Crowned Lawping Eggs
At sunrise one morning I found a Crowned Lapwing nest. In sandy, slightly gravelly soil, the birds had scooped out a small, circular, crater. Two of the most stunning eggs lay on the gravel bed. Their base colour was a mustardy, khaki. On top of this were uneven patches blue-gray and larger, splotches of either dark blue or browny-black depending on the temperature of the light. In the picture, parts of the same egg lie in the shade and other parts are lit by the golden light of the rising sun – allowing you to see both colors. There were also a few white marks that I presumed to be the uric acid part of the bird’s faeces.
Two Crowned Lapwing eggs lying in their shallow scrape nest.
These colours create a brilliant disguise. I had searched a small area numerous times, before I discovered them. These eggs are remarkable. The developing embryo is hidden and protected by the shell from the outside world. However, it can breathe; via minute pores it exchanges gasses and water vapour with that same outside world. This is risky, if gasses can get in and out, germs can too. The egg counters this this threat with a series of microbial barriers. Their contents, the yolk and albumen, supply both nutrients and water that are essential for the health and growth of the embryo. The construction of their shells is both sufficiently robust to support the full weight of the incubating adult and yet fragile enough to allow a newborn chick to crack it open.
Tim Birkhead desribes it well, “Evolution has done a fine job of devising ‘a self-contained life support system’ – what is essentially an external placenta and premature baby unit”.
Thomas Wentworth Higginson, was an American Unitarian minister, author, abolitionist, and soldier. He devoted much of his life to fighting for the right’s of freed slaves, women, and other disenfranchised people. His writing reveals a deep love for humanity, art, and nature. In 1862 he said, “I think that, if required on pain of death to name instantly the most perfect thing in the universe, I should risk my fate on a bird’s egg”.
Two Crowned Lapwing eggs lying on a bed of gravel. Note short grass next to nest.
Recently I have learnt a bit more about how eggs are created and it is fascinating. Birds copulate a few days and sometimes weeks before the egg is laid. This sperm is then held in storage tubes near the junction of the vagina and the uterus. When the egg is released it is engulfed within a flexible tube or funnel called the infundibulum. Initially, this tube appears far too small to receive the ovum, but it distends greatly and slowly moves over it in much the same manner that an egg-eating snake swallows an egg. As it travels down this tube it is met by hundreds, sometimes thousands of sperm that have earlier been released from storage and have now moved up to meet the descending ovum. Unlike mammalian fertilisation where a single sperm is required to start a new life. In birds it seems that several sperm, sometimes many, are required to successfully fertilise the ovum. The fertilised ovum now travels down the oviduct to an area called the magnum where the albumen (egg-white) is added to the ovum. Continuing its descent it moves on to a region called the isthmus where thousands of tiny glands exude fibres that cover the ovum in a flexible membrane, called the shell membrane, which although both pliable and permeable, is sufficiently firm to hold the egg in shape. Continuing its journey downwards it moves into the entrance to the uterus. In this ‘blood-rich’ region, calcium carbonate mounds, known as mammillary cores, so named because they resemble tiny breasts, are exuded or sprayed from the surrounding cells onto the shell membrane. The egg now enters the uterus proper where water is exuded or squirted between these hardened mounds.This penetrates the fibrous egg membrane and plumps up the egg. A further set of cells are now activated that pump concentrated calcium carbonate columns on top of the mammillary cores. These harden into upright pillars of calcite, known as the palisade layer. However, numerous tiny vertical shafts remain, places where neither mammillary cores nor calcite pillars exist. These pores allow the egg to breath; gasses and water vapour can enter and escape through the porous ‘egg membrane’. About twenty hours have passed since the egg entered the oviduct and the egg now has a porous shell, but the process is not finished. Yet another set of cells begin to spray colored dyes onto the surface of the egg. These mix with the outer layers of calcium carbonate which creates the ground color of the egg. Once this has been done and sometimes before another set of cells spray or ooze blotches, spots, streaks, and lines that further decorate the surface. This decoration is sometimes called ‘maculation’. The final act that completes the egg involves another set of cell or sprays that coat the surface with a sticky protein that is sometimes mixed with some pigment. This layer, called the cuticle and is the first layer of defense against germs. It covers the entire surface of the egg and quickly dries when it makes contact with the air. I don’t know if the description of ‘sprays or squirting’ of pigments and other substances used in the above description is accurate or not. I initially encountered it in Tim Birkhead’s book. He does write that the way the paint guns produce and project pigments onto the surface will be discussed in more detail later in the book. I have read his book and still do not fully understand the detail of how the egg shell and colours are created and laid down in the oviduct. However, the book is excellent, informative and easy to read.
‘The most perfect thing – inside (and outside) a bird’s egg’, by Tim Birkhead.