At the Project:
Your Control Sheet shows the earliest nests have reached incubation day 13 or 14,
when eggs normally start hatching. Carefully opening these boxes you should see
newly hatched young!  Be proud.  Your careful management has permitted these little
nestlings to be here at this place.  (Photo by Dick Stauffer).

Pick up a brood of nestlings one by one and cradle them in your cupped hand.
How do you pick up a tiny nestling?  Very gently and carefully lift them one at a time
out of the nest and transfer them to your other, cupped hand.  It may be scary for you
the first few times you do it, but remember they are tough little critters and lifting won't
hurt.  Get comfortable handling nestlings.  You'll be doing it a lot.  Also get used to
being defecated on!  Don't take it personally, it's a nestling's reflex.

And don't worry if the adults fly around overhead, and scream and dive on you.  After
you've counted the young and any unhatched eggs, returned the young to the box,
and moved off to the next, the parents will settle back down again.

What happens within an egg shortly before hatching starts?
  • The embryo grows to the point where it takes up nearly all the space.
  • It positions its body so its head is at the large end of the egg next to the air
  • A few days before hatching it uses its bill to poke a hole through the
    membranes into the air space.  Now it can begin to breathe with its lungs,
    although the CAM is still the main structure for gas exchange.
  • Most remaining albumen and yolk is consumed by the embryo.

How do bird eggs hatch?
  • Bird embryos use an "egg tooth," not a real tooth but a small sharp temporary
    structure on the top of their bills, to cut through their shells from the inside.
  • With the help of a special muscle in the back of its neck an embryo rubs its egg
    tooth against the shell, which has already been thinned and weakened as the
    growing embryo absorbed calcium from it for its bones.  Continued rubbing in
    one spot cuts a small hole in the shell (see below).

  • As it continues to rub with the egg tooth the embryo rotates its body, gradually
    cutting a circular ring around the middle of the shell as it turns (see egg at
    upper left in the photo below by Elinor Schindel).
  • When the cut is complete the hatching bird pushes against the large end of the
    shell with its head and shoulders, and against the narrow end with its legs and
    feet, forcing the eggshell halves apart.
  • Finally, the hatchling works itself free of shell membranes and separates itself
    completely from the shell halves.
  • The hatching process can take small songbirds several very tiring hours.

You may find "pipped" eggs, ones in the process of hatching, when you make
box checks.
  • A "pipped" egg is one that shows a few small holes or a thin line in the shell
    where the embryo has begun cutting.

Warning!  Do not touch pipping eggs!  Damaging the shell at this time could make
it difficult or impossible for the bird to hatch.

What happens to empty shells after hatching?  Where do they go?
  • In some species, including Tree Swallows, female birds eat the shells, probably
    to regain minerals drawn from their bodies when the shell was produced.
  • Click Here to watch a female Tree Swallow crushing and swallowing eggshells
    of her newly-hatched young in a video from Francois Paquette of Quebec.
  • Parents of some other species carry shells away and discard them.

You may find that not all eggs hatch.  Why is this?  
Do ones that didn't hatch appear different?
  • Some eggs may not have been fertilized due to mating difficulties or sterility of
    one of the pair of adults.
  • In other eggs embryonic death may have occurred due to genetic defects,
    exposure to cold, or attack by germs able to penetrate shells and membranes.
  • Still other unhatched eggs may be light in weight because air has leaked in,
    drying the contents and killing the embryo.
  • Some unhatched eggs will look odd.  Note the large white air spaces and darker
    small ends of the two eggs at right in the photo below.

  • If examined under light such eggs may seem clear and watery inside, with no
    sign of an embryo.  The egg below being candled with a flashlight by Erna
    Giesbrecht of Manitoba, was one of five similar eggs incubated for almost a
    month.  None hatched.

  • If an egg has not hatched after young in a nest have reached three days old,
    we assume it is not alive and remove it.

What are young Tree Swallows like when they hatch?
  • They are tiny, only weighing between about 1.3 and 1.8 gm.
  • They are mostly naked, except for little whiffs of down on backs and heads.
  • Some internal organs and the last of the yolk are visible through the thin skin of
    the abdomen.
  • Their eyes are large but completely closed.
  • Their beaks are large with fleshy yellow edges.
  • Their wings look like thin little paddles.
  • Their legs are much bigger than their wings.

Can they move?
  • Although mostly helpless and weakened by the hatching process they can raise
    their heads on wobbly necks, open their mouths, and wave their tiny wings.

