of birds, while performing sexual act, males mount on the back of the females and then twist their cloacae apertures down and round to meet that of the females. In this way they pass their sperms to the females, but some birds, like mammals possess true penis to facilitate internal fertilization. Ostriches, rheas and cassowaries are huge birds, they cannot mate in the way most birds do, so they have evolved true penises. Birds that mate in the water like ducks, geese and swans also have penises. Reason, if they perform their sexual act in the manner most birds do they will face the danger of water-seeping inside, which will interfere with the act of insemination.
Long-living birds, like gannets, lay one egg a year while those with short life, like partridges and pheasants, may lay on an average 15 to 16 eggs in a single clutch. In the same way eggs laid in the open may be colored in ways that hide them from enemies, but eggs laid in dark holes are generally white. The coloring, shape and usually also the number of eggs laid are characteristic for the given species.
There are birds that neither build any nest nor incubate even their own eggs, instead they deposit their eggs in the nest of other species; this phenomenon is known as social parasitism. Cuckoos or Asian Koels are the best and the most popular example of this behavior. When nestlings come out of the eggs they are raised by foster parents, in whose nest eggs were deposited, and they often pay very heavy price in the form of their own young. Parasitic species generally lay single egg in the nests of different hosts, but in the entire season they lay as many as 20-24 eggs, distributed among various hosts.
Since birds have originated from the reptilian stock, scientists believe that originally all bird eggs used to be white as their ancestor’s method was to bury them in sand or loose soil, the habit that still continues. Even to this day primitive families of birds like cormorants, pelicans, albatrosses etc. have retained pale color for their eggs besides more advanced species of hole-nesting birds. Since the eggs of these birds remain in the darkness of holes and cavities they have no need for cryptic coloring, however, there are some exceptions, like titmice, among the hole-nesters who do have colored eggs. Reasoning given by scientists in this case is originally they used to lay eggs in the open but gradually at some point of time they adapted themselves to the habit of hole-nesting
It has been observed that birds, which build cup-shaped nests in trees often lay pale blue eggs. For long, it was believed that this color was designed to imitate spots of sun on the leaves and so confuse egg-thieves, but recent studies have rejected this theory, saying that the color of the eggs makes little difference because predators usually find the nest before they have seen the eggs.
As has been already mentioned that eggs laid in open are normally colored or camouflaged in ways that hide them from enemies, but how the eggs of some species – which lay them in the open – get colored, is an interesting study. When the egg passes through oviduct, it receives colors from pigments secreted by cells situated in the walls of the oviduct, particularly of the uterus. These colors are deposited at different depths on the shell depending upon the position of the egg in the oviduct at the time of secretion.
Eggshell, which itself is a secretion that takes place in the uterus consists mainly of proteins and minerals (largely calcium carbonate which is present in the form of calcite). It gets its much of the ground color from the pigments in the spongy layer of the shell and must therefore be secreted at the upper end of the oviduct, while the blotches, speckles and scrawls on the surface are secreted lower down, shortly before laying. Colors come from blood haemoglobin and bile pigments, but shades are derived by overlapping of colors. The birds whose eggs are stationary in the oviduct while colors are being deposited have spotted eggs, but those whose eggs move about inside have streaked coloration.
Although all bird eggs are oval, but still there are major differences between those laid by various species. The eggs that are rounded types are generally laid by birds that nest in holes, or in deep cups. In, other words where their rolling will not pose a hazard of falling down. For example owl’s eggs are rounder than most and those of swifts and swallows are long and narrow, whereas guillemots, which nest on precarious rock ledges, lay sharply tapered eggs that roll in tight circles and do not fall off easily.
It has been observed that the time gap between the laying of eggs also varies in different birds. Usually the first egg is laid as soon as the nest is completed, but some birds are always in a hurry and start laying even before the nest is completed. Some prefer to delay it by a day or two. Many species are very particular about the time of day for egg laying, for instance pigeons and their relatives – doves – prefer early afternoon, whereas songbirds choose early morning.
Incubation refers to the process by which birds hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the maintenance of constant temperature required for its development over a specific period. Especially in domestic fowl, the act of sitting on eggs to incubate them is called brooding. The action or behavioral tendency to sit on a clutch of eggs is also called brooding, and most egg-laying breeds of poultry have had this behavior selectively bred out of them to increase production.
