August 28 2014 Ramble Report

As Mr. Rogers said, It’s a
beautiful day in the neighborhood, and that certainly described this morning as
22 Ramblers gathered to seek what we could find. The temperature today was in
the mid 60s, marvelously cool for our early ramble. 

The link to Don Hunter’s album
for this ramble is here.

Reminder: All the
rambles for the remainder of the year will begin at 8:30 AM. Don’t forget to
update your schedules. 

Reminder:
There is a trail walk at Sandy Creek Nature Center this coming Weds.,
Sept. 3, at 9:00 AM. Meet at the Education & Visitor’s Center.
Coffee and homemade goodies afterwards.

Announcement:

Ben Tonks’ Eagle scout project to construct a walkway over the
muddy marsh at the lower part of the Orange Trail still needs about $400 more
to purchase materials. If you’ve already sent Ben a check he thanks you, but if
you’ve put it off or forgotten, here’s what you need to do: Please send your
personal check made out to: Benjamin S.
Tonks, 270 Hunnicutt Dr., Athens GA 306061708

Continue reading

August 14 2014 Ramble Report

It was a beautiful morning with
temperature in the 60s, and 22 ramblers showed up.

Don Hunter’s photo album of today’s ramble is here.

Ed Wilde read a few selections from the book Keeping the bees : why all bees are at risk
and what we can do to save them
by Laurence Packer (Harper Collins,
Toronto, 2010) (The author was recently interviewed by Terry Gross on Fresh Air.):

“There
are more species of bees than there are of birds and reptiles combined; there
are more species of bees and wasps combined than there are of plants. We can
better estimate changes in ecological conditions with insects than we can with
the more popular birds and mammals simply because there are so many more
species to give us the information we need.” pg 15


… bees may be the proverbial canaries in the coal mine of the globe’s
terrestrial habitats …I believe they are particularly good at indicating the
state of the environment in areas that have been considerably influenced by
human activity.” pg 4

“Less
than 5% of [all bees] make any honey at all, and only a small fraction of those
make honey in large enough quantities for us to be able to use it.” pg 42

“The
single most bee-diverse couple of hundred square kilometers on the planet . . .
is in the SonoranDesert on the Mexico-Arizona border over 500 hundred
species.” pg 141

Rosemary read the Mary Oliver poem: Look and See, from the collection Why I Wake Early, Beacon Press, 2004.

Look and
See

This morning,
at waterside, a sparrow flew

to a water rock and landed, by error, on the back

of an eider duck; lightly it fluttered off, amused.

The duck, too, was not provoked, but, you might say, was

laughing.

This
afternoon a gull sailing over

our house was casually scratching

its stomach of white feathers with one

pink foot as it flew.

Oh Lord,
how shining and festive is your gift to us, if we

only look, and see.

~ Mary
Oliver ~

The route today was out the mulched trail
to the Dunson Native Flora Garden, then out the white trail under the power
line to the river. There we took a left on the Orange Trail which we followed
to the Orange Spur bridge and trail to the Flower Garden and back to the
Visitor Center.

Our first stop was a line of dead tree
branches arranged through and around trees. Natalie, an education specialist
with SBG, told us it was an art installation created during a recent
“Festival Day.”

Horse balm flower buds

We stopped to discuss the northern horse
balm, which has yet to bloom.  Its
straggly yellow flowers are slow in coming. 
Carol and I saw them in full bloom last weekend along the Blue Ridge
Parkway. The curator was thinking that the plant had suffered a good deal of
caterpillar damage, but the plants in the mountains had also suffered that
damage.  Insects seem to like the leaves.

Soapwort Gentian

Currently there are few flowers blooming
in the Dunson Native Flora Garden but the curator, Joey Allen, is trying to
plant some that will flower at this time of year.  One recent planting is harvest bells, or
soapwort gentian, Gentiana saponaria,
that was in full bloom.  Someone asked if
its closed flower ever opened up, and the answer was that it does not. Bumble bees
that pollinate it have to push their way in. Somehow we got to talking about
the fringed gentian, Gentianopsis crinita,
that grows on the road verges around Track Rock Gap near Young Harris.  At least it used to do so.  Even though the Department of Transportation
marked off the plant areas with posts to prevent mowing during its growing
season, recent retirees moving into the area are neatniks and want the verges
always mowed.  They mowed the verges
themselves and just about wiped out the plants in that area.  Other safeguarding sites have been found.  Hugh commented that some believe the fringed
gentian to be the most beautiful of our native wildflowers.  Later Dale challenges that with his belief
about the most beautiful wildflower.

