Thursday, January 30, 2014

Exploring a New Urban Wild Place (2)

Yesterday another free morning, a dusting of snow on the ground, and time to explore the wild place I first entered two weeks ago.  That time I entered from the north, struggling through briars and striving to avoid being in sight of surrounding homes, but also to keep my feet dry on the slightly swampy land.  This time I would enter the wood from the south end, where there was a easier walking and a bit more elbow-room. 

There are a couple of possible entries, both related to the newest residential development to invade the woods.  I parked on a side street a few hundred feet from my chosen entry, switched on the little gps I use for boating, and walked in, noting that a few of the houses had cars in the driveway--possibly observant homeowners at home.   

Scattered stands of American beech, still holding dried leaves, glow in the sun.
In my family at this season we call them "lantern trees."

A thick stand of bullbriar makes this more open area more difficult to walk through that it appears. 

One of the ubiquitous stone walls is visible among the trees.

The tree in the center was marked with a snowball at 5 feet so I could estimate its total height at 45-50 feet.  Though one of the tallest trees in this part of the wood, it is dwarfed by several in the northern part.

This was drier and more open ground with well-spaced trees and a low, continuous cover of something like huckleberry bushes, and I went quite a way--crunching noisily in the snow-covered leaves--before I was comfortably obscured by trees.  My course in the beginning was restricted by housing to both left and right, but these faded out of sight faster than on my earlier visit.  I crossed several stone walls in the first minutes, and then came to a boulder pile that I first guessed was the result of fairly recent construction.  Then I noticed the tree growing out of the top of it, showing it was probably mostly natural, or at least fairly old.  The top of the pile (perhaps ten feet above the level of the nearby woods) made a good vantage for a look around, and I took panoramic photos that showed the ground for a hundred or more feet in all directions.

Panoramas taken from the top of the rocks.  (See flag in aerial images.)

It was pleasanter crunching in the dry leaves and wading through the low huckleberries than the wet, briary scramble of two weeks ago, and I covered considerably more ground in consequence. This part of the wood may be younger than the north end, judging by the smaller height and diameter of the largest trees, but it was a pleasant place.  A few minutes after leaving the pile, I happened on a well-trodden path.  It led a good distance, passing a sort of blind along the way.  (I later discovered a hunter's tree stand--the third I've noticed in this wood.)*

A hunte's blind?  A young teenager's fort?

Although I rather resent finding trash, structures, and other signs human use, I had no objection to the frequent stone walls that spoke of human occupation in times gone by.  The walls represent a stimulating mystery: how long ago were they built, and for what purpose? what can they tell us about the lives of these earlier owners?   In general, stone walls in New England less fences, than the result of frustration born of attempting to plow the rocky ground: the stones cleared from the fields are piled roughly up at the borders of field or property to become boundaries in themselves.  But the particulars of these walls remains a mystery for now.  

The path.

And the path rather gladdened my heart, since it meant easier travel.   I followed both ways until getting too close to houses on the borders, and found one structure, made of old pallets leaned against trees and piled with branches--a fort, or perhaps a hunting blind.   There were not many more signs of human visitation here than in the north, especially considering I saw more of it than I had before, but I did find another tree stand.  Someone must expect deer. 

This new development is primarily McMansions.  I don't know which disturbs me more: 
mcmansion owners oblivious to nature, or those inspired to take a proprietary interest in nature. 

I came out at another possible access point, walking through the city's newest McMansion development. 

I returned home interested to put my gps track on Google Earth so I could see where I'd been, and also explore the land from above. 

 Southern two-thirds of the Wild Place, with the new development at upper right at different stages.  Upper image is from 4/9/2008, while lower is from 8/24/2013.  I tried to roam pretty widely, but avoided getting too close to houses, and went back and forth several times on the path.  The man-made clearing that appears at top center is the same that I visited in my earlier entry from the north.