Most eggs lose about 20 per cent of their weight due to loss of water through porous surface of the shell, which also allows important ventilation. To avoid this, maintenance of correct humidity is also important during incubation.
In ninety per cent of birds both partners share the task of parental care, whereas in mammals, such percentage is only three. Reason, male birds are equally well equipped to do so as females, the case is not so in mammals. For instance young of the birds eat all that can be brought by either parent. Another reason is, if male helps in rearing his offspring, he might well succeed in raising more offspring than if he leaves the task solely to the female.
Incubation is a period between the laying of the last egg in a clutch to the hatching of that egg. Birds being warm-blood need to keep their eggs at a constant, relatively high temperature after they leave female’s body. Therefore all birds, barring a few exceptions like megapodes and parasitic species keep their eggs at the proper temperature by sitting on them so that their body heat is transferred directly to the developing embryo inside the egg.
For the purpose of incubation most of the birds develop a special brood patch. This is an area adjacent to the abdominal portion, which is relatively free of feathers all year round and the skin here is too slightly thick with relatively high density of blood vessels.
Bird eggs require temperature between 34 degrees C to 39 degrees C (93-102 degrees F) during incubation depending upon species; however, about 80 per cent of the species maintain constant temperature of 35 degrees C. This is achieved only after a warm-up period, the length of which may be related to tightness of sit and time needed for the incubation patch to take shape.
Now the question is how long a bird can sit on its eggs at a stretch? Answer, this period varies greatly from under an hour for many passerines, to about 64 days continuous sitting by emperor penguins. Most seabirds, like gulls, sit for several hours whereas offshore-feeding birds such as shearwaters sit 3 to 15 days at a stretch.
The process of coming out of a young one from an egg is known as ‘hatching’, which can take from about 45 minutes to 6-7 days depending upon the species. Preparation for the chick to come out of the egg starts quite early, during the course of incubation. Development of embryo and the evaporation of moisture from within the egg, through the pores of eggshell starts simultaneously and by the time, embryo is fully developed, moisture evaporates and an air pocket is formed at the blunt end of the egg, between the inner and outer shell membranes. The developing chick for the purpose of breathing while inside the egg uses this air; however, it still relies on a part of the embryo called the allantois for oxygen exchange.
To crack the eggshell for coming out, the chicks develop temporary ‘egg tooth’ on the tip of upper mandible and a special hatching muscle. At the same time, eggshells too become quite brittle, because they lose their strength in the form of minerals and other nutrients, which work as inputs for developing the skeletons of chicks.
Before cracking the eggshell embryo realigns its body, lies along the length of the egg facing the blunt end and with the convulsive thrusts strikes the bill against the shell. The horny ‘egg tooth’ soon cracks or ‘pips’ the shell. After emerging from it, the hatching muscle becomes useless and withers away whereas the ‘egg tooth’ either drops off or is re-absorbed into the bill depending upon the species. In some species parents also help their offspring in coming out of the egg.
Birds are divided in two groups on the basis of the behavior of their nestlings at birth. One group is called nidifugous. Their young start roaming or following their parents within a few hours of coming out from the eggs. They start feeding themselves immediately, while their parents just guide them to the food and provide the protection. Other birds are called nidicolous, whose nestlings are helpless at birth and they are provided with all kinds of parental care for a certain period of time.
The time spent by nestlings in the nest varies greatly. Young of some large raptors, e.g. vultures, remain in the nest for about a hundred days and those of certain species of albatrosses as long as 250 days. In some species the period is very short, e.g. gulls, specially herring and common gull’s nestlings may leave the nest after a few hours of hatching.
Young ones of every species need parental care in one form or the other. Even the nidifugous offspring need parental care in the form of protection and guidance for food. The nidicolous hatchlings have various methods for demanding food; uttering loud cries is the most common of them. In songbirds, during the first days of their lives, featherless and blind nestlings cannot identify their parents by any visible trait. Hence, for their yet undeveloped brain, a slight shake of the nest is a signal to gape for food. It means the mother has flown in.
Keeping nest clean is also part of parental care exercise otherwise it may lead to spreading of disease among nestlings. Different species adopt different methods for cleaning nests, e.g. some remove droppings with their beaks and either throw them out or swallow them. In some species, like raptors, nestlings drop their faeces over the edge of the nest or squirt it out. Only few species are those who do not clean their nests.