C.T. White Lecture for 1996


Birds’ eggs: form and function


Peter F. Woodall

Dept. of Anatomical Sciences, University of Queensland, Brisbane, Q. 4072.



Introduction

Eggs are almost universal in the animal kingdom, bees, birds and mammals all have eggs and superficially they all look very much alike. William Harvey in his book De Generatione Animalium published in 1651 shows all manner of organisms from plants and insects to birds and mammals emerging from an egg with the caption “ex ovo omnia” (all from the egg).

But looking at eggs in more detail reveals many differences. Where there is limited amounts of yolk, as in most mammals, cleavage of the zygote is total and all the yolk is incorporated with the individual cells (blastomeres). But where there is more yolk, cleavage is partial and limited to the yolk-free area at the animal pole and a cellular blastoderm becomes separated from the yolk mass. This is the situation found in reptiles, birds and the mammals that lay eggs.

There was a major change as vertebrates moved out of the water and they (the reptiles) developed amniotic eggs. This means is that the embryo develops in a fluid-filled sac, the amnion, which surrounds and protects it. Many reptiles and all birds have developed a cleidoic (closed box) egg which is sealed from the environment. allowing only limited gaseous exchange and thus protecting the egg from desiccation.


The Development of Birds Eggs

Even though birds’ eggs, in the form of chicken eggs, are one of the most common animal items in our diets, and Australia produces nearly 3 billion eggs per annum, few people are aware of the intricate process needed to make an egg. The avian reproductive system has a number of unique features:




Ovary

There is generally only a single functional left ovary, except in the kiwi & birds of prey which have two ovaries, and this is where the yolk is laid down and the ovum or germ cell develops. Ovulation, the process of releasing the ovum, occurs into a small pocket in the abdomen, and the ovum moves into a funnel and down the single oviduct.


Oviduct

The oviduct consists of a long tube which adds a succession of products to the ovum as it passes through. The first part or infundibulum (=funnel) is where fertilisation takes place if the bird has mated. The infundibulum also adds the thick white or chalaza. The next part of the oviduct is the magnum where the majority of the egg white or albumen is laid down. This is followed by a narrow isthmus which lays down the shell membranes. The egg spends most of its time in the uterus or shell gland where initially water is added in a process called “plumping” followed by the addition of the shell and shell colours. This is a highly active part of the oviduct and every 15 mins the uterus takes from the blood a weight of calcium equal to the total amount circulating in the blood. The final part of the oviduct is the vagina which adds a thin outer layer, the cuticle. The vagina is also the site of sperm glands, which store sperm for 15-20 days after mating before there is a reduction in fertility. Sperm are released just before egg the ovulates and make their way up the oviduct to arrive at infundibulum for fertilisation. The controlling mechanisms which regulate this are not known. (King and McLelland 1984).


Egg colour

One of the fascinating things about eggs, and which no doubt fuelled the passion for egg collecting in earlier times, was the wonderful variety of colour and pattern to be found in birds’ eggs. As a young boy, interested in birds, I avoided the egg collecting passion by taking photos of them rather than taking the eggs themselves.

In general, birds which lay in dark situations, holes, hollows or enclosed nests, have pale or white eggs. While those laid in the open have a much greater variety of colours and patterns. So bee-eaters, kingfishers and owls have pale white or cream eggs but jacanas and lapwings have elaborate markings to camouflage their eggs.

An interesting aspect of egg colour is the way in which cuckoos mimic their hosts’ eggs. As a school boy I studied Red Bishop birds Euplectes orix in Zimbabwe. They built woven nests in the reed beds and laid pale blue eggs. The Diederik Cuckoo Chrysococcyx caprius was a frequent parasite of the Red Bishops

and when it laid an egg in their nest it was slightly larger but closely matched the unmarked Red Bishop eggs (Woodall 1971). However, when the same species of cuckoo parasitised a nearby nest of a Masked Weaver Ploceus velatus, it laid a mottled egg, closely resembling those of the weavers (Fig. 1).

For this to happen there must be two populations or demes of the Diederik Cuckoo which specialise on different host species.

Fig ! A & B -Clutches of eggs


Fig.1a. A clutch of Red Bishop eggs with a matching Diederik Cuckoo’s egg

1b A clutch of Masked Weaver eggs with a matching Diederik Cuckoo’s egg.

Function of the egg

The eggs we normally see in the refrigerator seem lifeless and inanimate,

and indeed they are, unfertilised, they have no development. However, for most birds’ eggs it is quite the opposite. They are the focus of great metabolic activity as the embryo develops inside.

Development of the embryo requires a constant supply of warmth or the embryo will die. This warmth is generally supplied by the incubating adults, which develop a very vascular and featherless “brood patch” which is applied to the eggs. For most birds a relatively constant temperature of 34oC is maintained for up to 80 days in the Royal albatross, 21 days in hens and 10-14 days in small birds.

In very hot climates birds have to take care that the eggs are not overheated.

This may take the form of simply shading them but some years ago I witnessed another form of cooling where a White-fronted Sandplover Charadrius alexandrinus was watched dipping its breast feathers in the water and then taking the dripping feathers back to its nest on a sandbank in the Zambesi River. I have also watched a Red-capped Plover Charadrius ruficapillus perform a similar action at Beachmere.

The embryo inside the egg is growing rapidly and like any living animal it needs a good supply of oxygen. The movement of gases through the eggshell of a bird has been the subject of much investigation and indeed it is one of the few areas of biology that have given rise to physical laws, in this case on the diffusion of gases.