A surprise came when I chose the most recent image that showed the woods bare of leaves and zoomed in:  the rough, tumbledown stone walls I had crossed were clearly visible, were spaced out in almost a grid, and were about as straight as a ruler could have made them.  I chose one place where walls met at an apparent right angle and measured it: again, as near to 90 degrees as I could determine.  However long ago those walls were built, the surveying behind them seemed sound.  Besides how far in I got, my gps track shows several things: the location of the rocks I stood atop (marked as waypoint 135), the path I found (the long diagonal I doubled back on), and how close I was able to come to the border of the wood without much risk of being observed. 

I look forward to further exploration in late spring when the leaves are out.

*I assumed Brockton would not allow discharge of firearms within city limits.  But to my surprise, it IS legal to discharge a gun except  "in or upon any street or public place, or within 100 yards thereof, or in any building or within 100 yards of any building without permission of the city council."  (

Wednesday, January 29, 2014

Morning After Light Snow (and a rant)

Squirrel tracks.

How did I know they were squirrel tracks?

The night and earlier morning gently dusted us with a dry inch of snow--enough to show who's been by.  I walked through the back yard and all the way to the back of the tiny wood, but saw signs of only two visitors: a squirrel and probably a rabbit.  No coy wolf--though it's true the snow was only hours old, and an urban eastern coyote would typically roam a three-square-mile area.  I took the regular photos of my Wild Place.  

(or Might As Well Put This Here as Anywhere Else!)
Many Americans despise the metric system of measurements--the Systeme Internacional.  I do not, but neither do I follow it slavishly.  You may have noticed me mix the measurements even within the same post, as I did in "Domestic Science." 

I grew up with the English--or "customary"--units of measure: inches, feet and  miles, pints, quarts and gallons, ounces, pounds and tons, and so forth.  I well remember being taught that America was going metric, so we'd better learn it; this was in third grade in about 1967.  You'll have noticed by now we've been easing into it pretty slowly!   (As far as I am aware, the only time most of us use metric units is to buy soda!)   I am pretty comfortable with millimeters, meters and kilometers, though, as well as grams, kilograms and tonnes, and milliliters and liters.  They are universally used in the sciences which are my background.

Metric units have two great advantages over customary units.  First, they are more fundamentally derived: the meter comes (originally) from French measurements of the distance from the north pole to the equator through Paris.  A cube 1 centimeter on a side contained a volume of 1 milliliter, and this volume of water has a mass of one gram--all neat and tidy!  The second and a very practical advantage came when you had to do any kind of math: all the units related directly to each other, and all by powers of ten.  For example, a distance of 1000 meters was one kilometer, while a meter in turn contained 1000 millimeters.  A liter bottle of water has a mass of one kilogram.  Quick: since a mile is 5280 feet and a foot is 12 inches, figure out how many inches are in a mile!   Doing math in the metric system, by contrast, reduces to merely keeping track of the decimal point!

On the other hand, having physically-derived units doesn't make a system practical, and having units that relate by powers of ten limits, somewhat, their flexibility in size.  for example, I am 6'2" tall.  In the metric system, my height would usually be represented as 188cm --in my opinion, a rather unwieldy number of digits.  Feet and inches are better sized for many everyday purposes than centimeters and meters.  Furthermore, breaking down units fractionally has its own elegant simplicity: if you want a smaller unit, just halve the one you've got by doubling the denominator: 1/2 inch, then 1/4 inch, then 1/8 inch, and so on, limited only by the steadiness of your hands, the acuity of your eyesight, and your technology.  The problem  is it does force one to get down and diry with fractions, which still don't come easily to me.  Here's an example I wrestle with frequently. Put these drill bits in ascending order of size: 5/32, 1/8, 11/64. 

Finally, although the Celsius scale of temperature measurement is elegant in dividing the temperature difference between the melting point and boiling point of water into 100 equal parts, those parts--the Celsius degrees--are almost twice as big as Fahrenheit degrees, and they force you to use negative numbers in winter.