We stopped by the devil’s walking stick
which was past blooming.  The leaves on
this plant are the largest in North America. 
The leaflets themselves are small, but the whole doubly-compound leaf
can be up to 64″ long. Next to it was a silky dogwood in fruit, and below
it was sweet spire, Itea virginica,
in bloom. A nearby dogwood seemed to be attacked by the disease affecting
dogwood trees in the Southeast.

A passionflower (Passiflora incarnata) drew lots of discussion.  Dale said that to him this is the

Nectar droplets on Passionflower extra-floral nectaries

most
beautiful native wildflower. We talked about the three lobed shape of the
leaves, and the fact that a flower only opens for a day. Both the Variegated
fritillary and the Gulf fritillary butterflies lay their eggs on this plant.
The chrysalis of the Variegated fritillary can survive our winters, but the
Gulf fritillary cannot. So in spring only the Variegated fritillary is present.
The Gulf fritillary population is replenished by migrants dispersing out of
Florida, where the chrysalis stage can survive over winter.  The passionflower has a way of defending
itself from the caterpillars of these fritillaries. It recruits ants by means
of nectaries located at the base of each leaf blade. These nectaries secrete a
sugar solution like the one found in the flowers, attracting ants.  Once on the plant the ants roam about and kill
and eat any eggs or small caterpillars they find. They are not 100% effective,
otherwise we would never see any Gulf fritillaries.

By this time we were at the wet area of
the Dunson Garden where the clustered stems of horsetails, among the oldest
plants in evolutionary development, were growing.

Cardinal flower

The next point of interest was the
dramatic red cardinal flower, Lobelia cardinalis.  Crossing from the road into the Garden we
checked out the seashore mallow, Kosteletzkya
virginica
, but it was only in bud, and not blooming.  The swamp rose-mallow was still
vigorous.  It came in shades from pink to
white, but they all had the red blotch at the center of the petals with the
typical hibiscus column of style and attached anthers.  Behind us someone was asking about the mint,
which was Monarda punctata,
horsemint or

Wild Senna flowers

spotted beebalm, and which was just budding.  Avis asked about the rattlesnake master, Eryngium yuccifolium next to the swamp
mallow.  Sue Wilde wanted to know about a
yellow flowered plant she had found in the area which is not cultivated.  Wow!  I
had been looking for this plant for weeks. 
It is southern wild senna, Senna
marilandica
.  Next to it was a
beautiful rose pink, Sabatia angularis,
and all around it was the first blooming of white wingstems. They turned out

Rose Pink

to
be white crownbeard, Verbesina virginica with
its straggly white flowers.  Also on the
same slope was the still-blooming mountain mint, Pycnanthemum incanum.

Don pointed out an example of spittle bug
covered with a large mass of its white foam. He challenged Dale to sample it,
but Don himself tried it and thought it tasted bland.

On our way to the white trail under the
power line, Dale found Virgin’s Bower, Clematis
virginiana
, and pokeweed, Phytolacca
americana
, in all stages from flower to fruit.

Under the power line along the white
trail Andie showed us the yellow flag where she had marked the spot where she
had observed a box turtle laying its eggs in a hole.  The turtles don’t seem to have hatched
yet.  We also saw more southern wild
senna, as well as golden aster, Heterotheca
latifolia
, that was just starting to bloom.

Going through the gate to the floodplain
we observed the beautiful, but small red morning glory, Ipomea coccinea.

In the flood plain the late flowering
boneset was just beginning to bud everywhere. 
Camphorweed, Pluchea odorata,
was also starting to bloom.  We could
just see the faint pinkish color to the buds at the top of the plant.  Someone pulled off from one of the plants
both a leaf-footed bug and a Lynx spider. 
Dale put them in the same box, so I asked whether they were friends or
enemies.  No, they were indifferent to
each other.

There was so much to see and talk about
here.  St. Andrews Cross, Hypericum crus-andreae, was blooming low
to the ground.  Although not in bloom, we
could recognized the opposite leaved wing stem, Verbesina occidentalis.  We
decided the huge thistles were native because the leaves were grey
underneath.  The sunflowers were Helianthus hirsutus.  Then there was a cricket.  One long lasting false dandelion, or Carolina
desert chickory, Pyrrhopappus
carolinianus
was still blooming. 
Also low to the ground were a number of blooming leafy elephants foot, Elephantopus carolinianus.

