Most of the series brings to life peer-reviewed scientific research, or at least plausible interpretations of the data scientists have unearthed. However, when IFLScience asked two scientists who had studied the extinction event, they considered this part of the documentary unlikely to be accurate.

The series’ artwork is so impressive, and Morgan Freeman’s narration so soothing, it’s easy to overlook a glaring problem with the tale of avian survival it tells. However, almost all birds today are, like mammals, initially dependent on parental care. If the birds of the end-Cretaceous were similar, and all adults were killed by the effects of the impact, how would the hatchlings survive with no one to guide them initially?

Do all birds need a parent?

The majority of birds would not even get to hatch if their parents were killed in a disaster like the asteroid strike. Most species need someone to incubate the eggs until they are ready to break out. Sometimes the parents take turns, while other species leave it up to just one parent (not always the mother, as emus prove). Cuckoos and their relatives even outsource the job to members of other bird families, and use mafia tactics to get them to oblige. 

There are, however, some birds whose eggs do not require continuous care, for example building nests that provide the warmth through the breakdown of organic material. 

However, getting to hatching is only part of the problem. Many birds are like us, so undeveloped they cannot initially get around on their own, a trait known as being altricial. In contrast precocial species are those who can see and have some defense against predators from birth or hatching. Nevertheless, even normal precocial young could not survive entirely on their own.

Superprecocial birds, particularly megapodes, are able to run after prey immediately from birth, and some can even fly the day they hatch. Yet even the megapodes usually get some parental support, for example the fathers regulating the heat of their nesting mound by adding or removing decaying plants to keep the eggs in a narrow temperature window.

Irrespective of whether any modern species could survive if both parents were killed in a global catastrophe, Dr Melanie During of the University of Uppsala told IFLScience we can’t assume birds’ modern behavior reflects that of their ancestors 66 million years ago. On the other hand, it would be somewhat surprising if the survivors of the mass extinction were superprecocial and yet almost all their descendants have become dependent at birth.

Not the only problem

However, During thinks there are bigger problems with the “survived in the egg” story. “Shells are not that protective against infrared radiation,” During told IFLScience. “People think the fallout after the event was the biggest problem,” During added. While she stresses, “I don’t want to suggest there wasn’t an effect,” from years of post-impact darkness, During says the initial heatwave was the main killer, at least in North America and Europe where she has done most of her research. “If you were exposed you were dead,” During says. Survivors were those that were shielded in some way, such as being underground. “Most dinosaurs did not lay eggs in sheltered locations,” During said, but she can’t rule out the possibility some of the era’s birds did.

We’re still in the time of the dinosaurs.

Dr Melanie During

During led research at the remarkable Tanis deposit in modern day North Dakota that showed the impact occurred in late Spring, and studied the variety of ways in which death came to a location thousands of kilometers from the impact site.

So how did they do it?

During said the birds that survived were probably in the southern hemisphere. Not only were most of these, particularly those in Australia, much further from the impact site and less exposed to the first blast, but the impact occurred in their autumn. Birds that were hibernating at the time would have been protected from the original blast, and may have woken up as plant recovery was already beginning. 

No modern birds truly hibernate, often surviving winter by migrating instead. Many, however, enter torpor, a less extreme and shorter-lasting state in winter. For the common poorwill (Phalaenoptilus nuttallii) this means spending weeks sleeping without the need to feed. Any bird doing this in a location that protected them against the blast of infrared radiation might have made a good start on surviving the cataclysm. 

Moreover, while some survivors may have found themselves marooned on a southern continent where they made it through, birds would have had little trouble colonizing the north, once vegetation had recovered.

During noted, “Ecosystem recovery [after the impact] was twice as fast in the southern hemisphere as in the north.” She stressed to IFLScience this does not mean far more species survived there, but she said this is likely, although unproven. 

The difficulty in confirming the southern refuge hypothesis is that, During said, “Australia has very few rocks younger than 80 million years old.” Consequently, the record of who was around immediately after the impact, or even a few hundred thousand years later, is almost entirely lacking. It’s almost impossible to know what records Antarctica kept, even if polar species survived the extended darkness from falling ash. South America and Africa outside South Africa have been poorly explored by palaeontologists, and so far little has been found from the relevant era.

In contrast, During described tripping over T. rex and hadrosaur bones in North America from shortly before the impact, and finding mammal remains from almost immediately after, along with the specimens preserved from the day of death.

It’s all in the beak  

Dr Derek Larson of the Royal BC Museum has looked not at where birds survived, but what characteristics assisted their survival. Larson was first author of a paper that compared the beaks of bird-sized dinosaurs in the late Cretaceous with those found afterwards. Larson and co-authors found wide and stable diversity in the mouthparts of small dinosaurs prior to the extinction event. 

The maniraptorans, who resembled the surviving birds in size but had teeth, died out, while some toothless birds made it through to be today’s birds’ ancestors.

This, the authors proposed, was probably because the toothless birds were frequently seed-eaters. If you dodged being roasted by the initial radiation, seeds were the food supply most likely to remain available for a long time, so it makes sense that a few of those with a taste for them survived. Meanwhile, predators and fruit-eaters had nothing to go on once the original supply rotted. “This pattern is observed in modern fire succession communities, where granivorous birds are the first avians to re-occupy disturbed habitats due to food resource accessibility,” Larson and co-authors wrote.

“I also don’t think that surviving only in eggs was likely. I am not familiar with any evidence that has been published supporting that idea,” Larson told IFLScience. “The three main factors that seem to come up again and again (including in our paper) for surviving the extinction were: body size, habitat, and diet.” Smaller animals often do better when the environment changes drastically, but Larson thinks this was particularly the case for this extinction event because they were better suited to “seeking shelter from the impact and subsequent firestorm.” Subsequently those that could live on seeds dropped before the disaster survived until forests regrew.

Whatever their secret, birds’ success means, During claims, “We’re still in the time of the dinosaurs. There are almost 10,000 species of surviving dinosaurs, and a mere 6,000 or so of us mammals.” She does ruefully admit, however, that mammalian influence on the planet is probably larger, thanks to one species.