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How Some Birds Evolved to Build Protected Nests

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Birds' nests are as diverse as the creatures who build them. The variety of locations, shapes, and sizes they’re built in and the materials they’re made from can be staggering. Hamerkops, for example, spend weeks assembling thousands of twigs into a massive nest that’s insulated and waterproofed with mud. Then there’s the horned coot, which makes its nest on top of an island of pebbles that it piles up in shallow lakes. Meanwhile, the edible-nest swiftlet builds its home entirely from hardened layers of its own saliva.

Even for someone more focused on birds’ brains, like neurobiologist Zach Hall was during his Ph.D. work, the diversity of nests didn’t go unnoticed—nor did the fact that the evolution of such wildly different nest types isn’t well researched. With a new study, Hall thinks he’s figured out how one variety of nest, a protected dome-shaped one, evolved. As they say in real estate, it’s all about location, location, location, and dome nests appear to have evolved as some birds transitioned from nesting in trees to nesting on or near the ground and needed a different kind of nest more suited to the new terrain. 

It’s not exactly a new idea, Hall says. Biologist Nicholas Collias suggested it nearly 20 years ago to explain why some some birds from a family called the Old World babblers build dome nests while others build open, cup-shaped ones. At the time, though, Collias didn’t have enough information or the right techniques to test his hypothesis. With a more complete picture of babblers’ evolutionary history and relationships now available, Hall and his team decided to see if the idea held up. 

The researchers gathered descriptions of the nests built by 155 different species of babbler and mapped their heights and structures (either cup- or dome-shaped) to the birds’ family tree. They found that Collias was on to something. Across the babbler family, species that build dome nests live closer to the ground than their cup-building relatives, and as the birds diverged from ancestors that built cup-shaped nests high in the trees, dome-shaped nests coevolved with nest height as some species ventured closer to the ground.

While they confirmed that nest height and structure go hand in hand, the researchers are still left with the question of which trait arose first and influenced the other. Nesting closer to the ground could have driven some species to add domes to their nests for protection against predators, as Collias suggested (though the team says it can’t yet rule out other influences, like parasites or the need to keep nests warm on shady forest floors), but the domes might have also come first and allowed the species that built them to stake out new nesting sites. The team notes, though, that the first situation is more likely. Changing nest height is an easier leap than changing nest shape, and more consistent with what researchers have seen in other birds. 

There’s still more work to be done, but Hall’s team is confident that their approach can answer the remaining questions about babblers and be used with other birds to uncover the reasons they build their nests they way they do.

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Big Questions
Why Do Cats Freak Out After Pooping?
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Cats often exhibit some very peculiar behavior, from getting into deadly combat situations with their own tail to pouncing on unsuspecting humans. Among their most curious habits: running from their litter box like a greyhound after moving their bowels. Are they running from their own fecal matter? Has waste elimination prompted a sense of euphoria?

Experts—if anyone is said to qualify as an expert in post-poop moods—aren’t exactly sure, but they’ve presented a number of entertaining theories. From a biological standpoint, some animal behaviorists suspect that a cat bolting after a deposit might stem from fears that a predator could track them based on the smell of their waste. But researchers are quick to note that they haven’t observed cats run from their BMs in the wild.

Biology also has a little bit to do with another theory, which postulates that cats used to getting their rear ends licked by their mother after defecating as kittens are showing off their independence by sprinting away, their butts having taken on self-cleaning properties in adulthood.

Not convinced? You might find another idea more plausible: Both humans and cats have a vagus nerve running from their brain stem. In both species, the nerve can be stimulated by defecation, leading to a pleasurable sensation and what some have labeled “poo-phoria,” or post-poop elation. In running, the cat may simply be working off excess energy brought on by stimulation of the nerve.

Less interesting is the notion that notoriously hygienic cats may simply want to shake off excess litter or fecal matter by running a 100-meter dash, or that a digestive problem has led to some discomfort they’re attempting to flee from. The fact is, so little research has been done in the field of pooping cat mania that there’s no universally accepted answer. Like so much of what makes cats tick, a definitive motivation will have to remain a mystery.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Animals
Watch a School of Humpback Whales 'Fish' Using Nets Made of Bubbles 

Just like humans, humpback whales catch many fish at once by using nets—but instead of being woven from fibers, their nets are made of bubbles.

Unique to humpbacks, the behavior known as bubble-net feeding was recently captured in a dramatic drone video that was created by GoPro and spotted by Smithsonian. The footage features a school of whales swimming off Maskelyne Island in British Columbia, Canada, in pursuit of food. The whales dive down, and a large circle of bubbles forms on the water's surface. Then, the marine mammals burst into the air, like circus animals jumping through a ring, and appear to swallow their meal.

The video offers a phenomenal aerial view of the feeding whales, but it only captures part of the underwater ritual. It begins with the group's leader, who locates schools of fish and krill and homes in on them. Then, it spirals to the water's surface while expelling air from its blowhole. This action creates the bubble ring, which works like a net to contain the prey.

Another whale emits a loud "trumpeting feeding call," which may stun and frighten the fish into forming tighter schools. Then, the rest of the whales herd the fish upwards and burst forth from the water, their mouths open wide to receive the fruits of their labor.

Watch the intricate—and beautiful—feeding process below:

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