Birdwatching: Young and their development
Technically, the young birds from the time they hatch until they can fly are called ‘pulli’. But I shall call them ‘nestlings’ whilst they are in the nest, ‘chicks’ until they can fly and then ‘juveniles’ until they moult. During the incubation period the egg gradually becomes lighter and the shell weaker. A day or so before hatching the embryo develops two structures which help it to break out of the shell: firstly, a strong muscle at the back of the neck, and secondly, a blunt, horny, calcareous spike — the egg-tooth – on the tip of the upper bill; both disappear within a few days of hatching.
The first external evidence of hatching is a star-shaped crack that appears on the side of the egg near the larger end which is caused by the embryo’s egg-tooth pressing against the inside of the egg-shell. The beak moves very slowly and very spasmodically around the inside of the egg and gradually the top is cut off. Just before it breaks out the embryo’s first calls establish contact with its parents. Depending upon the species, and the weather, hatching may take from as little as an hour or two to two or three days; grebes hatch out within a few hours.
Once hatched, nestlings of different species show different degrees and methods of development: one group known as ‘altricial’ young are helpless when hatched. They are also called ‘nidicolous’ in that they remain in the nest for an extended period. They are entirely dependent on their parents for food and have a brightly-coloured gape surrounded by a soft and puffy light-coloured edge which shows the adults where to put the food. They are usually naked at hatching or have only a sparse down on the dorsal regions which may possibly help to keep them warm and also hide them.
On the other hand, ‘precocial’ young are capable of walking or swimming soon after hatching. They may be dependent on their parents for food for a day or so or may immediately join the parents in searching for food. These young birds are also called ‘nidifugous’ in that they leave the nest almost immediately after hatching. They are covered with a dense down which is cryptically coloured, and, when the parent calls the alarm notes, they ‘freeze’ and become extremely difficult to see.
The majority of altricial species can fly at about seventeen days and become actually independent of their parents at about twenty-eight days. Some species of altricial nestlings leave the nest before they can fly and this departure from the nest is correlated with both the safety of the nest and the size of the bird. Nestlings hatched in a hole undergo a longer period of nest life than a chick hatched in an open nest and, as a general rule, the larger the species the longer it takes to develop. The Cuckoo is one notable exception; its young develop very rapidly because some development of the embryo may start while the egg is within the female. As a result the egg, which is laid later than its hosts’, often hatches out first.
The weight of the nestlings of most passerines at hatching is approximately 66% of the weight of the fresh eggs, and about 6% to 8% of the weight of the adult female. However, it increases rapidly during nest life until at the time of leaving the nestlings weigh only 20% to 30% less than the female and in some species they are somewhat heavier than their parents.
The young of small perching birds pass through five stages of development. During the first three or four days the nestlings grow rapidly, their feather quills are visible beneth the skin and gradually force their way through. Their chief instinctive activities are stretching up their heads and neck and gaping when a parent brings food and, second, stretching out to defecate. They might also make their first food calls. In the second stage their eyes open and their weight increases rapidly. They begin to preen the opening feather quills, and the control of body temperature is established. At birth the nestlings are cold-blooded like reptiles. As soon as the female leaves the nest, their temperature drops towards air temperature. I studied the change in temperature control in nestling Wheatears – by inserting a small resistance thermometer among the young in the nest I was able to see how the difference in temperatures gradually decreased between the maximum, when the female was brooding, and the minimum recorded temperature after she had been away to feed. At the same time the percentage of time that the female spent brooding the young also decreased. When the nestlings were six-days old, and had a thin covering of feathers, the female no longer brooded them and their temperature remained constant.
In the third stage, more motor co-ordinations appear: the nestlings cower in fear; they can stretch their wings upwards and sideways; they can scratch their heads, shake themselves, fan their wings and flutter them when begging. A new series of call notes also appears. They are now capable of regulating their body temperature, and are well-feathered individuals. Some may be independent of the nest and their nest mates and be able to care for their feathers and move about to escape enemies. They can inform their parents of their whereabouts and respond to the parents’ alarm notes. At this stage they will leave the nest if disturbed prematurely.
During the fourth stage, the nestlings that still remain leave the nest. At first, behaviour is characterized by silence, except when calling for food, and by general immobility. Their chief advance is the acquisition of flight. They also begin to show various independent feeding activities such as wiping the bill, pecking at objects, and picking up food.
The fifth and final stage of their development begins with the attainment of flight. They may still pursue the parents for food and this is the chief time when the young are conditioned by parental behaviour as to what they should fear and can be frequently heard calling in fear. They also develop antagonistic attitudes, such as threat postures and fighting. Finally they become independent of their parents.
The nearly-hatched precocial chicks, such as Lapwings, Moorhens and so on, by contrast with the development of nidicolous or altricial young birds, are rather well-proportioned except for the wings which are relatively small and undeveloped. The chicks are thickly covered with down feathers called neossoptiles on all the feather tracts. As in the altricial young the white egg-tooth is conspicuous but by contrast the rictal region is not enlarged and puffy and therefore not obvious to the parents. The chick shows quick response to visual and auditory stimuli, the eyes are open and both vision and hearing are already developed. The body temperature is partly established but the birds still need brooding for a few days.
Depending on the species, the weight of the precocial chick when it hatches ranges between I% and 6% of that of the adult female. This decreases between hatching and the first feed but thereafter the young bird’s weight increases steadily. The main feathers appear about five to six days after hatching on the scapular and the tail feather tracts. In aquatic species feathers are first evident on ventral feather tracts. The feathers on the wings and back appear between the first and the third weeks.
Most precocial chicks remain from about three to twenty-four hours in the nest after they have hatched, while they dry off. They rapidly become increasingly active, standing up and walking and running about. If they are waterbirds they may even swim. They peck at objects in the first stages of learning in order to discover what they can eat. When they rest they customarily sit on the tarsi with their heels touching the ground. Feeding begins on the first day out of the nest, and quite surprisingly, they search for, find and pick up food independently of their parents. They can already respond to the food-calls of parents and take food from the parents’ bills or from the ground where the parents have placed it. Many aquatic chicks are more dependent and require assistance from parents in gathering food: young grebes, for instance, wait on the surface until their parents emerge with food. During the first three days after hatching the young birds begin to display virtually all the activities associated with the parents, except for breeding and flying. They sun themselves, swim, use feet for scratching themselves. They can call alarm notes, ‘freeze’ and peck at each other. Harmless fighting occurs and a definite social bond is evident. In many of these birds the brood persists as a unit until autumn or winter comes along.
What really has happened is that with precocial birds the first three stages of development of the altricial birds have already passed in the egg.