Therefore, I will continue to mix my systems in these posts: metric if much math is involved, English if it's much more convenient in size, and Fahrenheit pretty much always!

Saturday, January 25, 2014

What is a Winter..

What is a winter unless you have risen and gone abroad frequently before sunrise and by starlight?     Journal, Jan 23, 1854,  H.D. Thoreau

Out at 6:15am this morning to see International Space Station pass overhead in pre-dawn darkness, but gaps in low, fast-moving clouds weren't enough even to catch a glimpse of it.  Temp was up to a tropical 220F.  Took photos of woods and house from far-back anyway (+2.0 exposure).  Went back to bed.  By late morning, temp was above freezing, wind had risen, and crowns of trees were sometimes impressively in motion.  Since trees were often out of sync, twigs near neighboring trees must take a beating.  Does the clashing of tree branches in the wind have a role in controlling crown shape?  I'd always assumed that light was the only factor.  Gotta see what's on the web.

Went out around noon with tripod and took a good deal of video of tree crowns.  (The stars of the show are the white ash and white oak at the edge of the woods.)  

Our eastern coyote (coy wolf) had wandered through at least a few times since the snow ended on Wednesday, but tracks came no nearer to the house than the farback right against the woods.  I want to name her--even if I do share her with a quarter of Brockton.  What will it be?

Finally, took a set of standard photos for my Wild Place (in the little patch of woods out back).  It had been awhile.  Someday maybe I'll arrange all my Wild Place photos into a sort of time series "flip book."

A FEW DAYS AGO, during the latest cold snap, I decided to investigate something I'd long been curious about.  You may have heard that snow is actually a blessing to plants and small critters during very cold weather, since snow is a good insulator.  The Inuit, also, make their temporary winter shelters of snow--igloos.  (Of course, the Inuit could only  enjoy a balmy 320F indoors without melting their home, but outside would be much colder, and windy to boot.)  I collected a few food thermometers--the cheap mechanical probe-with-a-dial-at-one-end type--and ventured forth over several days to compare air temperatures with the temperatures at the ground surface, under six to eight inches of snow.  

Ideally, I'd do multiple measurements in several different environments with different depths of snow cover, or at different air temperatures.  In the end, the uncertain accuracy* of these little thermometers restricted useful data to just a few moderately trustworthy measurements in several places near the house.  The bottom line was that the temperature at the ground surface was very near the freezing point no matter what the air temperature was, down to at least 150F.

The downside of snow cover is that it also prevents the soil from warming up when the air temperature is warmer than 320F.

Water is funny stuff.  Because it is so familiar, we don't always realize what a peculiar chemical it is.  Their slight electrical charges cause water molecules cling to one another (and other molecules) giving water the property of absorbing enormous amounts of energy when it goes from solid to liquid, or liquid to gas, and giving off that energy when changing in the other direction.  (This is the reason ice is so useful for keeping your drink cold: even on the hottest day, the ice in your soda will, in the process of melting, absorb whatever heat it encounters, keeping your drink very near the freezing point until the amount of ice is no longer adequate to keep up with it.) 

In other words, the kinetic energy of heat entering your drink is absorbed in the process of breaking molecules of water free from the ice crystals of which they are a part--all without any further temperature change.  The amount of heat needed to turn one kilogram of ice (at 320F) into one kilogram of water (still at 320F) is 80 Calories.  (Those are "big C" or food calories, like those on nutrition labels.)

So by the same process snow or ice cover will keep the ground at the freezing point until it is entirely gone.  By the same principle, if a comfort-loving Inuit were to try to heat his igloo above freezing, he couldn't succeed until his roof fell in!

*Testing them in boiling water and ice water (which have known temperatures of 2120F and 320F) showed them to be off by half-a-dozen degrees at the high end, and a few degrees at the low end.  My further frustration with inconsistent readings was finally solved when I discovered the needles sometimes got stuck and needed a little tap to read properly.

