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ACCORDING to Navajo legend, North American Indians learned of the medicinal
powers of osha roots from brown bears. We now know that bears do indeed seek out
these roots, which they chew to a frothy pulp, before grooming their pelts with
it. The concoction, it seems, may help them keep parasites at bay.

Nature is full of herbalists. Indian elephants and wild boars both eat plant
roots that ward off parasitic worms. In Panama, coatis鈥攔elatives of the
raccoon鈥攇room themselves and each other several times a day with sap from
the Trattinnickia tree. Local Amerindians also recognise the medicinal value of
these menthol-scented trees. Meanwhile, Western pharmacologists are tapping into
the medical know-how of chimpanzees and gorillas in the hunt for new drugs. But
recent research suggests that the title of nature鈥檚 apothecary could go to the
humble bird.

鈥淏irds are flying Petri dishes, teaming with microbes inside and out,鈥� says
Dale Clayton, a parasite ecologist from the University of Utah. His interest in
how birds combat invaders has helped reveal how skilled they are at plundering
nature鈥檚 medicine chest. Many species fumigate their nests with plants to boost
the health of their offspring and prolong their survival. And birds commonly
groom their feathers with ants so they can benefit from the insects鈥� antibiotic
secretions. But perhaps the most bizarre therapy of all belongs to the owls that
nest with a live snake to keep parasites in check.

It has long been known that some birds add fresh green foliage to their
nests. One idea is that greenery helps keep the nest very humid, another is that
it shades chicks from direct sunlight. But both theories have their problems.
鈥淭he humidity-maintenance theory does not explain the fact that the behaviour is
as common in rainforests and maritime climates as it is in dry ones,鈥� says Peter
Wimberger from the University of Puget Sound in Washington state. He also points
out that you find greenery in shady nests beneath the forest canopy and in
spherical nests, where chicks are protected from the Sun.

Indeed, over the years it has become clear that birds nesting in well-shaded,
high-humidity tree cavities or covered nests use green foliage more often than
open-cup nesters. South American monk parakeets, for example, are rainforest
birds that spend all year in massive colonial nests resembling ramshackle
apartment blocks. Each nest is covered, and they are stacked one upon another,
sometimes 15 storeys tall. Monk parakeets bring green leaves to their nests from
the surrounding forest鈥攂ut only during the breeding season.

Back in the mid-1980s, Wimberger noticed that birds of prey also use greenery
at very specific times. They regularly replenish the sprigs in their nests while
their eggs are incubating and shortly after hatching. He realised that offspring
would be most vulnerable to parasites at this time, and wondered whether parents
might be taking advantage of the volatile defensive chemicals that some plants
have evolved to combat leaf infections and evade herbivores. It was the first
hint that greenery might be there to kill the bird鈥檚 own parasites.

Nests are a breeding ground for disease. Together with microorganisms, there
are bloodsucking parasites that leach the calories chicks need to thrive, and
that reduce the amount of oxygen-carrying haemoglobin in their blood. 鈥淭he
larvae are large relative to their hosts,鈥� Wimberger says. 鈥淚t does not take
many of them to weaken their host significantly.鈥� And some, including
hippoboscid fly larvae, overwinter in nests and emerge just as the eggs are
hatching鈥攚hen birds step up their efforts at collecting greenery.

Wimberger reasoned that if his medication idea was correct, the birds most
vulnerable to parasites would also be most likely to resort to herbal remedies.
Some birds鈥攆or example, those nesting in relatively rare tree cavities
made by other animals鈥攎ust reuse nests from a previous year. Other birds,
including many raptors, simply choose to use the same nest again and again. The
risk of infestation from parasites that can kill whole broods is much higher in
these nests, and so these birds would need to evolve more sophisticated ways to
fight disease than the ones building pristine nests from scratch each
season.

Nest freshener

To see whether bird species that reused nests were more likely to bring home
foliage, Wimberger reviewed 49 species of North American and European birds of
prey. What he found confirmed his suspicions. Nearly 80 per cent of
nest-reusers, including bald eagles, northern goshawks and common buzzards,
ferried fresh greens to their nests. Of those birds that build new nests each
year, such as the tawny owl and dark chanting goshawk, barely 40 per cent use
green leaves.