Toward the river it seemed like we were
walking in a canyon created by the 12 foot high giant ironweed, Vernonia gigantea.  The staff will probably have to cut down much
of the vegetation in the flood plain because a number of tree saplings have
sprung up.  One in particular that
pioneers in the floodplain is box elder, Acer
negundo
.

Privet has been removed!

We made the left turn to go down Orange
trail.  The big reason for rambling along
here was to see the huge effort that has been made by the contractor hired to
remove the privet.  Large tree-like
stumps that were left after sawing down the privet were painted with a kind of
Round-up that would not poison the amphibians inhabiting this environment.

After leaving this environment we entered
the area still

Cross vine

surrounded by privet. 
Nevertheless, there was lots to see: 
cross vine (Bignonia capreolata),  Virginia dayflower (Commelina virginica), sweet autumn clematis (Clematis terniflora) [which differs from the Virgin’s bower by
having only one leaflet

Virginia dayflower

instead of three], greenbrier (Smilax rotundifolia), river oats or fish on a pole (Chasmanthium latifolium), and river cane
(Arundinaria gigantea).  One person suggested that river cane was very
aggressive.  One wishes it were really
true because that would be a wonderful substitution for the privet.  But as I understand river cane it spreads
slowly and blooms only rarely and then dies. 
At the time of Bartram there were wonderful

Sweet Autumn Clematis  

canebrakes along
streams.  Other findings along the river
were poison ivy, and a green ash sapling, which enabled us to point out the
opposite compound leaves for the green ash, which is the dominant canopy tree
in the floodplain.  It was a good way to
see the leaves up close.  Other trees
identified were musclewood, hop hornbeam, and chalk maple.

Looking at the man-made dam in the old
beaver pond area, Hugh talked about why the Garden installed the dam after the
beaver left.  The reason was to purify
the water coming down the stream along the Orange Trail.  At that time there was a University pig farm
near the headwaters of the stream, so the water definitely needed
purifying.  Today Horticulture has taken
over that land and the pig farm is gone. 
In the middle of the swampy area were a large number of water hemlock, Cicuta maculata.  Dale and Hugh had a discussion about whether
this was the poisonous plant used to provide the drink which killed
Socrates.  Hugh thought it was; Dale
thought it was the poison hemlock (Conium
maculatum
).  After returning home, we
both looked it up and Dale was correct.

Clambering over the piled wood at the
very muddy spot going around the beaver pond area, Hugh talked about Ben Tonk’s
eagle scout project to build a board walk here. 
At this point he is looking for financing for the project which will
cost $1739.01.  So far, he has received
almost $500.  I suggested that the
Ramblers might want to provide some money for the project.  There seemed to be general agreement to do
that.  As soon as we receive from Ben a
PDF of his project and design with where to send money, we will send it out to
everyone, so that those who wish to contribute can do so.

Here we spotted a netted chain fern.
Carol identified it by the fertile frond with its linear, alternate, widely
spaced segments. The spores were still green, not brown yet, so hard to
distinguish at a distance. On the way upstream we saw jump seed, Polygonum virginianum..  Jumpseed gets its name from the way in which
the mature fruit jumps from the plant when its persistent style is pushed. As
we passed the stand of green headed or cut leaf coneflower (Rudbeckia laciniata) we wondered why it
is not blooming so much this year.  it
looked like there had been a few blooms, but they were gone.  Speculation is that the canopy is closing
over and reducing light to this spot.  A
greenbrier (Smilax bon-ax) that has
leaves that look like cinnamon vine, but this smilax has thorns.

On the way up the slope to the Flower
Garden the most excitement was generated by the crane fly orchid, Tipularia discolor.  During the winter we see its leaves, which
are green on top and purple underneath, everywhere,.  The leaves disappear in late spring, and the
flower pops up in July.  Since the color
of the flower is a yellowish tan it blends easily into the background and is
easily missed walking by it.

In the Flower Garden we were all
enthralled by the beautiful purple berries on a cultivar of beauty berry (Callicarpa sp.).  From there it was just minutes to Donderos’
and refreshment and conversation.

Hugh

SUMMARY OF OBSERVED SPECIES:

Common Name

Scientific Name

In the Dunson
Garden

Northern
Horse Balm

Collinsonia canadensis

Harvestbells

Gentiana saponaria

Devil’s
Walking Stick

Aralia spinosa

Silky
Dogwood

Cornus amomum

Sweet
Spire

Itea virginica

Assassin
bug

Family Reduviidae  

Passionflower

Passiflora incarnata

Horse
tails

Equisetum sp.

