This week dramatic winter skies loomed over the meadows. Very little moved in the woods – a distant Fox Squirrel running across a log, a Black-capped Chickadee in a small tree. A few birds called – the usual – crows, woodpeckers, tree sparrows, blue jays. As the winter solstice approached, the park felt empty – but it wasn’t, of course. Despite shorter days, life goes on in all kinds of ways throughout the park.
Wrapping my scarf more tightly and shoving my mittened hands in the pockets of my wool coat, I wondered about my chilly wildlife companions in the park. How can birds stand having bare feet in the winter? And what else is under the pond ice, besides those active muskrats in last week’s blog?
Cold Feet, Warm Heart: Birds’ Feet in Winter
Well, first, it turns out that birds’ feet don’t have many nerves, like our human feet do, so they probably don’t feel the cold the way we do. Glad to hear that. But they still have to keep them from freezing, right? According to Cornell Ornithology Lab, birds DO have cold feet, barely above freezing sometimes.
But, luckily, they don’t have much fluid in the cells of their feet and their metabolism is so fast, that the blood doesn’t stay in their feet long enough to freeze. The blood vessels in their legs are close together, too, so that warm blood going to the feet partially warms the cold blood headed for the heart – which keeps the bird’s body and organs warm despite cold feet. And of course, on cold days, birds fluff their feathers creating insulating air pockets which hold their body heat, just like a quilted down jacket does for us. This Mourning Dove (Zenaida macroura) in one of the park’s wetlands this week may look like it’s swallowed a soccer ball – but it’s just fluffed its feathers to keep warm.
So, good news. Nature’s provided birds with effective strategies for dealing with the cold even without wool socks, boots, or down jackets.
Now, Who Else is Under the Ice? Turtles!
Remember the “Von Trapp Family” group of Painted Turtles (Chrysemys picta) from a blog last spring? How do you suppose they’re coping with winter in the ice cold pond?
Reptiles can’t regulate their temperature internally like last week’s muskrats can. When cold weather makes the body temperature of Painted Turtles drop, they get sluggish and stop eating . They look for a spot at the bottom of a pond, usually close to shore but deep enough not to freeze to the bottom and then bury themselves in the mud. The mud protects them from being easily seen by predators and shallower water means quicker warming in the spring. Their very slow hibernating metabolism requires very little oxygen – which is good, since they have to stop breathing through their lungs for months on end when trapped under the ice. The turtles’ skin evidently can absorb enough oxygen from the cold water to keep them alive at this slowed rate – even when buried in mud! If that oxygen runs out, they can live just on sugars and body fat without oxygen, and some scientists report that the “carbonate buffers” in their shells neutralize some of the resulting build-up of lactic acid that could be harmful to them. That may be how the turtle’s ancestors survived the global winter that scientist believe followed the extinction of the dinosaurs.
And What about Baby Painted Turtles? Glad You Asked…
Incredibly, some Painted Turtle hatchlings survive even when half of the fluids in their bodies freeze solid! As you know, turtles lay their eggs on land, buried in sand or dirt. Painted Turtle babies that hatch in late summer sometimes stay where they are and overwinter in their underground nests together, living on body fat for a whole winter without eating. It’s cold down there – as low in some places as 25° F. It’s believed that some hatchlings produce more glucose and other compounds than others, and those substances act like an anti-freeze to prevent them from completely freezing even in very cold weather. So some of those babies, after being literally half-frozen this winter, will thaw in the spring and go on with their lives! Nature’s ahead of us, as usual, with this reptilian version of hatchling cryonics.
The Big Guys – the Common Snapping Turtles (Chelydra serpentina).
These normally quite solitary animals often hibernate together in the mud below the ice, occasionally forming piles on top of one another, perhaps to share body heat. Others burrow singly into the mud under the ice in places where the water doesn’t freeze to the bottom. Their body temperature drops to 34°, just above freezing and their hearts beat extremely slowly. Though they stop breathing with their lungs for months at a time, they can lift their heads out of the muck , open their mouths and absorb oxygen through the membranes of their mouth and throat. They too can live on body fat ’til spring, perhaps with the help of their shells and skeleton.
The Snapper hatchlings born in the fall head straight for water, like this little one heading for Paint Creek from the Wet Prairie on the Paint Creek Trail in September.
If the weather is too cold when they hatch, these little ones may also try to overwinter in their nests. If the ground freezes too far, however, it’s still unclear whether they can survive when half-frozen like the Painted Turtle hatchlings.
Turtles are vulnerable in extremely cold weather. Sometimes hibernating turtles in shaIlow water or hatchlings in their earthen nests freeze or are found by hungry predators. But enough turtles survive, of course, to give us the pleasure of seeing them gliding through the marsh or sunbathing on a log in the Center Pond the following summer.
Outside, as the longest night of the year arrives, birds tuck into tree holes, crevices or gather on limbs with feathers fluffed for insulation, oblivious (we hope) of their cold but unfrozen feet. And turtles snooze , piled together under the ice or burrowed in the mud or earth until spring. And we humans? Well, we bundle up, turn up the heat and fight off the torpor of the shortest days with brightly lit celebrations with our kin in our wooden or brick abodes . Have a delightful holiday with the people you love and This Week at Bear Creek will be back in the new year – unless, of course, I see something so amazing that I just HAVE to share it with all of you before then!
*Footnote: As well as Oakland Township Stewardship Manager, Dr. Ben VanderWeide, my sources for information are as follows: Ritland, D. B., & Brower, L. P. (1991);Stokes Nature Guides: A Guide to Bird Behavior Volumes 1-3, Allaboutbirds.org, the website of the Cornell Ornithology Lab at Cornell University; Wikipedia; http://www.butterfliesandmoths.org; Herbarium of the University of Michigan at michiganflora.net; various Michigan Field Guides by Stan Tekiela; Butterflies of Michigan Field Guide by Jaret C. Daniels; University of Wisconsin's Bug Lady at www4.uwm.edu/fieldstation/naturalhistory/bugoftheweek/ for insect info http://www.migrationresearch.org/mbo/id/rbgr.html for migration info; invaluable wildflower identification from local expert, Maryann Whitman; experienced birder Ruth Glass, bird walk leader at Stoney Creek Metro Park for bird identification; Birds of North America Online; Audubon.org, Nature in Winter by Donald Stokes, Trees in My Forest by Bernd Heinrich, Winter World by Bernd Heinrich, Savannah River Ecology Lab (Univ of Georgia), Tortoise Trust website www.tortoisetrust.org.