At the Project:
Egg laying is almost over.  Most clutches are complete.  When you check boxes open
them just a crack to make sure certain no adult is inside.  If there is, gently close the
door, then move away and wait for the bird to exit.  This may take awhile so you
might want to check elsewhere, then return.  Never force an adult off its nest.



















Concepts:
A freshly fertilized egg does not have a tiny preformed bird inside that simply has to
grow larger.  And it does not contain a master plan or blueprint.  Instead, a fertilized
bird egg starts as one undifferentiated cell with the genetic instructions encoded in
DNA for building a bird, plus sources of fuel and raw materials to build it with.  But if
you took a fertilized egg home and put it in a cup on a shelf what would happen?

The bird embryo would not develop.  The egg would die.

An egg needs more than a plan, fuel, and raw materials in order to grow from a
single cell into the young creature we will see at hatching.
  • Embryonic growth and development is a fast-paced chemical process.






  • Embryonic cells must divide rapidly, migrate around, differentiate into more
    specialized cells, and form tissues and organs.






What starts up these chemical processes of embryonic growth and then keeps
development going?
  • Chemical reactions won't start or proceed to completion unless the ingredients
    are warm enough.  Embryonic development requires heat.
  • In Tree Swallow eggs embryonic development won't even begin unless egg
    temperatures are over about 26 degrees C (80 degrees F).
  • And for optimum embryonic development bird eggs must reach temperatures of
    about 38 degrees C (100 degrees F).  This is much higher than normal spring air
    temperatures.
  • But embryos within eggs can't generate or control their own heat.
  • So adult Tree Swallows must add heat to get their eggs to develop.
  • And once development has started egg temperatures must be kept within a
    certain range; too high or too low can kill the embryo.

How do Tree Swallows add heat to their eggs?
  • Females cover eggs with their bodies, conducting heat from themselves to the
    eggs and keeping them warm enough long enough for the embryos to grow.
  • This type of heat transfer and temperature regulation is called "incubation".
  • However, females still need to find food and maintain their own bodies, so
    having to incubate eggs places an extra energy demand on females.
  • During the time females are away the eggs cool down.  They must be reheated
    upon the females' return before embryonic development can begin again.
  • So incubating female Tree Swallows must strike a balance between their own
    self-maintenance and the growth of the embryos within their eggs.

How does nest design help heat transfer from female to egg?
  • The cup confines eggs in a tight group allowing good body coverage for
    efficient heat transfer and low heat loss.
  • Nest feathers may provide some insulation when eggs are not covered by
    females.

What happens if eggs are knocked out of the cup or pushed down into the nest
material?
  • Some songbirds retrieve dislodged eggs if they are within reach.  
  • Other species, including Tree Swallows, ignore partially buried eggs or eggs
    displaced out of the cup, even if in plain sight.  These eggs will die.

Don't male Tree Swallows incubate?
  • No, as in many songbirds, only female Tree Swallows incubate.
  • Females develop special temporary structures called brood patches for transfer
    of heat to eggs.
  • Hormones cause a large area of breast and belly skin to lose its feathers.  This
    skin becomes loose and swollen through water retention and expansion of
    blood vessels.  
  • As she settles onto her eggs an incubating female presses her bare brood patch
    skin firmly onto them, creating an efficient contact for transfer of her body
    heat to the eggs.
  • The brood patch below is typical of Tree Swallows.














When (after which egg is laid) do Tree Swallow females start incubating?
  • You'll get the answer when the eggs finally hatch.
  • If incubation starts with the first egg, they will hatch one per day over several
    days.
  • If incubation doesn't start until after all eggs are laid, they should all hatch the
    same day.
  • Incubation could also start somewhere in between these extremes.

How can you tell incubation has started?
There are behavioral cues.  For example:
  • Females begin staying inside boxes many minutes.  Try timing some.  
  • Incubating females develop a rhythm.  Periods on the eggs alternate with
    periods off.  They feed quickly and then return to the eggs.
  • In songbirds like Tree Swallows, where only females incubate, females average
    75-80% of daylight hours on their eggs.
  • If weather is cold or rainy females incubate even longer, because eggs need
    extra heat to maintain temperatures necessary for embryonic development.

