African Jacana (Actophilornis africanus)
These long-legged shorebirds build a flimsy nest that floats on water. When the bird steps onto the nest to incubate the eggs, the whole thing sinks. Fortunately, the eggshells are waterproof.
Fork-tailed Storm-petrel (Oceanodroma furcata)
This Alaskan bird's eggs must survive on their own for long stretches in cold weather, while their parents fly long distances scouting for food. Impressively, most of the eggs survive. They have a high tolerance for cold, and can also delay their hatching if the parents are away a lot.
Common Murre (Uria aalge)
Despite breeding high up on cliffs, these black-and-white seabirds lay their eggs directly onto the rocks, without building a nest to hold them in place. It sounds reckless, but the eggs are a long, pointed shape that stops them from rolling off the cliff.
What's more, the eggshells have cone-like structures that make the eggs "self-cleaning". This is useful, because murre colonies are tightly packed so the eggs get showered in guano, as well as salty water from the neighbouring sea. When water lands on an egg, its water-repelling shell causes the water to gather into spherical drops. As the drops roll off the egg, they clean it.
Malleefowl (Leipoa ocellata)
Malleefowl bury their eggs in a mound of compost layered with sand. This keeps the developing chick warm, but it also needs to breathe. So the eggshells have large pores and are extremely thin.
The eggs are also quite large, relative to the size of the bird, which allows the chick to become quite fully developed. That way, it can move and feed on its own after hatching, when its parents will not be around. In addition, the eggs have a high yolk content. This lasts through their long incubation period, and helps the chicks grow strong so they can make their own way out of the mound.
Hoopoe (Upupa epops)
After building its nest in a tree cavity, a hoopoe produces a stinky brown secretion and coats its eggs with it.
Bacteria in the secretion make antimicrobial proteins, which guard the egg against infections. The eggshell has specialized tiny craters to hold the secretion in place.
American Robin (Turdus migratorius)
The blue-green colour of a robin's eggs may signal to the male that the female, and therefore her offspring, is healthy. Chicks emerging from eggs with vivid hues are more likely to be fed by males than chicks from pale eggs.
Black Tern (Chlidonias niger)
This bird builds floating nests out of damp vegetation. As a result, its eggs are exposed to more humidity than those of ground-nesting terns, whose nests are dry.
This ought to mean that the egg should lose less water, because the difference between the water vapour levels of the egg and its surroundings is small. That is a problem, as for the chick to develop normally, the egg must lose about 15% of its mass in water. So the eggshell has lots of holes: it has 30 more pores per sq cm than a similar-sized egg of a ground-nesting tern.
Japanese Quail (Coturnix japonica)
A female Japanese quail is selective about where she lays her eggs. She chooses a background that matches either the colour of her eggs or their pattern, whichever is more striking.
If her eggs have only a faint pattern, the female chooses a site that matches their colour. But if they have a strong pattern, she goes for a site that blends with it, and which hides the contour of the egg. This means the female must know the pattern of her own eggs.
Great Tinamou (Tinamus major)
While a great tinamou is a drab brown, its eggs are glossy and coloured. That is odd: it is a ground-nesting bird, and they normally camouflage their eggs.
But the tinamou's conspicuous eggs serve a purpose. They draw the attention of other females and signal them to use the same nesting site. The more eggs are piled up all together, the better the odds that any given egg will survive, as a predator simply can't eat them all.
Red-winged Blackbird (Agelaius phoeniceus)
If a red-winged blackbird breeds at high altitudes, its eggs have a smaller pore area than if it was breeding near sea level.
This ensures that the eggs don't get dehydrated, which is a risk high up where the atmospheric pressure is low.
Common Ostrich (Struthio camelus)
Ostriches lay the largest eggs of any living bird. However, compared to the bird's size they are quite small. The eggs are round with thick shells, giving them enough strength to bear the weight of the incubating bird without being crushed.
Even though they are laid on the ground the eggs are white. This makes them more conspicuous to predators, but crucially it also helps keep them cool in the heat of the day.
Snowy Owl (Bubo scandiacus or Nyctea scandiaca)
The snowy owl nests during the Arctic summer. Its nest is a scrape dug out of earth, where it incubates its eggs for about a month. The 24-hour daylight contains harmful ultraviolet radiation, but fortunately the thick eggshell acts like a filter controlling which kinds of light gets into the egg. Much of the dangerous ultraviolet radiation is blocked, while the beneficial visible light is allowed in.
Common Cuckoo (Cuculus canorus)
The common cuckoo famously lays its eggs in other birds' nests. Their eggs mimic those of their hosts in colour and pattern, tricking the hosts into accepting them as their own.
Cuckoo eggs also have unusually strong eggshells, so that even if the host bird figures out what has happened, it will find it difficult to puncture the egg. As a result, cuckoo chicks have a hard time getting out of their eggs. They start pecking at the shell sooner, and are heavier than the host chicks when they hatch.