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?

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.

♦

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.

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, and voles might
move along their trails.  If I move too,
my 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 I couldn’t
stay still.  I rarely caught voles
then, and my parents had
to feed me. 

But I
am a wind hoverer
now.  I am a killer, calm-eyed,
ready, absolutely steady, and working
like hell to hover in 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
discriminate voles bumping
grass and wind bumping it would be
difficult, even if kestrels were standing
still, and they can do it.  But if kestrels’
eyes were moving too, everything in
both eyes’ visual fields
would be moving.  Imagine
how much harder it would
be to tell the difference.

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 our 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 processing 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 March 2021