Monday, January 20, 2014

Winter Day on the Water

The youth gets together his materials to build a bridge to the moon, or perchance a palace or temple on the earth, and at length the middle-aged man concludes to build a woodshed with them.   
Journal  July 14, 1852  H.D.Thoreau

Assonet Bay is just a widening of the Assonet River, tributary to the Taunton River (left edge of image).
Our route took us under the highway, and west to the entrance to the river proper.

January is the only month I had never been on the water.  When I saw the MLK Day weekend coming up, I got a minor case of adventure itch, and determined, if possible, to introduce my 13-year-old son Stephen to winter boating.  My original idea had been an overnight trip in our little enclosed sailboat, Surprise--probably in the relatively quiet waters of upper Narragansett Bay.  Then I began to wonder if a kayak camping trip might be possible--if my two-man kayak, Serendipity, would hold enough gear.  A test fit of all our bulkiest gear proved it would.  A night out on the shore of shallow little Assonet Bay would test our gear and mettle with a minimum of risk.  Now I had to watch weather and opportunity. 

Nope, not Surprise.

Nope, not Serendipity.

Musketaquid--a skin-on-frame kayak--with, and without, her skin.

The weekend arrived, but circumstances made the trip impossible.  I didn't regret it too much: it was raining steadily anyway.  With only Monday remaining of the weekend, I decided to at least put the kayaks in and try to check out camping sites for a future trip.

After a very lazy holiday morning, we finally set out in the afternoon with two kayaks atop the minivan for the half-hour ride to Assonet Bay.  The big kayak, Serendipity, stayed behind; for this day trip, Stephen would paddle Speedbump, and i would have Musketaquid.

Winter boating takes fussy preparation to be safe.  On the one hand, the air temperature was in the forties, and we would generate a fair amount of body heat as we paddled.  (It's harder to keep warm sailing, since there is less effort involved.)  On the other hand, even a little wind would make the air seem colder--especially if we got a little wet; and the water was a chilly 38oF.  Stephen and I were in ski pants and fleece jackets and hats, with neoprene gloves that would protect hands from wind and wet.  I wore tall rubber boots to keep my feet dry getting in and out of the kayak. 

Of course we wore life jackets, and each of us had his cell phone in a waterproof box clipped to it.  --falling overboard in deep water this cold without flotation is nearly a death sentence, since even a strong swimmer would find his muscles cramping up in just minutes, and drown long before hypothermia became an issue.  

A few years back, Stephen and I and his older brother had capsized our new two-man kayak on a November paddle in wind and waves; this had already impressed on Stephen the importance of getting out of cold water immediately.  Although wet clothes don't insulate very well, actual submersion in water draws heat out of the body at a tremendous rate.  Here in southern New England, it is perfectly possible to get hypothermia if you're in the water long enough even in the middle of summer.  To be wet is automatically to be cold, so I carried a change of clothes in a little dry bag in case one of us upset and took an unexpected dip.  As a final precaution, we would be in the shallows hugging the shore most of the way. 

With a gentle headwind, modest waves, and Stephen's cold hands, we only got about two miles out.  It was fortunate we turned around at that point, since my patching of my own boat, the skin-on-frame Musketaquid, had somehow failed to stop the leaks,  and I had taken on a good deal of water. 

Bufflehead (The Cornell Lab of Ornithology)

As I approached them, first one then another of two small flocks of small birds took off from the water.  One of these birds quacked as it took to the air, while a second, already left behind, dove beneath the water to escape.  On the theory that, if it swims like a duck, quacks like a duck, and dives like a duck, it is likely, in fact, to be a duck, I consulted my Sibley Field Guide after we returned.  Among the diving ducks, only one has the observed white cheeks and prominent white markings visible in flight: the bufflehead.  I figure I can pretty safely add this new bird to my experiences, and put a notch in my binoculars.  (Unfortunately, I was too busy paddling to keep my camera at the ready.)