The eggshell is not solid but in fact has a large number of tiny holes or pores which allow the passage of gas but not liquids. The oxygen entering the egg and the CO2 leaving it balance one another, but during incubation there is a continual loss of weight and this is due to the loss of moisture. With its hard shell the egg cannot decrease in volume, so the loss of moisture is accommodated for by the formation of the air space between the shell membranes (Ar et al. 1974).

This provides an indication of how old an egg is. A fresh egg sinks in water while an older egg floats higher in the water. But this is also of vital importance to the young chick because the air space provides its first breath of air before it breaks through the egg shell.


Control and Ecology

The controlling mechanisms for this process of egg formation are a very complicated process. Initially, they come from the nervous system, which provides links to the environment and gives information on aspects such as day length, rainfall and temperature which may be used as stimuli for the start of reproduction. These signals are conveyed through the brain to endocrine organs such as the neurohypophysis and adenohypophysis which make up the pituitary.

The pituitary produces hormones such as follicle stimulating hormone (FSH) and luteinizing hormone (LH) which regulate the development of the ovary and oviduct and the production of eggs.

There are additional nervous and hormonal controlling mechanisms which regulate the transfer of lipids from the liver to the ovary for the formation of yolk and the transfer of calcium ions from stores in the bones to the egg shell.


Breeding Season

Unlike the domestic fowl, which lays eggs for most of the year, most wild birds have well defined breeding seasons and their reproductive organs show marked changes in size between the breeding and non-breeding season.

The start and duration of the breeding season it obviously of critical importance to wild birds and I have been interested in this topic for many years. In the tropics, birds have a long and poorly defined breeding seasons but in temperate regions the breeding season is generally shorter and more clearly delimited. The availability of food resources is usually given as the reason for this.

But birds don’t always follow this pattern, my study of breeding in the Noisy Pitta (Woodall 1994) showed that in tropical regions the breeding season was shorter and much more defined than in the subtropical and temperate areas of Australia.

Again food supplies may determine this and the food supply is probably largely determined by rainfall. Cape York has a high rainfall total (1729mm) but most of this falls between December-March, compared with a lower total rainfall for southern Queensland (1414 mm) which is more spread out between October and May.


Clutch size

The number of eggs birds lay varies from one to over twenty. Birds can be divided into determinate layers, which lay a predetermined number of eggs and then stop; even if some of the eggs are removed, and indeterminate layers which will continue laying for a variable length of time if some of their eggs are removed. The domestic fowl fits into the latter category and will keep laying for most of the year if its eggs are continually removed and a sparrow laid 50 eggs in succession when one egg was removed each day.

The clutch size varies between species, and also within one species from year to year and from location to location. Generally tropical birds lay smaller clutches than temperate species, but this has seldom been demonstrated in Australian birds. However, my study of the Noisy Pitta did show this pattern, with pittas on Cape York laying clutches of 2-3 eggs, while pittas around Brisbane lay clutches of 3-4 eggs on average (Woodall 1994).


Egg Size

Egg size generally varies with body size so small birds lay small eggs and large birds lay large eggs. However, there is a tactical choice between producing many small eggs or fewer larger eggs.

A large egg is advantageous because it provides the newly hatched young with a bigger reserve of food, and it allows the young to hatch at a later stage of development. Birds which have a high survival rate and live for a long time generally lay fewer, large eggs while those birds that have a higher mortality rate and lower life expectancy will generally lay more, smaller eggs.

Most birds lay eggs which are between 1/10th and 1/30th of body weight but there are always exceptions, and probably the most famous is the Kiwi. It lays a huge egg for its body size. nearly 1/4 of its body weight, so a 2.2 kg kiwi has an egg nearly equivalent to the egg of a 13 kg moa.

The kiwi seems a scaled down version of a moa which has retained the large egg. The large egg has advantages but it does require 10 weeks of incubation, compared with 3 weeks in a fowl. While incubating, the adult is vulnerable to predators, but there were few native predators in New Zealand until the introduction of exotic carnivores which have greatly reduced the kiwis populations.

During this long incubation there is also the added danger of microbial or fungal attack of the egg, but the level of anti-bacterial enzymes in the albumen is particularly high in the kiwi. The very long incubation has also meant that the porosity of the egg has had to be reduced to prevent excessive water loss (Calder 1979).


Conclusion

The humble birds’ egg, so commonplace in our daily lives through the hen’s egg, is in fact a masterpiece of construction. It provides a wide range of functions, both the immediate needs of protection and gaseous exchange for the developing embryo and also the long-term needs of continuing the survival of the species through an optimal mix of egg and clutch size.


References


AR, A., PAGANELLI, C.V., REEVES, R.B., GREEN, D.G. & RAHN, H. (1974).

The avian egg: water vapour conductance, shell thickness and functional

pore area. Condor 76: 153-8.

CALDER, W.A.III (1979). The kiwi and egg design: evolution as a package deal.

BioScience 29: 461-467.

KING, A.S. & McLELLAND, J. (1984). Birds, their structure and function. 2nd ed.

Bailliere Tindall, London.

WOODALL, P.F. (1971). Notes on a Rhodesian colony of the Red Bishop. Ostrich

42: 205-210.

WOODALL, P.F. (1994). Breeding season and clutch size of the Noisy Pitta

Pitta versicolor in tropical and subtropical Australia. Emu 94: 273-277.