Incubation
by Stan Tekiela
© NatureSmart
May 12, 2006
In the past three columns
we have look at and explored many aspects of
bird nests and bird eggs. Now in part four I
want to look at the next logical topic--incubation.
All birds lay external eggs that need to be
kept warm throughout their development. This
process is called incubation. It may not be
an easy process but it is a process that has
stood the test of time and seems to work well
for the avian world.
One interesting aspect of incubation is why
at least one or two species of bird have not
evolved to give birth to live young. As I mentioned
last time, there are a couple mammal species
which lay and incubate external eggs, so it
reasonable to think that at least one species
of bird may give birth to live young. Snakes
and other reptiles, which reproduce mainly with
external eggs, have many species that give birth
to live young. So why not birds?
Apparently the advantages of laying eggs have
out-weighed the advantages of reproducing with
live young. For one thing, with eggs, both parents
(or at least in some species) can help incubate
or at least keep the eggs warm, thus reducing
the burden of incubation. In addition reproducing
with eggs would allow the parents to abandon
their eggs in the face of deadly danger such
as floods, fire, or predators. This would allow
the parents to survive and reproduce another
brood in another location at a later date.
The temperature needed for the development
of the embryo within the egg varies across the
species. In general, the temperature for incubation
in most birds runs between 95 and 100 degrees
F. Since air temperatures rarely remain a constant
at these levels in the northern states addition
heat needs to be provided by the incubating
adult. There are some birds such as the Wilson’s
Plover which nests near the Gulf of Mexico or
the Black-necked Stilts in the Salton Sea of
Southern California actually need to cool their
eggs during the day but that is another story.
Incubating eggs seems like a simple enough
process. You just sit on the eggs to keep them
warm. Right? Wrong. I think we all know the
fabulous insulating properties of feathers--down
feathers in particular. Down feathers are the
small fluffy inner feathers of a bird that you
usually cannot see. If you have ever seen a
bird during a snow storm you may have noticed
that the snow flakes pile up on the bird without
melting. This is another testament to the insulating
values of feathers. No body heat is lost through
the feathers. So will just sitting on the eggs
be enough to transfer enough body heat from
the parent to the eggs? NO!
A few days before the female (and in some
species the male) begins incubating her clutch
of eggs her body undergoes a great transformation.
The small down feathers on the breast and belly
begin to fall out. In most species of duck and
goose, the mother pulls or plucks out the down
feathers in this region creating an incubation
patch (also called a brood patch). In addition
to the loss of down feathers the skin becomes
swollen through the retention of water in the
tissues and more importantly the expansion of
the blood vessels that feed blood to the skin.
The brood patch takes up much of the area
of the belly and breast but is actually concealed
by the contour feathers which cover the bird.
In other words, the larger, often colored feathers
that cover a bird’s body hide the bare
skin beneath where the down feathers once were.
So the inner layer of a birds two layers of
feathers has been removed to expose the brood
patch.
To incubate a bird spreads apart the contour
feathers to expose its brood patch beneath.
This allows direct contact to the eggs with
the mother’s body thus allowing direct
transfer of body heat. Most birds have a body
core temperature somewhere between 98-105 F
which allows the mother to rapidly warm her
eggs and maintain the warm temperatures over
a long period of time.
How long a bird must sit on there eggs will
be the subject next time. Until then…
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