John L. Koprowski, a biologist and professor at the University of Arizona, with a Pallas’s squirrel at a Squirrel Zoo near Gifu, Japan. Squirrels are among the animals Koprowski studies. (H. Saito)

Tell me something about squirrel nests, those big leaf balls in the upper branches of trees. Where is the entrance? Why so high? Are they mainly for moms and newborns? In winter, how many squirrels can crowd into a nest at the same time?

Marie Wanner, Waldorf, Md.

Wonderful questions, and especially timely, given that today marks the start of The Washington Post’s sixth annual Squirrel Week.

Before we get to squirrel nests, let’s step back and ask a broader question: Why squirrels? Why study squirrels? Why write about squirrels? With so many things going on in the world, why Squirrel Week?

Because squirrels are interesting. Because squirrels are cute. Because, from a scientific point of view, squirrels are worth studying.

“Growing up in Cleveland, Ohio, they were my connection to the natural world,” said John L. Koprowski, a professor of wildlife and fisheries science at the University of Arizona and co-author of “North American Tree Squirrels” (Smithsonian Books).

As a kid, John spent hours following squirrels around. He studied them as an undergraduate in college. “To look at foraging or habitat use or any behavior, they’re excellent models,” he said.

Each time John advanced up the academic ladder — to his master’s, to his doctorate — he thought he would trade squirrels for polar bears or mountain lions. But the squirrels kept pulling him back.

“I just see them as being incredibly valuable in a number of different ways,” he said.

They also are a lot like us. “I think squirrels value the forest in much the same way we do,” John said. And when squirrels start disappearing, it’s a good warning that forests are, too.

Now for those nests: The leafy nest a squirrel builds high in a tree is called a drey. It’s up high to avoid predators, though the location is a bit of a trade-off. It puts squirrels farther from mammals such as foxes but closer to hawks and owls.

Dreys are marvels of engineering. There are three layers. The outermost layer is made of leaves and twigs. Next is a tightly woven layer of bark and vines that provides some insulation. And inside that is the soft center where the squirrels snuggle. It is composed of “fluffy stuff,” John said: everything from shredded paper to bedding pulled from old couches left on the curb.

The drey entrance is typically toward the bottom, often positioned on the opposite side from the prevailing wind. There is no door, per se — “Mostly it’s just a hole,” John said — but on cold winter days, you’ll often see the squirrel plug the hole.

Dreys aren’t the only places tree squirrels live. Squirrels also take advantage of natural cavities in trees.

“Some species prefer to give birth to their offspring in a cavity,” John said. “We’re not sure if that’s because it’s more sturdy, a little more predator-proof or a little more watertight. Newborns are often very susceptible to changes in temperature. They get pneumonia easily.”

As many irritated homeowners know, squirrels also like that big cavity at the top of your house called an attic.

Don’t forget that some squirrels — ground squirrels, to be exact — live in burrows. Ground squirrels include chipmunks, marmots and groundhogs (duh).

Eastern gray squirrels — the species most common in the Washington area — have something in common with jet-setting billionaires: They maintain multiple residences, between five and 10, John said.

Researchers aren’t sure if that’s to reduce the amount of their scent in any one nest, something that might attract predators, or to reduce parasites.

“Squirrels carry lots of fleas and ticks,” John said. “By moving around, you don’t build up big [parasite] populations in the bedding” of any one nest.

Dreys are used year-round — tree squirrels don’t hibernate — although with so many to choose from, some end up abandoned. Both sexes build dreys, though male and female squirrels intermingle only occasionally. (“Father and mother are only together for about 30 seconds on one day,” said John. “That’s it. With mammals, if you can’t nurse, you’re not much of a help.”)

Related females will nest in groups. “You can have three generations of females nesting together,” John said. “During the day, they’re all spread out doing their own thing. During the evening is when the really interesting stuff happens.”

The squirrels come home. Males disperse when they’re grown, but when it’s very cold they are sometimes given sanctuary in the drey of an unrelated female. On especially frigid nights, as many as eight or nine squirrels will nest together.

But there is a definite hierarchy. Said John: “The first thing in the morning, who gets kicked out? The warm bodies they used the night before. The unrelated males get kicked out right away.”

The related females sleep in, then they emerge to stretch in the sun, grooming one another before starting their day.

Tomorrow: The great squirrel purge of Lafayette Square.

Twitter: @johnkelly

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