© ANDRZEJ KRAUZENOTEBOOK
What researchers haven’t been able
to pinpoint is how exactly the herbs exert
an influence over baby birds’ health. To
answer this question, Helga Gwinner of
the Max Planck Institute for Ornithol-
ogy has been studying a wild European
starling (Sturnus vulgaris) colony near
Ammersee, a lake in Germany. Her past
experiments have shown that the hatch-
lings of starlings that add herbs to their
nests have higher red blood cell counts,
stronger immune function, and fewer
bacteria than hatchlings in nonherbed
nests. Ye t she and her colleagues have also
found that herbs don’t appear to directly
reduce the number of nest parasites such
as mites, fleas, and lice—thus ruling out
one leading hypothesis for how herbs
confer benefits upon hatchlings.
To get to the bottom of the mys-
tery, Gwinner recently set up an exper-
iment to test another hypothesis: that
the herbs improve offspring health by
keeping eggs in the nest warm. Herbs
might provide direct thermal benefits
through improved insulating proper-
ties, the team reasoned, or through heat
produced by decomposition of the mate-
rial, keeping the eggs toasty while parent
birds are out of the nest.
During the 2006 and 2007 mating
seasons, Gwinner and her colleagues
attached 53 bird boxes to trees near
Ammersee, watched males build nests,
and waited for the first clutches of eggs
to be laid. Once the eggs in the nests
were ready for incubation, the research-
ers swapped out the original nests with
36 artificial ones. Half of these contained
herbs, such as hogweed, cow parsley, and
goutweed, and half were made of just dry
grass to act as controls. Gwinner’s team
also added a “dummy” egg—a disguised
temperature sensor—to each nest to
record the eggs’ temperatures for roughly
13 days, the length of the incubation
period. From the egg temperatures, the
team could infer when the starling par-
ents left the nest, when they came back,
and how long they sat on their eggs.
The dummy eggs in the herby nests
did log higher average temperatures than
those in the plain nests over the course
of the study. But temperatures fell at the
same rate in the parents’ absence, suggest-
ing that the change was not due to physi-cal insulation by the nest. Instead, the egg
temperature data indicated that parents
in herbed nests spent more time sitting
on their eggs—at least 15 minutes longer
per day compared with birds in nests lack-
ing the aromatics. They also left the nest
less often, at least early in the incubation
period, and made shorter excursions out-
side (Proc R Soc B, 285:20180376, 2018).
The results show that parental behav-
ior, not insulation, is behind the higher
temperature in nests that incorporate
herby materials. “I think that the volatiles
of the nest herbs acted as a tranquilizer,”
Gwinner says. “Parents sat more quietly
on the eggs, nest temperatures dropped
less often below critical thresholds, so
the eggs and the embryos [within them]
experienced a more-constant and higher
temperature.”I think that the volatiles of
the nest herbs acted as a
tranquilizer.
—Helga Gwinner
Max Planck Institute for Ornithology