Since Wimberger鈥檚 ground-breaking study, other research has confirmed the
link between foliage use and nest reuse in songbirds. But if such behaviour
really does constitute herbal fumigation, the leaves brought to the nest ought
to be rich in volatile compounds that harm parasites. And this is what
researchers are finding. In India, for example, house sparrows nest throughout
the breeding season with leaves from the margosa or neem tree. Margosa leaves
contain sitosterol, which is a natural insect repellent and also disrupts
egg-laying in ticks and other blood-sucking parasites.

Another well documented example is the European starling. Larry Clark of the
Monell Chemical Senses Center in Philadelphia, and Russell Mason from Utah State
University, found that starlings are very choosy about the green leaves for
their nests. They shun ubiquitous plants like garlic mustard and instead opt for
locally rare species including wild carrot greens, agrimony, yarrow and
fleabane. Their preferred plants, it turns out, stop ticks, mites and lice from
hatching鈥攁nd kill disease-causing bacteria and fungi. Clark and Mason
discovered that they possess both a greater diversity and higher concentrations
of defensive volatile compounds than the plants starlings ignore on their
foliage-gathering trips.

Starlings reuse their nests and the pattern of leaf collecting is reminiscent
of spring cleaning. Courting males interweave their nests with greenery, which
they like to show off to prospective partners by flitting about with a sprig in
their beak. Females are less interested in leaf collection. And males cease
bringing greens to the nest once the eggs appear. Clark and Mason also found
that chicks from nests fumigated by wild carrot greens had more oxygen-carrying
haemoglobin in their blood than chicks reared in unprotected nests, suggesting
that the leaves were effective at killing off resident bloodsuckers.
Surprisingly, though, there was no significant weight differences between the
two groups.

Sexy sprigs

Not all researchers are convinced by this interpretation. Peter Fauth from
the University of Maryland suspects that male starlings attract a mate with
displays of greenery. This would explain why they lose interest in bringing
fresh sprigs once the eggs are produced. What鈥檚 more, when he took away the
green leaves, mortality rates in chicks did not rise. However, the team washed
their nest boxes with chlorine bleach just before the experiment, to prevent
鈥渆ctoparasite contamination鈥� from the previous year, which could explain why
Fauth鈥檚 team found no benefits for chicks reared in nests containing fresh
leaves.

Even so, Fauth is not the only one to see greenery as part of the courtship
ritual. Helga Gwinner of the Max Planck Institute in Andechs, Germany, found
that males who pile up the greenery in their nests are also more likely to
secure more than one mate in a single season. Gwinner concludes that in
starlings, greenery is a sexually selected 鈥渙rnament鈥�. If so, then nest foliage
may follow the same mate attracting rules as peacock trains: the more, the
better. Males who don鈥檛 adorn their nests with enough greenery will be beaten in
the chase for additional mates by their more resourceful neighbours.

Of course, just because greenery attracts mates doesn鈥檛 mean it doesn鈥檛 also
fumigate the nest. Females may simply prefer mates that protect their nests well
against parasites, points out Enrique Bucher of the University of Cordoba. Even
Gwinner confirmed that starlings prefer to adorn their nests with leaves
containing natural antibiotics and insecticides.

If nests are a hotbed for disease then so, it appears, is plumage. A new
study by Edward Burtt and Jann Ichida of Ohio Wesleyan University shows that
bacteria can destroy adult plumage more quickly than anybody suspected. In test
tubes, chicken feathers inoculated with bacteria from wild birds鈥� feathers
rapidly fell to pieces. After only a few days, keratin-destroying enzymes
released by the bacteria had reduced the chicken feathers to half-millimetre
shreds. Burtt and Ichida did point out that feathers on wild birds may not be
humid enough to support the high levels of bacterial growth they saw in their
test tubes. Still, bacteria are clearly capable of degrading adult plumage.