With the tide near low as we returned, the muddy shallows left us and the boats black, but we finally piled back into the car, pleasantly tired, after two hours on the water.

I was reminded of a few things along the way.  Boots and neoprene gloves will keep you dry, but not necessarily warm.  A dripping paddle is very annoying, and something is needed to keep sleeves dry as well as one's lap.

Friday, January 17, 2014


Pity the community with no sidewalks.  It's a neighborhood in which no one goes anywhere except by car, and probably has little of interest within walking distance, anyway.  A neighborhood where neighbors don't know each other.
I live in a neighborhood right on the main road.  This is sometimes more curse than blessing.  Traffic is fast-moving, and can be noisy.  You take your life in your hands crossing the street.  With the house set close to the road and our bedroom facing it, traffic noise comes right in the open windows of summer all night long.  The sidewalk in front of the house is well-frequented, and some youth are thoughtlessly loud late at night.  But I also have three supermarkets, two pharmacies, and several fast food outlets within less than a mile's walk.  D.W. Field Park is just over half a mile away.  If the traffic can be distracting on this side of our block, on the other side of it I heard birds singing only yesterday.  Because we are on a designated school route, our sidewalks are plowed by cute little snowplows after every winter storm.

But even in my neighborhood, so well-suited for walking, many do not walk.  Walking the dogs the other day I was struck by seeing lichens on the sidewalk.  Lichens are delicate, slow-growing creatures.  Yet here is a sidewalk only a few hundred feet from my door that is dotted with them.  Surely they could not survive if that sidewalk got much use.  I myself don't walk nearly as much as I should; it simply takes too long to get places.  Or so I imagine.  Whole weeks go by in which I go no farther on foot than is needed to walk the dogs.  

 The pale greenish spots are probably some species of Parmelia.
Regular foot traffic should have killed it.

Wednesday, January 15, 2014

Exploring a New Urban Wild Place

Did a little "wilderness" exploration today.

Although Brockton, Massachusetts is a city, and a fairly old one, it still has considerable green space.  I'll bet other cities do, too.  Of course, there is D.W. Field Park only a half-mile walk north of me, a beautiful place designed by Frederick Law Olmsted (a father of landscape design who also laid out NYC's Central Park among others).  Brockton is proud of this park and rightly so.  I have walked its roads and paths less and less often over the years.  Although green, D.W. Field is certainly not a Wild Place, save in neglected corners.  Of course the municipal golf course isn't even in contention. 

The best way I know to find a Wild Place is Google Earth.  If you haven't got this software, get it now.  The free version is so wonderful that I'm not seriously tempted to pay for the fancy version.  Google Earth cleverly and almost seamlessly integrates satellite photos into a globe that allows you to zoom-in anywhere on the planet and explore.  Over the US, the resolution of the photos is good enough to spot cars on the street, though seeing pedestrians is a challenge.  You can do many other cool things, from seeing street-level views taken by ground cameras to exploring the ocean floor, but the most important features, for today's purpose, is the ability to overlay the satellite images with map information, and the ability to work backwards and forwards among views photographed at different times and seasons, going back years. 

First I let the whole city fill the screen so I could easily see the green spaces.  The biggest of these--at about 40 acres--looked like a great place to explore.  Next I needed access to it without attracting attention from neighbors.  Zooming in on this space and overlaying streets and their names, I found some likely access points.   Then, since the canopies of trees obscured the view too much, I worked backwards until I found a view taken in early spring, when most trees were leafless.  This showed me areas where the woods came right to the street, so I wouldn't have to trespass on someone's lawn to get in.  I saved this view as an image file, printed it, and tucked it in my pocket along with camera and binoculars, then set out by car in the sunny warmth of late morning. 

I have no idea who owns this land, so I would have to be discreet.

