Hints from these experiments and others also suggest that
nest herbs affect the internal development of the eggs before lay-
ing, particularly the concentration of androgens in each yolk. In
a previous study, Gwinner and her colleagues collected and cal-
culated the quantity of herbs male birds had added to the nests
and then measured yolk testosterone. “We found a positive cor-
relation of yolk testosterone and the amount of green nest mate-
rial,” Gwinner says, explaining that the presence of herbs in the
nest may somehow induce females to increase yolk testosterone
concentration in their eggs during development. Higher concen-
trations of testosterone, as well as increased temperatures, could
have influenced nestling growth in the experiment.
Rubalcaba, who was not involved in the study, notes that
adding aromatic green plants is clearly good for the offspring.
Gwinner’s finding of improved egg incubation might not be
the only factor to boost the health of the baby birds in herbed
nests, he says.
It’s possible that herbs might also protect the immune sys-
tems of the parent birds, notes Michel Raymond, an evolution-
ary biologist at the Institute of Evolutionary Sciences in Mont-
pellier, France, who has previously studied birds’ herb use but
was not involved in this study. Although Gwinner’s team had
earlier found no evidence that the herbs affected parasitic load,
the authors note in the current paper that the greater heating
of eggs in herbed nests could prevent bacteria from growing
on the parents, keeping them healthier. If so, “the energy saved
(the immune system is costly) could be invested parentally,”
Raymond writes in an email to The Scientist. If the parents are
healthier, they can invest more energy in caring for the chicks,
making them fitter in the long run.
—Ashley Yeager
Ratted Out
When working on archaeological digs in the Gambier Islands of
French Polynesia, zooarchaeologist Jillian Swift has her attention
trained on the ground. Nevertheless, she’s often struck by a strange
lack of activity overhead: “It’s kind of eerie, where there’s just not
a single bird in the s k y.”
This avian silence fell over some of the places where Swift
works—such as several small islands clustered near Mangareva,
the central island of the Gambiers—centuries ago, after humans
and their attendant rats arrived, often making quick meals of
local birds’ eggs and young. But now, the bones of these wayfar-
ing rodents and their descendants offer researchers such as Swift,
who’s currently based at the Max Planck Institute for the Science
of Human History in Germany, a unique insight into the history
of human settlements. That’s because the composition of those
bones—specifically, the ratios of isotopes of elements such as car-
bon and nitrogen—are “really sensitive to environmental change,”
she says. Using those ratios to make inferences about what the
animals would have been eating when they were alive, researchers
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