Here's another way to tell if incubation has started:
  • Touch an egg from each clutch to your lips or cheek.  If it's cool, it isn't being
    incubated.  If warm, incubation has started.







How long does incubation last?  When will eggs hatch?
  • Tree Swallow eggs need about 14 days of incubation for embryo growth and
    development to reach the hatching stage.

Songbird eggs lose about 20% of their weight between laying and hatching.  Why?
  • Some weight is lost as fuel is burned during embryonic metabolic processes.
  • Some water evaporates through shell membranes and shell, and is replaced by
    air from outside.
  • The air space in the egg get larger as incubation proceeds.

How do fats, proteins, minerals and water get from the yolk and albumen to the
growing embryo?
  • A network of blood vessels (arteries, veins, and capillaries) grow out from the
    developing embryo, and bring back the nutrients necessary for development.
  • Click here to view a photo of a fertile Tree Swallow egg being "candled".  This
    egg has been incubated a few days and clearly shows the web of blood vessels
    extending out from the small embryo on the surface of the yolk.

How does an embryo "breathe" within its shell?
  • Songbird eggshells are not completely solid.  The shells have microscopic pores
    through which oxygen and carbon dioxide gases can pass in and out.
  • Also, as an embryo grows a special membrane, the "CAM" or "Chorioallantoic
    Membrane", develops just under the inner shell membrane.  
  • The CAM has lots of capillaries where blood can absorb airborne oxygen that
    has passed in through the shell pores.
  • Carbon dioxide waste produced by embryonic metabolism can be also passed in
    the other direction, from the CAM's blood out through the shell pores.

How does a growing embryo get rid of solid wastes it produces?
  • The embryo produces a membranous sac, called the "Allantois", that balloons
    out from the embryo's gut.
  • As the embryo grows solid wastes that can't pass through the shell are diverted
    into this sac, where they accumulate.
  • Just before hatching the wastes in the allantois dry up.

Incubating songbirds turn eggs over periodically with their bills.  How is this done
and why?
  • When turning its eggs an incubating bird rises up on its legs, arches its neck so
    its head looks down, then draws its bill backwards through the eggs.  This
    motion rearranges the eggs so those out on the outside are brought into the
    middle.
  • Regular turning ensures that all eggs in the clutch are incubated evenly and
    that all surfaces of each egg receive equal warmth.
  • Turning also moves an egg's contents, keeping membranes and embryo from
    sticking to interior shell surfaces, which could cause embryo death.
  • Songbirds may turn their eggs as often as seven to eight times an hour.

What happens to the eggs at night?
  • Before incubation neither male or female swallows stay in boxes at night unless
    it's extremely cold.  They usually roost in nearby trees, shrubs or marsh
    vegetation.
  • However, once incubation starts females stay in boxes overnight.
  • Staying in cavities overnight is dangerous.  Females can be trapped in boxes
    that lack predator guards by nocturnal predators like raccoons, possums, or
    cats.

Female Tree Swallows are very busy now incubating and foraging.  It seems they do
all the work.  What do male Tree Swallows do during  incubation?  Are males and
females together often at boxes now?
  • During incubation it's typical to see pair members relieving each other at the
    nests.  Males guard boxes while females leave to feed.
  • Although males don't incubate, they may perch at holes and peek in, or enter,
    perhaps to "inspect" things.
  • More often males perch on boxes or poles, preening and giving occasional Up
    and Down Songs to passing swallows.  Chatter Calls aren't heard much now,
    except if floater females approach and males want to attract them.
  • Nest females usually fly directly to the hole and duck right into boxes.
  • Returning swallows often give Gurgles (also called Contact Calls).  At this signal
    the other normally leaves, so a pair are seldom together.
  • Females who have finished an interval of incubating, but whose males haven't
    returned yet, may perch in entrances waiting to be relieved.











Questions for the next Topic:  Takeovers.
  • Why are "floater" Tree Swallows still intruding at established nests?
  • Why are incubating females especially vulnerable to having their nests taken
    over by other females?









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Incubation
Learn About Birds at Tree Swallow Nest Box Projects