“Bird remains in a cat’s guts don’t mean that it was
hunting and catching birds in trees.” — JERRY D. HARRIS
Mere fear shrinks bird families
By Susan Milius
Nothing to fear but fear itself can actually be dangerous for nesting birds.
Song sparrows protected from attack
but subjected to recordings of preda-
tor yowls and leaf-crunching noises
raised 40 percent fewer offspring in a
year compared with neighbors hearing
innocuous broadcasts, says population
ecologist Liana Zanette of
the University of Western
Working on small, uninhabited
islands off the western coast of Can-
ada, Zanette and her colleagues set
out cameras to identify which preda-
tors were feasting on the vulnerable,
open-cup nests of song sparrows there.
Researchers then devised obstacles to
those predators, fencing 24 nest sites
to keep away raccoons and building lit-
tle teepees swathed in netting to keep
Ontario in Canada. Preda-
tors do not need to kill a
single creature to have a big
effect, she says.
out ravens and owls while
allowing tiny sparrows to
dart through.
Scary noises, broadcast where the sparrows
nested in the wild, took a
toll throughout the breeding season, Zanette and her
colleagues report December
Predator sounds
spook song sparrow
parents so much that
they slack off in feed-
ing their nestlings.
9 in Science. The alarmed
sparrows laid fewer eggs to
begin with, and the parents proved so
skittish and cautious that they reared
a smaller percentage of hatchlings than
neighbors did.
Biologists have tended to focus on the
direct effects of predators killing prey,
says evolutionary ecologist Thomas
Martin of the U.S. Geological Survey in
Missoula, Mont., who was not part of
the sparrow research. This new study,
he says, suggests theorists have underestimated the impact of predators.
For half the birds, researchers played recordings of some predator
sound every few minutes
in random order around
the clock for four days at
a time, allowing four-day
sound vacations between
broadcast stints to keep
sparrows from getting
used to the recordings. The
sounds stretched over a whole breeding
season of raising two batches of young
per family. The other half of the sparrows heard unalarming broadcasts,
such as recorded goose honks and loon
wails.
Before this, no one had tracked effects
of predator sounds during an entire bird
breeding season, Martin says. And, he
adds, the study neatly isolates the effects
of predator risk.
“Predators shape everything,”
Zanette says. Wolves that eat elk give
more plants a chance to survive, which
in turn changes which other creatures
thrive.
What ornithologist Sönke Eggers
of the University of Agricultural Sciences in Uppsala, Sweden, would like to
know now is whether those chicks lost to
indirect effects would have been eaten
anyway in the real world.
Previous work, including Zanette’s,
suggested that fear of predators could
affect the number or size of eggs birds
lay. Yet separating the effects of fear
from those of actual predator attacks
took years of preparation, she says.
Zanette predicts that fear does take
a toll on real-world nests beyond direct
attacks from predators. A predator’s
noises can scare every little bird within
earshot even when the predator doesn’t
catch them all. s
Feathered dino
feasted on birds
A fossil Microraptor gui from northeastern China—still a dinosaur
despite winglike feathers on all four
limbs—has bird bones in its abdomen, report Jingmai O’Connor and her
colleagues at the Chinese Academy
of Sciences in Beijing. The position of
the bird feet and partial wing suggest
the dinosaur swallowed a now-extinct,
tree-perching bird whole, the researchers contend in the Dec. 6 Proceedings
of the National Academy of Sciences.
They propose that Microraptor (shown
dining in illustration above) frequented
trees and hunted deftly enough to
snag what was probably an adult bird.
Possible, but not the only possibility,
cautions paleontologist Jerry D. Harris
of Dixie State College in St. George,
Utah. Structure analyses of Microraptor
and its relatives suggest ground-based hunting. Today’s cats certainly
get into trees, and perching birds visit
the ground, Harris says, but “bird
remains in a cat’s guts don’t mean
that it was hunting and catching birds
in trees.” — Susan Milius
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January 14, 2012 | SCIENCE NEWS | 13
