The Egg-citing Avian Nature
Easter is just around the corner; have you rustled up the dyes for festive egg hues yet? In the wild, birds could make it equally eggs-cellent without a drop of ink! Bird eggs share a broader palette than humans expect; colours and patterns vary not only from species to species, but also individual to individual! There are lots of complicated ecological interactions that determine the colours and patterns of an egg - it is more than sheer attractiveness!
To decrypt the enigma, let's start from the formation of a bird egg. Egg yolk (ovum) was the first form created in the ovary. Passing to the oviduct, fertilisation takes place when the female mates with a male. Egg white, moisture and a soft, stretchy membrane layer will be added onto the fertilised egg, sequentially, in the oviduct. The half-developed egg arrives at the uterus and is covered with calcium carbonate, the eggshell is then formed - it is almost finished!
Drawing to an end, a nice colour and texture toning marks the final step before an egg is laid. In the uterus, there are special glands “spraying” the eggshell with two pigments - biliverdin for blue and green; protoporphyrin for red and brown. The time and quantity of the spray works dependently on environmental factors, as well as the genes and physical condition of the mother, making each egg one of a kind. In addition to appearance, the texture of each bird egg differs from the finishing-up, for instance, a bright smooth shell is the product of an additional protein layer, while calcium carbonate coats the shell with white powder.
Environment the designer
Habitat decides how an egg looks. To avoid becoming dinner, ground-nesting birds, such as pheasants and woodcock, camouflage their eggs with striped or dotted patterns, blending them into the surrounding stones and fallen leaves. However, eggs laid by geese and ducks tend to be spotless and lighter in colour. Why? Camouflage plays a lesser role by virtue of the longer time parents spend during incubation. Parents may even cover their eggs with nesting materials. No wonder these eggs look less camouflaged, parental care is the best shield indeed!
How about birds that nest in tree holes or burrows? Considering the dark yet protective environment, hole-nesting birds (e.g. owls, woodpeckers) lay gleaming white eggs for easy location. Arboreal birds are not likely to be affected by the environmental factors above, hence, mothers tend to be more creative when decorating their eggs.
“Colour war” of eggs
What happens when a mother bird mixes its eggs with its neighbour's? Living in flocks, seabirds (e.g. Laridae) paint their eggs with unique patterns, just to let their eggs stand out from the crowd!
Furthermore, some birds such as cuckoos may foist the responsibility of rearing their own offspring by laying their eggs in nests of the others. Research revealed that whenever a brood parasite bird becomes a frequent guest to one specific species, a “colour war” will then begin. The “host” bird changes its eggs' pattern to be different from the brood parasite bird's, while the latter will subsequently mimic the new pattern of the host. Therefore, you may find a constant switch of colours and patterns within the same nest of eggs. Asian Koel (Eudynamys scolopaceus) is a cuckoo that's among the brood parasite birds in Hong Kong, despite not being choosy regarding their “hosts”.
The attention attractor
Speaking of impressive eggs, American Robin (Turdus migratorius) definitely hits the nail on the head - their eggs are tiffany blue! You may wonder why they are so bold and eye-catching in the deadly wild. The fact is, American Robin nests in shrubs, which provides excellent protection to their young. Moreover, the bluish eggshell not only acts as a natural sunscreen against harmful ultraviolet rays for the embryo, but also nurtures the baby by absorbing moderate heat more effectively than the white ones. Also, studies also demonstrate that the healthier the mother American Robin is, the more biliverdin it produces. Guess what? Father American Robin is eager to spend longer to take care of the brighter egg!