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Bird
Moms Manipulate Birth Order To Protect Sons
A mated pair of
house finches (Carpodacus mexicanus) and the mites
that infest their nests.
Credit: Copyright 2005 Alex
Badyaev
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By Mari N. Jensen
Protecting her kids from
peril is the job of every good mom.
When marauding mites
turn up in a house finch's nest, she shelters her sons from the
blood-suckers by laying male eggs later than those containing
their sturdier sisters, according to new research.
Making
sure the vulnerable baby boys are exposed to mites for a shorter
period allows both the sons and the daughters to survive long
enough to leave the nest.
"Sons are more sensitive to
the mites than daughters," said Alexander V. Badyaev of The
University of Arizona in Tucson. "Mothers minimize sons'
exposure to mites by laying male eggs later than female eggs. As
a result, the males are in the nest fewer days."
Even
so, the male chicks that grow up during mite season end up just
as big as ones from the mite-free time of the year.
It's
all mom's doing, Badyaev said.
Once breeding female
finches are exposed to mites, their bodies make hormonal changes
that affect the order of egg laying and accelerates the
development of their sons while they're still in the egg.
"We've
found a mechanism by which duration of growth can be adjusted to
a changing risk of mortality," said Badyaev, a UA assistant
professor of ecology and evolutionary biology. He added that this
is the first documentation that maternal manipulation of both
ovulation and growth influences the duration of development in
birds.
Badyaev and his colleagues' article, "Sex-Biased
Maternal Effects Reduce Ectoparasite Induced Mortality in a
Passerine Bird," is scheduled to be published in the early
online edition of the Proceedings of the National Academy of
Sciences the week of Sept. 18.
His co-authors are UA
graduate students Terri L. Hamstra and Kevin P. Oh and UA
research specialist Dana A. Acevedo Seaman. The David and Lucile
Packard Foundation, the National Science Foundation and the
Silliman Memorial Research Awards funded the research.
One
of Badyaev's interests is figuring out how the various
developmental periods of birds evolve and how birds can modify
those developmental periods to maximize the survival of their
young.
There's a trade-off between keeping the kids at
home longer so they grow big and strong and getting them out of
the nest quickly because nests are targets for predators and
parasites, he said.
Since 2002, Badyaev, Oh and their
colleagues have been intensively documenting the lives of a
population of house finches (Carpodacus mexicanus) on the UA
campus.
A
newly hatched house finch (Carpodacus mexicanus) and its
not-yet-hatched siblings. The numbers on the eggs reflect
the order in which they were laid. (C) 2003 Alex Badyaev.
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Throughout the year, the
researchers capture birds several times a week to band and
measure them and to take DNA and hormone samples. During the
breeding season, the researchers locate the nests, keep track of
activity in the nest, follow nestling growth and development, and
take DNA samples from the chicks.
The researchers have
also been counting the numbers of mites on the birds and
documented a seasonal pattern. When breeding starts in February,
the mites are absent. As winter turns to spring, mites start
showing up on the adult females, in their nests and on their
nestlings. The exact timing depends on the year. Mites can kill
nestlings.
"When it is safer inside the nest than
outside, then there's no need for young to leave the nest until
growth is complete, but when mortality risk of staying in the
nest is great, chicks need to complete their growth fast and get
out as soon as they can," Badyaev said. "What should a
mother do in the face of shifting mortality risk?"
"To
leave the nests sooner and still survive outside of nests, the
kids need to grow faster," Badyaev said. "But the
mechanisms which regulate nestling growth in relation to changing
mortality were not known."
So the researchers looked
to see how finch moms changed their child-rearing strategy so as
to always do best by their kids.
The birds lay one egg per
day. To successfully raise baby finches in the presence of mites,
the mothers altered the order in which male and female eggs were
laid.
When mites were absent, the chances of any
particular egg being male or female were even. But once mites
came into the picture, the mothers laid female eggs first and
male eggs last.
Males that grew during mite season did
more of their development in the egg before hatching. Their
mothers accelerated their sons' growth, both in the egg and after
they hatched.
"Mothers essentially hid their sons in
the eggs," Badyaev said.
It's remarkable that the
fledglings have such similar morphology with or without mites, he
said. "Mothers did that by modifying the order of laying of
male and female eggs and the pattern of their growth."
Source
/ credit: Arizona University
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