鈥淩esearchers have known for decades that the plumage harbours a diverse
community of bacteria and fungi,鈥� says Clayton. 鈥淯nfortunately, the influence of
these creatures on the birds themselves has received little attention.鈥� He and
others think that鈥檚 a mistake, because parasites are a major force in evolution.
And with feather-destroying bacteria, there is plenty at stake. Such bacteria
make flight less efficient and disrupt feathers鈥� capacity for insulation,
creating thermoregulatory stress. Clayton believes that adult preening,
dust-bathing and perhaps even annual moulting are evolutionary adaptations to
the survival challenges posed by microbial feather-busters. 鈥淒usting and sunning
may play a role in microbial defence by making the plumage too dry to support
bacteria,鈥� he speculates.

Army of cleaners

As well as basic hygiene habits like dusting, birds may also tap into
nature鈥檚 medicine chest. The unlikely verb 鈥渁nting鈥� describes how birds often
pick up ants and obsessively groom their feathers with them. 鈥淎nting,鈥� says
Clayton, 鈥渕ay reduce bacteria by allowing birds to acquire antibiotic secretions
from the metapleural glands of ants.鈥� Metapleural glands produce chemically
complex secretions that contain compounds such as auxin and beta-hydroxy fatty
acid. Several of these chemical components could well kill fungi and bacteria,
so anting may help birds fight off infections. There also appears to be a
positive correlation between anting and conditions of high humidity鈥攊n
which bacteria thrive.

Strangely enough, anting need not necessarily involve ants. Birds have been
seen anting with millipedes, marigold flowers and the tobacco in cigarette
butts. Grackles and starlings even anoint themselves with gardener鈥檚
mothballs鈥攎eant to repel caterpillars from the vegetables. All these
contain antibacterial agents. Marigold petals not only slow bacterial growth,
but contain sitosterol, the same chemical found in the margosa leaves used by
house sparrows in India.

Clayton once watched a grackle anting with pieces of a discarded lime fruit
in a city park. With Jennifer Vernon鈥攁 colleague when they were both at
the University of Oxford鈥攈e discovered that lime, too, fights parasites.
Lice taken from doves and kept overnight in a dish with a small piece of lime
died in large numbers, whereas almost all the lice in the control dish survived.
What鈥檚 more, most of the remaining lice in the experimental dish appeared to be
dying, even though they hadn鈥檛 come in contact with the lime.

鈥淎lthough bits of lime peel smeared by the bird on its feathers may kill some
lice,鈥� says Clayton, 鈥淚 suspect that it鈥檚 the vapour which is the most
significant killer. It is easy to imagine that vapour from bits of lime peel
smeared on the feathers could get trapped between the outer layers of plumage
and the skin.鈥� Birds trap air in just this way to stay warm. Clayton points out
that parasite-killing fumigants would be extremely effective if they could
permeate this insulating layer of air.

How birds select their medications is something of a mystery. There is
evidence that some species can learn from each other. This so-called social
learning is particularly advanced in birds living in large groups. There may
also be a genetic component to self-medication behaviours. Birds need not
consciously understand the benefits of bringing greenery to their nests, but as
long as the practice produces more surviving offspring, genes associated with it
will spread.

Whether the bird鈥檚 medical knowledge is learned or innate, we could still
benefit from it. Studying animal apothecaries could be a fast route to new
treatments for our own ailments. 鈥淛ust as modern medicine has benefited from the
medicinal practices of indigenous peoples, it can benefit from the medicinal
practices of other animals,鈥� says Clayton.

But there is one avian medical trick we are unlikely to emulate. Nearly a
fifth of all screech owl nests have a resident blind snake, an animal that
normally lives underground. Fred Gehlbach and Robert Baldridge from Baylor
University in Texas, found that these marooned snakes survive on a diet of
arthropod larvae, making life easier for the chicks. Could this be why parent
owls bring the snakes to their nest unharmed? Nobody鈥檚 sure. But chicks reared
in nests containing snakes do grow faster and are much more likely to survive
than those growing up without a snake in the family.