How d’ya like your eggs in the mornin’? I like mine without risks… Many species of bird have evolved ingenious mechanisms of egg laying, colouration, and nesting behaviour, to avoid predation and improve their brood’s chance of survival. If you have been lucky enough to see bird’s eggs you will know that they are startlingly varied in terms of size, colour and pattern, so why is there such difference in this life event that is universal across all bird species?
The environment in which a bird builds its nest is rarely predator free, as there are not many birds which are at the top of their food chain, even eagles and birds of prey are not immune to their eggs being stolen. One of the best ways for reducing the threat of predation to their eggs is to nest out of the way or high up in the top canopy of trees. But for ground nesting birds, as many of our shorebirds in Britain are, predation can be a big risk to their egg hatching success. To overcome this, these birds have adapted to produce eggs which match the ground substrate they lay on: and they do look remarkably like shingle and pebbles!
Birds that lay their eggs in holes or anywhere where dark, such as a kingfisher, are likely to have eggs that are either white or pale blue and this helps the birds locate them.
Egg pigmentation as a biological process is not as technical a mechanism as you might think! The production process of eggs resembles a miniature assembly line inside a female bird. Eggs receive their signature colour and patterning during the last few hours before they are laid. An egg’s story begins in a female bird’s single ovary. When an ovum is released into the oviduct and fertilized, it is just a protein-packed yolk. The albumen—the gelatinous egg white—is added next. The blobby mass then gets plumped up with water and encased in soft, stretchy membrane layers. The first globs of the calcium carbonate shell are then deposited on the exterior, with the mineral squirting from special cells lining the shell gland (uterus). Pigmentation, if any, comes next, with an overall protein coating added before the egg is laid. It takes about 24 hours to build a single egg.
University of Sheffield zoologist Tim Birkhead compares the pigmentation process to an array of “paint guns.” Each gun is genetically programmed to fire at a certain time so that the signature background colour and spotting of a species’ eggs is produced. Very Charlie and the Chocolate Factory!