Cardinal
flower

Lobelia cardinalis

Spittle
bug

Superfamily Cercopoidea

Swamp
rose-mallow

Hibiscus moscheutos

Spotted
Beebalm

Monarda punctata

In the Power
line ROW

Mountain
mint

Pycnanthemum incanum

American
senna

Senna marilandica

Common
rose pink

Sabatia angularis

White
crownbeard

Verbesina virginica

Virgin’s
Bower

Clematis virginiana

Pokeweed

Phytolacca americana

Late
blooming thoroughwort

Eupatorium serotinum

Camphorweed

Heterotheca latifolia

Leaf-footed
bug

Family Coreidae

Green
Lynx spider

Peucetia viridans

Reclining
St. Andrews Cross

Hypericum stragulum

Opposite
leaved wingstem

Verbesina sp.

“Giant”
thistle

Family Asteraceae

Hairy
Sunflower

Helianthus hirsutus

Cricket

Acheta sp.

Carolina
desert chickory

Pyrrhopappus carolinianus

Leafy
elephants foot

Elephantopus carolinianus

Giant
ironweed

Vernonia gigantean

Box
Elder

Acer negundo

In the Orange
Trail

Crossvine

Bignonia capreolata

Virginia
day-flower

Commelina virginica

Sweet
autumn clematis

Clematis terniflora

Common
greenbrier

Smilax rotundifolia

River
oats

Chasmanthium latifolium

River
cane

Arundinaria gigantean

Poison
ivy

Toxicodendron radicans

Green
ash sapling

Fraxinus pennsylvanica

Musclewood

Carpinus caroliniana

Hop
Hornbeam

Ostrya virginiana

Chalk
maple

Acer leucoderme

Water
hemlock

Cicuta douglasii

Beech
blight aphids

Grylloprociphilus imbricator

Netted
chain fern

Woodwardia areolata

Lady
fern

Athyrium filix-femina

Jumpseed

Polygonum virginianum

Saw
greenbrier

Smilax bona-nox

Cranefly
orchid

Tipularia discolor

Beauty
berry

Callicarpa americana

August 7 2014 Ramble Report

Another beautiful morning at the State Botanical Garden, not
too hot and not too cold, found 22 Ramblers ready to learn about Hummingbirds. 

But first, we presented Ronnie and Eva, our two

Eva
Ronnie

Junior Ramblers, with
certificates to commemorate their participation in our rambles this summer. We
enjoyed having them present and benefited greatly from their enthusiasm and
sharp-eyed powers of observation. We hope they will return in the future.

Don and Emily each contributed a reading today. Don’s is from A Short History of Nearly Everything by Bill Bryson; Emily’s is from Hummingbird Nest: A Journal of Poems by Kristine O’Connell George. Both readings can be found here.

Today we were pleased to have a guest leader, Judy Glenn,
who designed the Hummingbird trail for the State Botanical Garden. This trail
consists of numbered Red Markers, each with a Hummingbird silhouette, that are
strategically placed in spots all over the formal gardens. Each marked location
is where Judy has personally observed consistent hummingbird activity. These
are generally places where there are plants that are good nectar sources
(Lantana and sage, as well as others). The numbers on each marker correspond to numbered
locations on a map of the garden that is available in the Visitor’s Center. (Because
the maps are in short supply I’ve taken the liberty of copying one and placing
it here
on Google drive. My scanner could not accommodate the full width of the
brochure, so part of the illustrations are missing. You’ll have to pester the
garden to get a full sized map.)

Judy is a nature photographer as well as a hummingbird
enthusiast and has a blog
and a facebook page
that display her work.

Judy began by taking a group photo and then playing
recordings of Ruby-throated hummingbird vocalizations. She then told us how she laid out the trail and then presented some facts about our local
hummingbirds. We have only one species, the Ruby-throated hummingbird, here in Georgia. (There are occasional strays of western species, but these are very uncommon.)  Before she began Judy passed out pennies to a number of
ramblers and we discovered why — a penny weighs 2.5 grams while an adult
Ruby-throated hummingbird weighs approximately 3 grams, just slightly more
than the penny in our hand! The male Ruby-throats arrive in our area about the
middle of March, ahead of the females, and establish territories. Only the
males have the brilliant iridescent red throat; females and juvenile males have
white throat feathers. As the young males mature they gradually get a
sprinkling of red feathers in the throat region. Until then it is not possible
to identify the sex of the young birds. The red coloration is not due to
pigments. It is a structural color, like the colors seen in soap bubbles. Its
appearance also depends on your viewing angle. From the side the throat of a
male looks black. The male can move the feathers to flash the brilliant red in
the direction he wishes. Ruby-throats migrate to Mexico and Central America in
the fall.

