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CHAPEL HILL - Competing against older brothers and sisters can be tough work, as any youngest child will tell you.
new research from a biologist at the University of North Carolina at
Chapel Hill shows that when it comes to some birds, you should reserve
any underdog sympathies for the first born - or rather, first laid -
siblings as well.
The finding, published in the March 12 issue of PLoS ONE, runs somewhat
counter to common wisdom, which holds that baby birds that are laid
before their brood mates have a better chance of surviving long enough
to leave the nest.
But after studying a population of Lincoln's
sparrows in a remote stretch of Colorado's San Juan Mountains, Keith
Sockman, an assistant biology professor in UNC's College of Arts and
Sciences, has discovered that first-laid eggs are, in fact, the least
likely to hatch at all.
"I believe this is the first study to
follow siblings from laying through fledging and demonstrate that the
effect of laying order on hatching is very different from its effect
post-hatching," said Sockman.
It is a well-documented fact that
being born just a day or so later often sets the stage for a situation
whereby youngest hatchlings die. That's because they're too small to
compete against their feistier brood-mates for the limited resources
provided by their parents. Such competitive disparities caused by
hatching or birth order can be found in other animals - from beetles to
marsupials to humans - which sometimes produce their young in series,
then rear the resulting offspring simultaneously.
But Sockman says up until now, such observations have usually failed to take account of what happens to eggs before they hatch.
Lincoln's sparrows lay one egg per day, usually producing three to five
eggs in total. While carefully observing and tracking the tiny birds
for three breeding seasons, Sockman and his team of researchers noticed
that typically, mothers do not settle down and start incubating the
eggs right away, since they still have other concerns during the laying
cycle, such as foraging for food.
Sockman believes this
contributes to the lower probability that first-laid eggs will hatch at
all - but also helps to ensure that overall, a greater number of
reasonably healthy, strong and feisty chicks hatch and go on to develop
into young birds.
"At these elevations, conditions can be fairly
harsh even during the summer when Lincoln's sparrows breed," said
Sockman. "It's often freezing at night, which is hard on an
un-incubated egg, while daytime temperatures are warm enough to foster
the growth of harmful microbes. As a result, since the mother sparrow
isn't keeping them at the most optimal incubating temperature from day
one, first-laid eggs can be exposed to environmental conditions that
lower the chance those embryos will ever see the world outside their
"If the female did start incubating all her eggs as soon
as she laid them, it would increase the probability they'd all hatch.
But it would also give a huge head start to those first-laid eggs and
the chicks that emerge from them, putting their younger siblings at
even more of a competitive disadvantage once they begin battling for
food and their mother's attention," said Sockman. "It may also reduce
the number of eggs she is capable of laying."
careful balancing of this trade-off enables her to end up with three or
four relatively equally robust offspring, instead of one or two strong
hatchlings and several "runts of the litter," said Sockman.
now intends to examine what, if any repercussions laying order has once
young birds reach adulthood. "The severely competitive environment in
the nest may have consequences on the individual's ability to compete
for resources and mates the following year when it is reproductively
mature," said Sockman. University of North Carolina at Chapel Hill. March 2008.
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