Every year in early April I wander the meadow to see what changes were wrought by the long, snowy winter. And every year I almost step in the hole that serves as winter den to the local population of garter snakes.

Some folks find snakes fascinating and beautiful. In others, snakes inspire an especially large measure of fear and loathing. Harvard biologist E.O. Wilson addresses this response in his essay, “The Serpent,” from Biophilia (Harvard University Press, 1984):
“What is there in snakes anyway that makes them so repellent and fascinating? The answer in retrospect is deceptively simple: their ability to remain hidden, the power in their sinuous limbless bodies, and the threat from venom injected hypodermically through sharp hollow teeth. It pays in elementary survival to be interested in snakes and to respond emotionally to their generalized image, to go beyond ordinary caution and fear. The rule built into the brain in the form of a learning bias is: become alert quickly to any object with the serpentine gestalt. Overlearn this particular response in order to keep safe.”

I have no intent to press those with established opinions about snakes. As Wilson points out, humans have “an innate propensity to learn such fear quickly and easily past the age of five.” But admittedly, it’s pleasing when kids respond with open interest to animals—especially maligned species that are actually beneficial. Last year my son and I were walking in the meadow and came upon this same den of emerging garter snakes. He’d never seen anything like it. Rather than recoiling, he stopped to watch for a long time, plying me with questions. I explained that it’s always wise to give wild animals space, but that these small snakes are generally harmless and helpful to humans—among other things, they eat worms, leeches, slugs, and a variety of insects. They congregate together underground during the winter, hibernating in groups for warmth. To the unfamiliar observer, the snakes may look like a living ball of twine, rolling and swirling as they emerge from the entrance of a hibernation den. This behavior, delightfully creepy to watch, is evidence of the other practical purpose behind garter snakes' communal hibernation: males and females have immediate access to each other in spring. After mating, each snake retreats to a separate domain around the area. Summer is not a time for sociability. This is not the last we see of them, however. Later in the season we will inevitably encounter slender hatchlings, which appear suddenly underfoot in the yard like animated blades of grass or take their turn to sun on the deck. Watch out garden pests—here they come!

This morning I took a series of photographs as a downy woodpecker and blue jay fed outside the kitchen window. I noticed that both birds spent some "down time" on their respective feeders, appearing to rest for a bit between bouts of feeding. When I later downloaded the pictures, I found several shots showing each bird with its eyes partially or completely "closed."

The birds weren't actually napping. I was lucky enough to get photographs of their third eyelids—what biologists refer to as the nictating membrane. The term is drawn from the Latin word nictare ("to blink"). Birds have paired eyelids, as we do. These close vertically (from the top and bottom) when the bird sleeps. The nictating membrane is a separate structure located between the eyelids and the cornea. It usually remains hidden at the inner corner of the eye. The nictating membrane has two primary purposes: to clean and moisten the surface of the eye, and to protect it from injury.

A bird's sail-shaped nictating membrane is firmly affixed by ligaments at two points (the top of the eye and the side closest to the bill). The third attachment is movable, allowing it to sweep sideways over the cornea in a motion something like that of a windshield wiper. As I observed, in a quiet moment the bird may flick the nictating membrane open and shut a few times as a sort of preening action. The membrane is also closed when the bird needs protection from environmental hazards.

This structure shows a lot of adaptive variation. Raptors have a nearly transparent nictating membrane, allowing them to see while flying but also protecting the eye from injury caused by twigs, branches, or struggling prey. Imagine the benefits of such protection for fast-flying or pelagic (sea-faring) birds, which otherwise face the drying effects of wind and abrasion from small airborne particles. Woodpeckers and nuthatches have unusually thickened, opaque nictating membranes that protect their eyes from flying wood chips.

Like raptors, diving birds have a transparent nictating membrane. I'm very nearsighted and always appreciate the little bit of visual enhancement provided by goggles when I swim. I wondered if nictating membranes similarly improve the vision of birds swimming under water. Researchers looking at this question found that the membrane lies very close to the eye and has a curvature almost identical to that of the cornea (i.e., it is not faceted like my goggles). As a result, there is no significant refraction of light and no noticeable improvement to a bird's vision.

Nictating membranes are common in every vertebrate group except mammals, suggesting that the structure evolved in fishes and was lost much later by some mammals. Monotremes (the platypus and echidna, most "primitive" among the living mammals) have nictating membranes, as do marsupials. A few groups of placental mammals retain them, in particular those that are aquatic—seals and sea lions, manatees and dugongs, and beavers. For polar bears, nictating membranes serve the additional role of protecting against UV light that is so strong in the polar environment.

A tiny member of the loris family, found in West Africa, is the only primate with fully functional nictating membranes: able to clean the eye and move freely across it. But this structure has not been completely lost in other primates. Take a look in the mirror. See the pink, crescent-shaped blob at the inner corner of your eye? That is a nictating membrane—or what's left of it. Opthalmologists call this the plica semilunaris. It's often said to be vestigial, a scientific term for structures that have been retained through evolutionary time but lost their function. That's not an entirely accurate description in this case. Although the plica semilunaris has no ligamenture that permits movement, it still helps to clean the eye by producing fatty secretions to which pollen, dust, and other particles stick. These waste materials glom up and weep out. Nictare—blink, blink—and your eyes are cleaner. No goggles required.

Great-horned owls are the first of our winter-resident birds to nest, and hooting becomes most intense just before the female lays eggs. Devoted mates, the owls form pair-bonds that endure for years. Rather than migrating, the pair establishes and maintains a permanent territory. (Localized winter food shortages may break this pattern, prompting temporary southerly movements, or irruptions, toward better food sources.) Territorial boundaries are reaffirmed...

May 15. That's the date on my 2009 phenology calendar marking the arrival of ruby-throated hummingbirds in our yard. The timing is fairly consistent from year to year, with a little variation accounted for by late-arriving winter storms that hold the migrants down south a little longer. Once in a while I'm fortunate to see a male hummer in April, three or even five weeks ahead of schedule. These precocious fellows ride the heels of yellow-bellied sapsuckers, a variety of migratory woodpecker that makes the rounds of our yard in the earliest days of spring. Sapsuckers drill wells in the bark of maple trees to access their welling sap. Small insects throng to these puddles of sticky sweetness, and hummingbirds gobble up this free source of body-warming protein to get them through the lingering chilly nights. There is always the risk of an unexpected storm or freeze. The male hummingbirds that survive the trip have a distinct advantage: early arrival in their summer range and access to a wider choice of breeding territories.

Once the females appear, I often hear chittering calls among the trees. But it takes sharp eyes and patience to see the birds firsthand. Adult ruby-throated hummingbirds seek insect prey to feed their miniscule but voracious young, making only rushed visits to the nectar feeder for their own refreshment. I stop to look for them when I first come downstairs in those still-dim minutes before sunrise, or when the evening shadows are long over the feeders.

As the fledglings become independent in late summer, everything changes. Throughout the day males, females, and juveniles zip