The nearest of my possible points of entry turned out to be in a neighborhood marked by a "luxury homes" sign.  I "cased" the entry briefly, then parked my car a few dozen yards away across from a newly-built house with a "for sale" sign in the yard.  Then I walked in, whistling a happy tune, trying not to look like a suspicious person, and noting which nearby driveways had cars indicating likely people at home.  

It was late morning, so most people should be in school or at work.  Even so, I tried to balance moving quickly to get out of sight of houses against moving unobtrusively through the bushes and brambles.  In a few minutes I was fairly well-obscured, though there was almost nowhere during my walk I was truly in the clear.  Next time I will leave the car at home to be more unobtrusive still, seclusion will be much more convincing after the trees leaf out.

Mind you, I don't think of myself as the criminal type.  I return money to cashiers when they undercharge me, and pick up after our dogs.  On the other hand, I am a bit of a rebel when it comes to open land.  (Some of the best hours of my childhood were spent in an old-field that adjoined my parents' land.  When as a teenager I discovered that land would be subdivided for house lots, I did a little sabotage, pulling up stakes and breaking pipes that had been sunk to assess groundwater depth.  It accomplished nothing, of course, except to make me feel less impotent.)  I knew I was trespassing on someone's land, hence the precautions.  But my main concern was not moral, but simply to avoid being seen, scolded, and forbidden to return.  

This wood is a delight.  It is a mixture of oaks, red maple, and white pine, with a shrubby understory that tends toward brambles.  It is trackless, and only two things--a discarded mylar balloon and some inflatable vinyl thing--gave evidence that humans sometimes came here.  Big trees, and fallen trees in various stages of decay, showed that these woods had not been managed for a long time--a conclusion strengthened by the presence of prince's pine--a clubmoss that favors old woods.  Of course, there has been no wilderness worthy of the name in southern New England for probably two centuries, and the hum of the nearby highway made it clear this was very near civilization.  But just as plainly I was in a little bit of The Wild.  Thoreau would certainly have found nothing wilder in the Concord of 150 years ago.

I was suddenly brought up short in my pleasure at discovering that this wood was my own hidden wild.  A contradiction.  I profess to be a big believer in fighting so-called Nature Deficit Disorder, yet was personally delighted that this wood, surrounded by homes, was unvisited.  Lack of trails was surely evidence that the surrounding families lived in-door lives with little but electronic experience of the natural world.  Yet I well remember my first paddle to a little island in Nippenicket Pond years ago, and discovering to my dismay the well-established maze of trails, rope swing, fire ring, and trash that marked a well-loved neighborhood haunt.  On posts scattered around the shoreline, signs proclaimed the Rules for visitors.  I haven't been back much since.  Of course when I was young this is how I claimed and settled the old-field of my childhood, establishing trails, and forts and hiding-places.  It is an unfortunate contradiction that we want to "have our wilderness and share it, too."  I will have to try to welcome the personal struggle, if more people get out into these woods.

After perhaps an hour wandering the woods, I headed home.  I'll certainly be back during the growing season!

Big trees.

 Prince's pine is not a pine at all, but a clubmoss or Lycopod.

White Pine mom and kids.

Cloud patterns seemed to emanate from the sun.

It looks like we are in the middle of nowhere.  Small pools were common, and 
parts of the land may be wet enough to qualify as wetlands, precluding development--I hope!

Bordering the Wild Place, a field over an acre in size is kept clear--for what?  A sheet metal storage building out of sight at the far end probably holds heavy equipment.  South of here is a small housing development that appears on only the most recent Google Earth images, so some development is occurring.

In the woods bordering the field are at least two tree stands.  
Is hunting legal within city limits?

Sunday, January 12, 2014

Bird Mortality

During that first deep freeze of January--when the temperature one night bottomed out at 00F (and yes I know we don't hold a candle to the midwest), and I had a combination of electric pipe tape, 75W bulb, and fan keeping some semi-exposed water pipes from freezing--I began thinking about the birds.

