We have to move,” I announced
a dozen years
ago when the cell tower
went up behind the
firehouse a few hundred yards west of our
back deck. “The view is ruined.” What
had been an open sky above a line of oaks
and junipers now held a gray metal edifice
rising so high it hurt my neck to look at
the pipes, wires, and plates at the top.
“We’ll just have to get used to it,” my
wife said. “If you look that way,” my
daughter suggested, pointing south, “it’s
still pretty.” “And remember you would
get great reception,” my son added, “if
you ever broke down and bought a cell
A few weeks later Turkey Vultures set
up a roost on the tower. It may have been
only two or three individuals at first; I
didn’t want to know—or even look in that
direction. Vultures are among my favorite
birds, but celebrating their presence on
that tower seemed the equivalent of tying
on a party hat to visit a car wreck.
One winter morning months later,
while checking on a Bald Eagle nest miles
away, I discovered the tower and its occupants
were visible on the horizon beyond.
At that distance the structure didn’t
look quite so ugly, and through my scope
I counted seven vultures, preening and
wing-flapping at the start of their day.
Eventually, one lifted up and circled the
tower. A second bird joined it, and then a
third. Within a few minutes, all seven had drifted out of sight in a loose line. Were
they following each other somewhere?
In their 1973 article in Ibis, “The importance
of certain assemblages of birds
as ‘information centers’ for food-finding,”
P. Ward and A. Zahavi sparked a debate
that has continued now for nearly 40 years.
Why do birds gather in groups at night?
The authors argued that the reason most
of us think of first—protection from predators—
might not be the primary benefit.
Especially in the case of vultures, corvids,
and other large birds that roost in groups,
protection seems an inadequate explanation.
Ward and Zahavi hypothesized that
the key value of these assemblages is actually
to share information. Birds come together
at night to learn from their fellows
where the feeding has been good during
One summer evening, three or four
years after the tower went up, my wife
hurried in from her garden to tell me she
thought she’d seen a Black Vulture land on
it. I hustled out to the deck, unfurling the
scope’s tripod on the way, and found the
bird where she pointed: perched on one
of the panels, among half a dozen of its
longer-tailed cousins. The bird looked so
relaxed, ruffling its feathers and preening
leisurely, that we wondered how many
nights it had been joining the roost without
Within a week or so, we found two
Black Vultures up on the tower among
the Turkey Vultures. Were they a pair?
Sometimes they stood side-by-side, but
frequently only one appeared, and just as
often, neither. Trying to understand their
behavior led me to notice how erratically
the Turkey Vultures themselves occupied
the tower. It’s an unpredictable pattern
that continues: some evenings as many as
nine vultures gather there, and they might
return for several nights running (assuming
it is the same individuals each night);
at other times only three or four appear.
Generally they arrive in the evening, rarely
earlier than late afternoon, and depart
the next day by midmorning at the latest,
more often by an hour or two after dawn.
The Black Vultures seem to appear only
when Turkey Vultures are also present,
and sometimes both species abandon the
tower for a week or more.
How is it that the birds know where the
assembly is happening each night? And if
they are exchanging information, how is it
Information sharing at a nesting colony
of swallows or terns is easy to conceptualize.
When parents are returning with food
for their young, the other adults need only
watch which direction the successful foragers
are coming from or simply follow
them to the food source as they depart.
The more frequently an adult returns
with food, the more promising the message
(“This way to food!”). The dynamics
are entirely different at a roost, however.
No young are being fed, the foraging is
over for the day, and there seems to be no
easy way for a bird to detect which of its
fellows has been successful.
It is well established that Black Vultures
use Turkey Vultures as scouts by day. The
latter are more efficient flyers—able to
stay aloft for longer periods with less effort—
and they can smell. All other things
being equal, Turkey Vultures find carrion
faster. Black Vultures take advantage
by watching for soaring Turkey Vultures
dropping down to feed, then following
them to the carcass and bullying the larger
but less agile birds on the ground. Black
Vultures also attempt to bully other Black
Vultures at feeding sites, of course, and
a large carcass can draw in hundreds of
Specialists often call such piracy “local
recruitment” and reserve “information
sharing” for more subtle behaviors. In
fact, the question “What is information?”
seems to be one of the controversies generated
by Ward and Zahavi’s hypothesis.
“Part of the problem,” ornithologist Patricia
Parker Rabenold wrote, “is the reluctance
of some authors to accept anything
short of a waggle-dance [as seen in honeybees]
or a detailed map as information.”
Rabenold investigated Ward and Zahavi’s
hypothesis in an extraordinary five-year
study of Black Vultures. “In communication
theory, information is that which
reduces the uncertainty of the recipient,”
she wrote in an article in Animal Behaviour
in 1987. “I would argue that any cue
that increases a bird’s foraging success that
is gained at a roost or colony qualifies as
information because it reduces the recipient’s
Rabenold trapped and wing-tagged 344
Black Vultures among a population of approximately
1,200 birds that shared seven
roosts in a study area of 250 square kilometers
in Chatham County, North Carolina.
From 1976 to 1982, she made 797
visits to the roosts as well as numerous visits
to baiting sites and other food sources.
Her conclusion? When it comes to
Black Vultures, at least, Ward and Zahavi’s
model is basically correct. The hidden
keys are age class and kin relationships.
Rabenold’s wing-tagged birds demonstrated
that younger birds follow adults
from the roosts in the morning and that
mates as well as parents and offspring often
gather together at roosts, follow each
other on foraging flights, and associate at
feeding sites. So, a young bird at a roost
will follow an adult (apparently just because
it is an older bird) to reduce its uncertainty
about where to find food. Family
members apparently also lead each other
to food sources.
The information to be gained, it seems,
comes right at the start of the day when
the birds depart. When they leave the
roost, they are following one another.
“Where that bird goes,” they seem to be
thinking, “I should also go.”
Rabenold put aside the topic of interspecific
information sharing between Turkey
Vultures and Black Vultures to focus
on Black Vultures alone, but I can’t help
wondering about our tower’s mixed species
roost. Based on Rabenold’s research,
I’d be willing to bet that our two Black
Vultures are a mated pair, and in our area
with few other members of their own species
to be found, they are especially dependent
on their Turkey Vulture roost mates
for directions each morning. Do they associate
with their cousins every night? If
so, where are the alternate roost sites, and
how do the birds of either species know
to move from one roost to the next? And,
finally, how many vultures of each species
does our immediate area support?
If we had a few more cell towers visible
from the back deck, we could investigate
these questions. Some people I know
might even be wondering why I don’t finally
break down and buy a cell phone to
help support the enterprise. Maybe some
questions are best left unanswered.