At the Project:
Egg laying is almost over.  Most clutches are complete.  When you check a box, open
it just a crack, because there's a good chance incubation has begun, and there's a
female Tree Swallow inside warming her eggs with her body's heat so the embryos
inside the eggs can develop.  If you do find an adult in a box, gently close the door,
then move away and wait for the bird to exit.  This may take awhile so you might want
to check other boxes, then return.  Please don't force an adult off its nest.

A newly fertilized egg does not have a tiny preformed bird inside that simply has to
grow larger, and it does
not contain a template or blueprint to build upon.  Instead, a
fertilized bird egg starts as one undifferentiated cell with the genetic instructions
encoded in DNA for building a bird, plus 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, in order to 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).  Of course this is much higher than
    normal spring air temperatures.
  • But embryos within eggs can generate very little heat of their own.
  • So adult Tree Swallows must add enough heat to get their eggs to develop.
  • And once development has started egg temperatures must be kept within a
    certain range for proper development; 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.
  • We call this type of heat transfer and temperature regulation "incubation."
  • However, females still need to find food and maintain their own bodies, so
    having to incubate eggs is a major extra demand.  
  • In fact, researchers have found incubating Tree Swallow females burn four times
    the energy of resting, non-incubating swallows.  Not surprisingly, incubating
    females lose significant amounts of weight during incubation.
  • When a female is away her eggs cool down, so they must be rewarmed when
    she returns so embryonic development can continue at the optimum rate.
  • So, although incubating female Tree Swallows may appear to be relaxing, just
    sitting around on their nests, their bodies are actually working hard, and must
    strike a balance between their own self-maintenance and the growth of the
    embryos within their eggs.
  • Click here to watch one female's typical incubation behavior on YouTube, using
    video from Francois Paquette of Quebec's in-box camera.

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

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
  • In Tree Swallows the "defeathering" of the area that will become the brood patch
    begins approximately 4-6 days before the start of egg-laying.
  • 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
  • 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 for longer intervals.  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.  
  • A recent study found female Tree Swallows incubated over 40 times per day.
  • 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 probably 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.  
  • 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.
  • The photo below of a Tree Swallow egg being "candled" was taken by Noah
    Hamm of the Golondrinas de las Americas project.  This egg has been
    incubated a few days and clearly shows the web of blood vessels extending out
    from the small embryo developing 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.  It will act like a little "trash bag" inside the egg.
  • 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
  • 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 many times an hour.  Tree Swallows certainly do.

What happens to the eggs at night?
  • Male Tree Swallows do not sleep in boxes overnight unless it's extremely cold.  
    They usually roost in nearby trees, shrubs or marsh vegetation.
  • Females may begin sleeping in boxes once they start laying eggs, and when
    incubation starts females remain in boxes overnight warming eggs as a rule.
  • However, roosting in cavities is dangerous.  Females can be trapped  by
    nocturnal predators like raccoons, possums, or cats.  Equipping boxes with
    predator guards reduces the chance of this happening.

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 usually guard boxes when females leave to feed.
  • Although males don't incubate, they may perch at entrance holes and peek in,
    or enter, perhaps to "inspect" things.
  • Occasionally, males cover and turn eggs, as you can see if you click here for a
    YouTube video, but males lack the brood patch necessary for true incubation.
  • Male Tree Swallows don't ever bring food to females during nesting.
  • Males often perch on their boxes or poles, preening and giving occasional
    Songs to passing swallows.  Chatter Calls aren't heard much now, except if
    floater females approach and males want to attract them.
  • Resident females usually fly directly to the hole and duck into their boxes.
  • Returning male swallows often give Gurgles (also called Contact Calls).  At this
    signal the female normally exits and 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:  Nest Takeovers by Female Tree Swallows.
  • 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?

Incubation and
Embryonic Development
Learn About Birds at Tree Swallow Nest Box Projects