The Sopranos of the Bird World
How come big birds don't take a whack at little birds harassing them? Do the
big birds feel threatened or just annoyed? Lanney, Sandia Park, New Mexico
 Mobbing
undoubtedly bothers big birds because it works: the large predator moves on. For
example, a carrion crow, looking for eggs or chicks, cruises through the nesting
area of black-headed gulls. Screaming gulls dive bomb the crow,
defecating. The crow, terribly distracted, can't find eggs or
chicks. She moves along, and the gulls stop mobbing her.
Cooper's hawk, Toronto, Canada. Courtesy of Wikipedia.
Occasionally, the big bird does take a whack at
tormentors. In 1946, Hawk Mountain curator Maurice Braun reported seeing a
golden eagle capture a red-shouldered hawk harassing him. Here's
his account:
"Lying on my back and scanning the zenith with my 7 x 50 binocular, I
picked up a small hawk making frequent passes at a much larger, dark bird. The
smaller bird persisted in annoying its fellow traveler. I switched to an
18-power glass. The dark bird proved to be an adult golden eagle. It made a
sudden thrust forward, executed an Immelmann turn [half looped over, then half
rolled to reverse direction] as effortlessly as a fly
landing on a ceiling, and then, to my amazement, it seized the smaller hawk,
which seemed to put up a momentary, hopeless struggle. Down came the two birds
precipitously, the eagle with set wings and clutching its victim. As the eagle
plunged to earth, the wings of the smaller bird were fully outstretched, and I
glimpsed the ruddy breast of the red-shouldered hawk. The eagle, still
clutching its prey, disappeared into the densely wooded flank of the ridge."
In fact, ornithologist
Eberhard Curio has
documented 35 cases where the big bird attacked, captured or killed little
birds mobbing her, the predator. "Taken together, these anecdotes
strongly support the hypothesis that mobbing birds are at
deadly risk," he concludes. Curio is a professor of biology at
Ruhr-University.
Why are predator attacks statistically small — though not negligible? The predator normally does not strike the mobbers because
maneuvering mid air costs energy, and does not benefit him sufficiently. In this case, the
eagle made a kill. He gained a meal. But most little birds are far
more maneuverable than the big one, and are in a state of full alert.
Furthermore, as Curio points out, mobbers are careful, which minimizes
big-bird whacks. The molesters generally
mob or distract the predator to protect their young, and, therefore, cannot
afford to take great risks.
Moreover, the little ones don't usually hit the big bird, especially outside
of the breeding season, and,
therefore, don't threaten it. Ornithologist
Millicent Ficken
recalls only
one such attack in thirty years; she is a professor emeritus in biological
sciences at the University
of Wisconsin-Milwaukee. A group of Steller's jays mobbed a hawk, confusing
him; one struck the
hawk on its back. The hawk fell to the ground, but stood up, then flew off.
But "birds of most species mob much more aggressively during the breeding
season," emails
D. H. Shedd, Thoresen Professor of Biology at Randolph College
in Lynchburg, Virginia. During the breeding season many bird species
strike a predator, especially an owl (less risky than hitting hawks and the
like). "I've often seen chickadees, robins and blue jays strike an owl
during the breeding season." Outside the breeding season, it is less
common, because "if the young aren't around to benefit, the risk of approaching
a predator that closely is too high."
So, a predator does strike back — even killing — but usually saves
energy by escaping the mob.
Further Reading
Predator harassment implies a real deadly risk: a reply to Hennessy,
Eberhard Curio and K. Regelmann (1986). _Ethology, 72, 75-78_.
Golden Eagle captures red-shouldered hawk by Maurice Broun, 1947. Auk
64: 317-318.
Rarely do predators strike the object of their wrath, National Wildlife,
Feb-March 2000
Bald eagle kills crow chasing a hawk by Bruce D. Ostrow, the Wilson Journal
of Ornithology, 12/01/2006
Kruuk, H. 1964. Predators and
Anti-Predators of the Black-headed Gull (Larus ridibundus L.). Leiden,
The Netherlands: E.J. Brill.
(Answered Feb. 11, 2008)
Readers' Answers
- Big birds usually do not have the same aerial
maneuverability as smaller, agile birds simply due to the laws of physics
involved in size and flight. Hence big birds cannot always reply to nuisance
birds with great efficiency unless they are well experienced or in larger
numbers (like the Harris hawks that are social birds of prey and hunt in
numbers — but they are a rarity or like a murder of crows).
Small birds also tend to fly above a larger bird and peck at the series of
muscles that conduct the wing motion while in flight. That peck often disturbs
the muscles from functioning in the larger bird's back and such disturbance
interrupts flight (and it most probably hurts). You'll often see these birds
drop a little in flight before regaining control. In a series of successive
pecks this attack can be quite annoying to flight stability and it seems
difficult for a larger bird to recover.
Fleeing is more reasonable than expending unnecessary energy thwarting a small
pest that is simply more versatile in air. Plus, juvenile/young large birds
are often inexperienced in dealing with smaller birds who are viciously
determined to protect their nest and/or territory. Older large birds often
have learned to use such flight techniques as barrel rolls to flash their
talons to warn smaller birds they are a bigger threat and mean business. I've seen
many golden eagles and red tailed hawks use this technique to
get crows and ravens off their backs.
Adriana C. Croghan, New York, USA
- Larger birds, particularly individuals of predatory or quasi-predatory
species such as eagles, hawks and falcons, depend upon having distance between
themselves and their prey when commencing an attack. They are not close-range
'in-fighters'. By approaching the larger bird and attacking repeatedly while
remaining close, the smaller bird negates the larger bird's instinctive
offense strategy and often forces it to flee in confusion. And, of course, the
larger bird definitely feels threatened.
Someone, World
- Smaller birds are much more maneuverable than the larger birds they are
attacking. The smaller birds can cause appreciable damage to larger birds by
pecking them in the head from behind. During the period of nesting and feeding
young, smaller birds attack larger birds to drive them from their territories.
This often starts a daisy chain reaction.
I once saw a hawk chased by a crow that was chased by a grackle that was chased
by a blackbird that was chased by a kingbird that was chased by a sparrow that
was chased by a canary. The whole stream of birds moved like an undulating
snake in the sky. Once nestlings are safely fledged these same birds will fly
right by their former protagonists without any concern.
Russ Schnell, Boulder, Colorado, USA
- My best guess, from having seen small birds harassing
eagles and turkey buzzards, is small birds can deftly outmaneuver the large
bird in midair. Kinda like the
difference between a Caddy and a Maserati.
G. A. Buehner, Narrowsburg, New York, USA
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