SPRING 2022 | THE SCIENTIST 19DAVID RANKIN JUAN LUIS CELIS-DIEZ
containing sugar water laced with one of
three chemicals: cuticular hydrocarbons
found on honey bees’ exoskeletons; for-
mic acid, a defense chemical released by
Formica ants; and (Z)-9-hexadecenal, the
ant aggregation pheromone they’d used
in the bee study. The birds avoided feed-
ers that smelled of the latter two, but not
those that smelled of honey bees or a com-
mon food additive (Behav Ecol Sociobiol,
75:132, 2021). “For specific contexts such
as danger, they are responding to odors
that are associated with... chemicals pro-
duced by insects,” says Wilson-Rankin.
“They might not have many [olfactory]
neurons, but they use those neurons to
avoid dangerous [situations].”
Other birds appear to use smell to
find food. In a study published the same
month as Wilson-Rankin’s hummingbird
experiments, conservation ecologist Juan
Luis Celis-Diez of the school of agricul-
ture at Pontificia Universidad Católica de
Valparaíso in Chile and colleagues found
that artificial prey worms placed on tree
branches were more likely to be attacked
by birds if dispensers of the aggregation
pheromone of carpenter worm (Chileco-
madia valdiviana) larvae were placed
among them (Biol Lett, 17: 20210360,
2021). According to Celis-Diez, the experi-
ment was inspired by a study in Spain from
a few years ago that used a similar design.
That study found that insectivorous birds
were attracted to a pheromone that female
winter moths (Operophtera brumata) use
to attract mates (PLOS ONE, 13:e0190415,
2018). “Smell in birds is more common
than we thought before,” Celis-Diez tells
The Scientist.
While many avian-focused scientific
manuscripts rehash the outdated belief
that birds can’t smell, or can’t smell
much, evidence to the contrary has been
building for some time, says Andrew
Iwaniuk, a neurobiologist at the Univer-
sity of Lethbridge in Canada. In 1960,
Johns Hopkins University’s Betsy Bang
looked at the relative size of the olfactory
bulbs in birds and found that some spe-
cies—in particular, vultures and a few
others—did have quite sizable structures.
More than half a century later, Iwaniukand his colleagues found that the turkey
vulture (Cathartes aura) has perhaps the
biggest olfactory bulb of all birds, along
with a high number of olfactory neurons
that link the nasal cavity’s smell recep-
tors with the brain (Sci Rep, 7:17408,
2017), features indicative of a particu-
larly sensitive sense of smell.
Alongside work on the anatomical basis
of smell in birds were functional studies
showing that vultures, and later wander-
ing albatrosses and other seabirds, can
pick up the scent of food—carrion in the
case of vultures, and chemicals released
by plankton on the ocean’s surface in the
case of the wandering albatross. Decades of
work starting in the 1960s has also demon-
strated the importance of smell for homing
pigeons’ navigation. Iwaniuk says there’s no
arguing that the scientific consensus hasshifted, and that scientists should adjust
their manuscript introductions accord-
ingly: instead of a statement about how
birds have traditionally been considered
anosmic, “just lead with, ‘We know that
birds can smell.’”
Still, it’s only recently that researchers
have begun to study smell in a much wider
range of bird species for a broad spectrum
of functions, including mate selection,
subspecies identification, and navigation.
“Now we’re in almost a renaissance, if you
will, in studying olfaction in birds,” Iwan-
iuk notes. “There’s more and more peopleSMELLY WORMS: Artificial prey worms suffered
more bird pecks when they were placed near a
dispenser (left side of image) of the aggregation
pheromone of carpenter worm (Chilecomadia
valdiviana) larvae.