Hummingbird nests are constructed from plant fibers like
milkweed fluff. These are collected and bound to a tree branch with silk from
spider webs. Sue mentioned that she had seen a hummingbird gathering spider
webs from her windows — an excellent reason not to wash those windows. After
the bundle of fiber is secured to the branch the female collects bits of lichen
and tucks it into the nest, camouflaging it. Then she sits in the walnut-size
nest and wiggles and turns about until a suitable depression is formed. She lays
one or two eggs, each the size of a small jelly bean. For the previous three
years Judy has recorded the last dates on which she has seen hummingbirds in
the garden: Oct. 1, 2 and 3.

After these introductory remarks Judy led us through the
garden, pointing out favorite flowers

Ramblers looking for hummingbirds

and perching sites at each of the 20
stations on the Hummingbird Trail. Because the hummers don’t like large groups
of people we caught only glimpses of them along the way; they fled our approach. So after
Judy showed us the location of the last station we broke up into smaller groups
to observe the hummers without disturbing them.

I’m going to attempt to answer some of the questions I was asked during the ramble, as well as mention some other aspects of hummingbird biology and physiology. Click on the links below if you’re interested.

How can hummingbirds hover?

Where do flowers get their nectar?

How do humming birds forage for nectar?


How much nectar does a hummingbird consume?

Why do hummingbirds have such a high metabolic rate?

What do hummingbirds do at night?

As usual, we left the garden and descended on Donderos’ in the Visitor Center for conversation and beverages.

Dale

What do hummingbirds do at night?

In a word, they fast. 

But
don’t they have to eat continuously to prevent starvation? And if the night is more
than a couple of hours long wouldn’t they starve? 

Yes, that’s true, but they have a
clever trick they can use to make their energy supplies last the entire night.
They allow their metabolism to slow and their body temperature to drop, a
condition called torpor. It’s like hibernation, but it only lasts overnight instead of over
several months. When daylight approaches they spontaneously warm up by
shivering their flight muscles. When they reach their normal operating
temperature they take off in search of food.

Twenty-five years ago I was
fortunate to spend a couple of days banding hummingbirds under the supervision
of the late Bill Calder. Bill had set up mist nets (nets with a mesh so fine
that it is not noticeable to birds) in a flower-filled, montane meadow at ~9000
ft. elevation in the Elk mountains of western Colorado. We arrived to set up
the nets early in morning, just as the sky was lightening but before the sun
was up. As we walked through the meadow we could hear the buzzing flight sounds
of numerous hummingbirds, already actively foraging for nectar in the very chilly early morning mountain air. 

As
soon as we deployed the nets we started to hear the distress calls of
hummingbirds trapped in the mesh. With our head lamps on we rushed to find the
captured hummers and gently extricated the birds from the mesh. We then placed them in bags
made from Bill’s wife’s nylon hosiery, and took them to Bill who weighed
them, checked them for bands and, if they were unbanded, clamped a tiny,
numbered band around one leg. They were then released. From capture to release
took at most five to ten minutes. We worked continuously for several hours
until the rate of capture fell to zero. By an hour or two after sunrise the
ravenous hummers had drunk their fill and could slip into their daily routine
patrolling their trap lines and stocking up on enough nectar to last them
through the next night.

Why do hummingbirds have such high metabolic rates?

One reason hummingbirds have such high metabolic rates is because they are so small. It hinges on the relationship
between body size and surface area. As the size of an animal increases its
relative surface area becomes smaller. And as animals become smaller their
relative surface area gets

Two equally warm cubeys. Which needs to eat relatively more?

larger. If this is not intuitive, imagine two
animals shaped like cubes. Call them “cubeys.” Look at the small
cubey on the left and compare it to the larger cubey. The larger cubey is twice
as big, if you measure bigness as height or length. Notice how many cubes make
up the larger cubey. There are 8 of them. Also notice that only 3 of the faces
of the little cubes in the larger cubey are exposed to the external surface,
but 6 faces are exposed in the smaller cubey. This means that the smaller cubey
loses heat twice as fast as the larger cubey because it has relatively more
surface area for its size than the larger cubey. The same holds true for real
animals. Their shapes may be different but the relationship between surface
area and volume (weight) holds true: larger animals have relatively less surface
area than smaller one. 

