It’s spring, and that means breeding season for birds. All around us, eggs are being laid and brooded by warm, feathery bodies; not all the chicks will make it out, but some will. The process of hatching is a tiny, daily miracle that connects our garden birds to their earliest ancestors, as zoologist Jules Howard explains in an extract from his new book, Infinite Life: An Epic New Story of Life on Earth

Hatching is perhaps the most coordinated and practised moment in the history of most bony land animals. In birds, trapped in their crystalline cases, it is an action that has been refined a little compared to other species, but the basics remain. Hatching is a process.

The defining moment for the chick actually happens in the period just before it hatches from the egg. At this moment, its limbs collected underneath its body and its beak tucked underneath its right wing, the pre- hatchling begins to squirm. The network of blood vessels that line the inner surface of the shell, that grasp at the oxygen entering the egg through its pores, begins to shut down. The blood in this network is reabsorbed into the body of the embryo, which starts to seal itself off at the umbilicus. The developed chick severs its connection with the yolk at this point, draining what is left of the yolk sac directly into its small intestine. The chick readies itself for its first breath now. To take it, the agitating chick moves its head out from underneath its wing and penetrates the hollow chamber at the end of the egg with its beak. This is the air space at the tip of the egg, visible when removing the shell of a boiled egg. It is here, before the chick hatches, that it fills its lungs for the first time. This breath is the single most important one in the life of a bird. Without it, the chick has little chance of breaking free from the egg; it energises and empowers the young life for what comes next.

Now, the chick pushes its beak hard against the internal surface of the roof of the egg. To break through, the chick is aided by the ‘egg tooth’, the calcified structure common to many amniote egg- layers. It pecks, once, twice; again, harder. Finally, a crack appears in the shell, small at first and then, with a few more precision pecks, it becomes larger. Through this hole, a great gust floods into the egg, connecting it more fully with the atmosphere. Air is pulled into the lungs of the chick, compressed, wet, dogged, but new. This oxygen boost empowers the chick, gifting it the energy it needs to continue its struggle. It pecks at the weakened edges of the hole now, rotating its body and pushing its shoulders and legs firmly against the sides of the shell to provide extra anchorage. The beak continues to work across the roof of the egg, opening up this, its gateway, to the chick’s new world.

Once the chick has climbed out of the shell, the adult bird, mother or father, deftly picks the eggshell up in its beak and drops it elsewhere, to remove its smell from the nest and deter egg predators. A short distance from the nest in which it was first placed, the eggshell sits on the floor, empty, alone and forgotten, the calcium atoms ready for another lap of the rock cycle. In time, perhaps, one in a million fragments of eggshells like these may become a fossil. Only a tinier fraction still will ever be seen or collected by human scientists. Most fossil egg fragments will erode to dust. But not all.

Much of what we know about the Cretaceous colonies of birds come from fossils found in western Romania, specifically the Sebes ̧ Formation of Transylvania. This spectacular layer of Cretaceous mudstone, announced to the world in 2012, was found to contain thousands of fragments of fossilised bird eggshell. In fact, 80 per cent of this accumulation consisted of smashed up, compressed layers of the stuff. One small subsection alone, less than the size of the open pages of this book, contained 150 fossil eggshell fragments. The skeletal remains of opposite birds from this and other Cretaceous sites show hatchlings with strong, calcified bones, wing feathers, big eyes and a large brain. These were chicks that were ready to fly within hours of their life beginning, suggesting a landscape in which the threat of predation was high: the sooner birds could get up to move around, the better their odds of survival. We see a similar pattern in hatchlings of mudflat birds today, including ducks, geese and waders.

I find it incredible to think that every single fragment of egg from the Romanian fossil assemblage had probably been seen and inspected by a parent bird, lost to history. It is exciting to consider that each one was generated in an unknowable egg tube; that each egg contained calcium atoms stripped from that forgotten Cretaceous world; that each species was a unique trial, a new evolutionary expression of egg-ness. Within a few million years, much of that expression would be curtailed, cut short, snuffed out by a random rock from outer space that grazed our planet’s surface.

The atmosphere changed immensely after the meteorite explosion that occurred 66 million years ago. For months, a choking sheet of dust painted the sky, preventing the sun’s light from reaching the surface of oceans and continents. In the seas, phytoplankton were starved and their photosynthesis went into a terminal decline. In their trillions, these life forms sank to the sea floor, dead. Some became sleeping cysts, but most did not survive. Food chains became incredibly simple for a while, almost like in the Ediacaran Period, hundreds of millions of years before. On land, plants withered and died, starving herbivorous animals, particularly the large dinosaurs. Those animals that depended on fish for their survival, some pterosaurs, some birds, found little to support themselves. There was food for those wily enough to locate it, of course: those able to home in on the dead bodies of others may have eked out a living for a while, for example. Perhaps months, perhaps years. But mostly there was death. For a short time, in geological terms, it would have looked like a dying planet.

The statistics for this mass extinction event are comparable to the end-Permian extinction crisis. Three quarters of animals and plants lost; almost every animal over 25 kilograms dead. This was the end of the road for the non-bird dinosaurs and for the Enantiornithes birds as well, although a fraction of ‘true’ birds survived and repopulated, in time. One day their eggs would find their way back into scrapes, into trees and the tallest cliff-side mountain tops. And these bird survivors would diverge into new lineages – the perching birds, the ground birds, ducks and their relatives and the ground-living fowl.

And what of the non-bird dinosaurs, those reptile rulers? Traditional hypotheses for their end include the fact that dinosaurs, many of whom were larger, probably had greater energetic needs and simply could not find enough food after the meteorite impact. Other suggestions for their disappearance are that, somehow, dinosaurs lacked sufficient adaptations for warm-bloodedness. That they perished in the cold climate brought about by the layer of dust that enveloped our atmosphere and shaded out the sun’s rays. These ideas are freely expressed in dinosaur documentaries and popular science books on the subject. More recently, a new hypothesis has entered the mix, however – one about eggs. Microscopic analyses of fossil dinosaur embryos, specifically counting the growth lines on teeth as if they were rings in a tree, show that dinosaur embryos took far longer to develop in their egg than the embryos of birds – sometimes six months or more. This finding lends weight to a new hypothesis for the decline of dinosaurs: that the long incubation time of dinosaur eggs added ‘lag’ to their re-population rates in the years after the meteorite impact. Because of their slow- growing eggs, it may have been that dinosaurs competed poorly against their rivals, the birds and mammals, which nudged them out ecologically, edging them ever closer to extinction. It’s possible that the dinosaurs may have remained to this day were it not for an apparently unsurmountable complication in how their eggs developed. We (the mammals) should be very grateful for this. We might never have had our time, otherwise.

Today, the birds that live in our neighbourhoods and that dot our skies continue to express the same forms of playful evolutionary creativity that have been their craft for more than 100 million years. The same journey out of the same egg tube; the patterns, dapplings, bruisings and pencil marks, noted and recognised; the same shell, cracked from within; the first connection between albumen-stained chick and weathered, tired-looking parents. Every song they sing each spring is an ode to more.

Long may that continue.

This is an extract from Infinite Life: An Epic New Story of Life On Earth (Elliott & Thompson) by Jules Howard, out in paperback 8th May, £10.99. It’s available to pre-order here.

Hatching

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