I remember my shock when I first learned of the tremendous mortality of birds in their first year, and especially of how they froze and starved in winter, with few birds surviving to their first birthday.  Birds have body temperatures higher than ours, and need to eat pretty constantly just to keep their tiny furnaces going.  With this in mind, the temperature staying in the single digits, and the ground covered in snow, I did what any empathic naturalist would: I brought home a big bag of bird food from the big box store and filled the feeder that dangled empty in front of our kitchen window.  For good measure, I poured another helping out on a nearby platform feeder that should be easily visible from the air.

Then I waited and watched, hoping the birds would discover the feeder quickly.  We don't feed year-round, and I've found it often takes many days before traffic begins to arrive.  But I figured that, with most food sources snowed under, and birds losing body heat, they would be motivated.  I waited some more, and got ready to write this post.

But try though I might, I wasn't able to support my beliefs.  Birds and Blooms has twelve winter bird myths, including that birds die in temperatures below zero, and that they depend on feeders.  Some thought-provoking ideas, but no citations, so I cannot say the author isn't just promulgating his own myths. 

It is true that mortality is very high in a bird's first year--but mostly among nestlings.  The high mortality only makes sense, since there is no more space or food (on average) this year than last.  As Dr. Mike Hounsome of Great Britain points out on a Countryside Info page, "if there was a pair [of robins] in your garden last year then there is a pair this year. But in the mean time that original pair has had, say, two broods of five young - that is ten new Robins. But by the start of the next breeding season there are only two."  Therefore, the mortality was about 80%. 
More authoritative information is harder to come  by.  A 1988 Stanford University page by Paul Ehrlich et al. lists maximum known lifespans for birds, and says that, in general, larger birds are longer-lived than smaller birds.  This page also points out how difficult it is to answering so simple a question as how mortality changes with age in animals that are always on the move.  (One nice thing about studying trees: they generally stay pretty much where you put 'em.)  The best data comes from bird banding studies, but these are difficult, expensive and chancy.

The population dynamics page of the same site (also by Ehrlich, et al,) has some nice examples of population studies, and includes the observation that the out-of-bounds growth of the human population is a huge cause of mortality and extinction.  "The human population has increased about 40-fold in the eighty generations since the time of Christ, as mortality rates have dropped without compensating declines in natality. One result of human population growth has been a decline in many bird populations, as Homo sapiens has hunted them, appropriated their food, and destroyed their habitats."

If there is a paucity of data about natural bird mortality, there is a great deal of interest in human-caused bird mortality.  By far the biggest cause is habitat destruction, as our population grows, and as we develops more and more of the earth's surface for our own purposes.  As causes of individual deaths (that is, after habitat destruction), SibleyGuides lists the human causes of bird death in steeply-declining order as: window strikes, feral cats, and high tension wires, with a bevy of other causes (wind turbines among the smallest) accounting for the small remainder.

But a recent report on Canadian bird mortality puts cats at the top of the list--responsible for three times more human-caused bird deaths than the eight other causes combined.  Scott R. Loss, et al, in another report, find that "Un-owned cats, as opposed to owned pets, cause the majority of this mortality. Our findings suggest that free-ranging cats cause substantially greater wildlife mortality than previously thought and are likely the single greatest source of anthropogenic mortality for US birds and mammals. Scientifically sound conservation and policy intervention is needed to reduce this impact.."  Certainly, domestic cats qualify as a very serious invasive alien species! 

 So it seems that winter isn't necessarily the killer I'd thought!  This has been quite an education!  And I still like seeing birds at my feeder, and not at all disappointed they aren't necessarily depending on us.  I am also glad I have become a dog-person!

SibleyGuide (see link above) figure published 2003--recent work sharply increases estimated mortality due to cats, chiefly those free-ranging because un-owned.