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Dark or light, round or oval, how every bird egg serves a unique purpose

The bewildering, beautiful colours, shapes and patterns of birds’ eggs are truly fascinating

eye 2020, ostrich eggEggsquisite: A decorated ostrich egg (Photo: Ranjit Lal)

While we’re all sourced from eggs, some moms in the animal kingdom prefer to keep their eggs outside their bodies. Which, I think, may be a really good idea. Think of the advantages: you can lay as many as you wish; if some get broken or are diseased, they can be replaced without affecting the mother’s health. Egg-laying moms have a range, too: from the bindaas moms (in the insect and piscine worlds) who lay thousands of eggs and forget about them, to birds who assiduously incubate their eggs, keeping them at exactly the right temperature until they hatch, and insect moms (wasps), who lay their eggs on anesthetised caterpillars or small arachnids so their babies have a stock of fresh meat to feed on after hatching. While some hip moms may not care for their eggs at all, others (crocodiles) guard their clutches ferociously. The great dinosaurs laid eggs, too, and from them came birds, to which our focus will shift now. All birds lay eggs — whether just a single one every two years (wandering albatross) or as many as 19 (grey partridge). The tiny bee-hummingbird’s eggs weigh 0.5 gm, while the ostrich’s is 1.5 kg (enough for a nine-person omelette!).

Birds’ eggs have fascinated us for hundreds of years. Collectors in the West back in the “bad old days”, engaged in cut-throat competition, collecting eggs from the nests of rare birds and then destroying them so others couldn’t get to them. It put many species in danger of extinction: mercifully the “hobby” has died a natural death, though sadly, sometimes, bird photographers these days indulge in the same reprehensible behaviour.

The bewildering, beautiful colours, shapes and patterns of birds’ eggs are truly fascinating. The default ground colour is white (thanks to calcium, which gives it strength), the eggs of many passerine (or perching) birds are blue or green or beautiful shades in between. Many are speckled and splotched in rich reds and browns. This patterning, apparently, adds to rigidity when the calcium may be deficient in the mom’s diet, and has antimicrobial properties. Eggs of non-passerine birds are usually white, barring the nesting ones on the ground (lapwings, etc.) whose eggs are cryptically coloured to camouflage with the stony ground they’re laid on, and, thus, easily trampled upon. Bigger eggs laid by bigger birds have bigger yolks (no-brainer) — when they hatch, the chicks are fairly chalta-phirta, able to run, swim and feed themselves, and called nidifugous although they need their moms only for protection.

While the yolk contains the nourishment for the embryo, the albumen surrounding it serves as a cushion, preventing harmful microbes from getting in. The eggshell is dotted with thousands of tiny pores through which the exchange of gases takes place. A typical chicken’s egg has 7,500 pores. The broad end of the egg contains the air sacs and most of the pores are concentrated here. Reason why there’s an empty space in this region, visible while peeling a hard-boiled egg. The broad end is also where the major parts of the embryo develops, its head and major organs, the tail being left to the narrower end. The oval shape of most eggs enables it to take up a naturally “heads up” posture and it is from here that the tiny chick breaks through using its “egg tooth”, made of calcium.

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Researchers have long argued about the shape and colour of birds’ eggs. Birds laying in risky places like cliff ledges lay eggs that are tapering and oval to prevent them from rolling off, those that lay in holes and hollows like owls and bee-eaters lay round eggs. But there’s more to this than that. It’s been discovered that the shape of eggs is highly dependent on its flight capabilities: if the bird is a fast flyer — like a falcon — its eggs will be streamlined to fit into its mom’s aerodynamic body shape perfectly.

Dark eggs absorb more sunlight, keep out ultraviolet rays and so are expected to be found more among birds living in colder climes. But dark eggs also have strong antimicrobial properties and hot and humid climes are hotbeds for harmful microbes. Researchers have finally concluded that dark eggs are better at thermo-regulation — in keeping the temperature at an optimal level for the development of the embryo and can be found in both hot and cold climes. Light or white eggs are a signal for predators – but then many birds still lay them!

The speckling and splotching of some sea birds that nest in colonies thousands strong are so specific that the mother can recognise her own; and, with some victims of brood parasitism, the mom keeps changing the patterns of her eggs to keep one step ahead of the nasty, waiting cuckoo.

First published on: 22-11-2020 at 06:30 IST
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