Lee’s Stories

Lee’s Stories

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posted on August 8, 2016 | Science and Nature
The Wind Hoverer

The Wind Hoverer

I was a kestrel,
hovering in the North Sea
wind,
inside the last dike in the
Netherlands, looking for voles. 
If the
wind had been calm or steady that day,
finding voles would have been easy.  Even
if I saw no voles, themselves, but saw grass
wiggle with their passage.
But when is wind
calm or steady inside the last dike?  When
don’t grasses wave and sway? When
don’t dikes buffet, tumble, and
bumble the wind, multi-
plying the difficulty
of hovering
in it?

To be
good for running,
voles’
trails must be narrow enough
to
hide them from kestrels
and wide enough
to let them run freely
but not
bump into grass and
wiggle it.

 

If I attend
to the evidence of my eyes,

when grass wiggles in a certain way,
especially if it leaves a certain
wake,
I drop and carry
away a meal.

As kestrels
understand the intransitive
verb “to hover”, it is to stay absolutely
stationary aloft, waiting, watching, ready
to drop, despite the roiling wind.
That, precisely, is their
challenge.

 

When wind moves,
grass moves.  V
oles might
move along their trails.  If I move too,
m
y own motion is part of my view, and that
makes it much less likely that I will detect them.
When I was young and eager, not yet a hunter
and learning to fly,
I bounced, flounced,
and flustered as I flew and
couldn’t
stay still.  I
rarely caught voles
in wind and
my parents
had to feed me. 

I am a wind hoverer
now.  I am
a killer, calm-eyed,
ready, absolutely steady,
and working
like hell to hover in the wind.


This story is based on
research by eminent Dutch
biologist Serge Daan.  Using high
speed films shot from 2 directions, He
showed that while wind hovering, kestrels
work  hard to keep their eyes stationary, with
respect to the ground, regardless of wind.  They
work so hard that sometimes their heads
are below their bodies, yet their eyes
remain nailed, precisely, to the
center of the earth.

Standing in my
lab with hummingbirds
flying around our heads, Daan
and I speculated about the develop
mental, psychological, ecological,
and evolutionary significance
of that remarkable
facility.

Using great
poetic license, the scenario
above expresses the most plausible of
the explanations we considered for kestrels
to invest so much life force in stationary wind
hovering. For them to separate voles bumping grass
from wind bumping it would be difficult even if
the kestrels are still.  If they are moving too,
what must happen in perception would be
an inordinately more difficult problem.
That makes it easy to appreciate
how hard they worked
to stay still.

Daan’s
contribution to the
conversation was his experi-
ence of kestrels and other animals,
his long record of careful measurement of
difficult things, and his desire to explain his new
discovery.  I had been reading about signal pro-
cessing in general, particularly in the face
of noise that can swamp signals
and make them difficult
to detect. 

One of my
graduate students was
using the mathematics of complex
wave forms to detect patterns at the time.
I had also been studying the development of
intelligent behaviour in hummingbirds and
human beings (students) at every age
from
elementary school children
to life long learners. All
of it helped us think
together.

 Like so
many ideas in science,
what we came up with is simple.
Stationary hovering simplifies
kestrels’ view and makes
finding voles easier
in wind.

I don’t know
whether anyone has written
about the problem from that perspective,
but someone probably has.  When animals ‘freeze’
in nature, it not only makes them harder to
detect but makes it easier to
detect us if we move.

 


On the Diaper-changing Behaviour of Robins
and The Deep Meaning of Creativity  are
also about adult birds feeding
young who can’t feed
themselves.
A Small
Matter of Maternity

is about species who don’t feed
their young but provide parental care.


Edited January 2019

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