This means that a small, warm-blooded animal will have
to generate more heat per unit of time than a larger, warm-blooded animal. To
do this it will have to eat relatively more for its size than the larger
animal. Thus the hummingbird consumes more than its weight of nectar in a day
while we get by with just a small fraction of our body weight. Notice that it is relative amounts that are involved. The hummingbird only weighs 2.5 grams, but consumes 3 or more  grams of nectar per day. We obviously eat more in absolute weight per day, but far, far less than a hummingbird relative to our body weight. 

This surface area/body size relationship also explains why you need to bundle up your babies and small children even when the weather is mild. A tiny infant or child will chill much faster than an adult because it loses heat more rapidly in relation to its body size than an adult would.

How much nectar does a hummingbird consume?

Hummingbirds drink more than
their weight in nectar each day. This requires hundreds of visits to flowers.
If they are deprived of nectar for just a few hours they are in danger of
starving. This high rate of consumption is required because they have the
highest recorded metabolic rate of any vertebrate, both while resting and
during activity. This high rate of metabolism is due to several factors: their
massive flight muscles, which make up 30% of their body weight and their tiny
size, which causes them lose heat more rapidly than a larger bird does.

How humming birds forage for nectar

Hummingbirds stake out a
group of flowers that are producing nectar and then defend them,
trying to prevent other hummingbirds from taking “their” nectar. The
flowers they defend are like a trap line. The “owner” checks each
blossom in a sequence, taking nectar when it is found and then moving on to the
next flower in line. This behavior is called “traplining,” in
reference to how humans hunt beaver and other fur-bearing animals by setting out a
large number of traps and them checking them in sequence at periodic intervals.
Hummingbirds can learn how long they have to wait to get more nectar from the
same kind of flower. So after emptying one blossom they visit others and don’t
return until they first has had enough time to replenish its supply of nectar.

How do flowers make nectar?

Plants use the energy in
sunlight to make sugar from carbon dioxide and water — the process that’s
called photosynthesis. Most of the sugar is made in the leaves, the plant organ
that is specialized to gather sunlight. From the leaves this sugar travels through
the plant’s conducting tissues to the other parts of the plant — the roots,
stems and flowers. These plant parts then remove the sugar from the conductive
tissues and use it to fuel all their metabolic processes. In the flowers there are specialized cells, usually found at the base of
the flower, around the ovary. These cells secrete the sugar into a sweet droplet of fluid that we call nectar. That’s what you taste when you
pick a honeysuckle blossom and suck on it. And that sugary solutions is what the hummingbird as
well as many insects are after. The flower produces nectar at a rate that
varies with the temperature and time of day, and the nectar will accumulate if no bird or insect visits visits the flower. So the amount of nectar present in the flower depends on how
rapidly it is produced and how often it is removed by hummingbirds or bees. When
most of the nectar is removed it takes a while for the supply to be
replenished.

How do hummingbirds hover?

Some birds, like hawks that are searching for prey, can
hover to a limited extent, but it is very awkward and they are unable to fly forward or backward while hovering. No other kind of bird can hover like a hummingbird, staying motionless (except for their moving wings) in a single spot and then flying forwards and/or backwards. This ability is
due, in part, to the shoulder joint. It is a ball-and-socket joint that only
one other type of bird has: the swifts, that are closely related to the hummingbirds. This shoulder joint
allows the wing to rotate 180 degrees and is the secret to the hummingbird’s
hovering ability. The wing can supply lift not only during the forward stroke,
but also on the back stroke, by flipping over. To see how this works hold your
arms straight out to your sides, palm facing down. Now tilt your hands up a
little (about 10 – 15 degrees above horizontal). Your hands should be tilted so that the thumb side is higher than the little finger side. Now move your arms forward, still keeping them straight. This is the same
motion you use when you are treading water. To continue treading water you
would rotate your hands downward so that the thumb side was lower than the little finger
side and then sweep your arms back. This makes the little finger side of your hand the leading edge on the backstroke. A hovering hummingbird is like a person
treading water, but it is “treading air.” And it doesn’t tilt its
hand like you did on the backstroke. Instead it rotates its entire wing
counter-clockwise as it begins its back stroke. It’s as if when treading water
you, instead of simply tilting your hand in a different direction, you actually
turned your arm over counter-clockwise, so that the thumb side was always the
leading edge. It is the ball-and-socket shoulder joint that permits the wing to
flip like this. Birds without that type of joint can’t flip their wings over so
they can’t generate the lift necessary on the backstroke to maintain their position in the air —
they can’t hover.