What is the purpose of these movements?
  • This is "begging," a set of behaviors the young instinctively "know" how to
    perform from the moment of hatching.  
  • Begging of songbird nestlings is usually a combination of postures, movements,
    and calls, which signal parents that the young are hungry.  
  • Begging's purpose is to stimulate the parents to bring food, because the
    highest priority for these tiny nestlings is to take in and process food.  To this
    end their digestive tracts are already well-developed, unlike most of the rest of
    their bodies.
  • For the rest of their time in the nest the nestlings' greatest and most persistent
    demand will be for food from their parents who, recognising they have young
    now, respond with a new set of behaviors geared to their care.  The thousands
    of feeding visits parents must make will be crucial if these young are to survive
    to fledging.
  • Although they sleep most of the time and can't see, newly hatched swallows will
    wake and beg if stimulated by sounds, touch, sudden shade, or nest movement
    since these sensations could mean an adult has arrived with food.
  • Newly hatched Tree Swallows don't vocalize at first, but within a short time they
    begin making high-pitched peeps as they beg.

What are the light yellow fleshy borders at the corners of their mouths for?
  • These borders are called "oral flanges" (see below).  They are sensitive, and
    if touched may stimulate the nestling to beg.  

  • Flanges have been shown to reflect ultraviolet light, invisible to us but which
    most birds can see.  This may help make a nestling's open mouth more
    noticeable to adults.
  • A nestling's gaping mouth with its colorful flange acts as a beacon to attracts its
    parents attention, and forms an effective target for food placement in the
    dimness of a typical Tree Swallow nest cavity.

You may find newly hatched nestlings whose down is damp and matted.  Why
is this?
  • Damp, matted down feathers on the head and back indicates the nestling
    hatched very recently.  Remember, the interior of the egg was mostly wet.
  • Within a few hours the down is dry and fluffy (see below).

How are baby Tree Swallows different from baby chickens or ducks?
  • Though they aren't really independent, baby chickens and ducks leave their
    nests after hatching.  They are mobile, able to see, and able to feed themselves
    almost immediately.  Young that are developmentally advanced in this way are
    called "Precocial."
  • By contrast typical songbird hatchlings like the Tree Swallow in the photo above
    can't do much more than beg, eat, digest, and defecate.  They must undergo
    much more development before leaving their nest.  Until then they are totally
    dependent on adults.  Young of this type are called "Altricial."

Why do eggs hatch after a particular number of days of incubation?
  • Each species' embryos hatch after they develop as much as they are
    genetically "programmed" to do within the egg.
  • By this time the egg's original fuel and raw materials have been nearly used up
    by the growing embryo.  Anything required for further growth and development
    must come from outside.  To obtain them the embryo must hatch.

Where will new fuel and raw material come from?
  • Tree Swallow nestlings are altricial.  They depend entirely on adults for fuel and
    raw materials, in the form of invertebrate food adults catch and bring.
  • Without sufficient food a nestling's development slows.  Prolonged food
    deprivation can mean stunted growth or death by starvation or hypothermia.

How long does it take for all of a clutch of Tree Swallow eggs to hatch?  
Do they all hatch the same day or not?
  • Hatch duration depends upon when females start incubating.
  • If a female waits until she is finished laying to start incubating, her eggs should
    hatch on the same day.
  • If a female begins incubation before she has finished laying, hatch will usually
    extend over more than one day.
  • Some female Tree Swallows finish laying before incubation starts.
  • However, some other female Tree Swallows begin incubating on the day before
    they lay their last egg.  In these nests the last egg laid usually hatches one day
    after the others.

Won't a nestling that hatches a day later be at a disadvantage?
  • Yes, it often is.
  • Slightly older and larger young can beg for food more vigorously and reach out
    for it more strongly.
  • If food is abundant adults can usually supply enough so all young develop and
    fledge, despite sibling rivalry.
  • However, when food is limited, bigger and stronger young will monopolize it and
    smaller, weaker ones may go without.  If this situation continues the smallest,
    which is often the last hatched, usually dies first.
  • Notice how much smaller the nesting at right below is than its nestmates.  It was
    from the last egg laid and hatched a day later than its five nestmates.  It will be
    at risk of starvation if food becomes scarce.

So why do some female Tree Swallows start incubating the day before they
lay their last egg?
  • This situation, where incubation begins before the last egg is laid, so that eggs
    don't all hatch at once, is called "Brood Reduction."
  • Brood reduction is a genetically based tactic that may increases the odds that
    at least some of a female's offspring survive if food is scarce.
  • Extreme brood reduction is seen in birds like owls, where incubation begins with
    the first egg, resulting in a very staggered hatch and nestlings of many sizes.  
    When food supplies are poor, older owl nestlings may eat their younger, smaller

What should you do if you find dead nestlings in a box?
  • Remove them and take them far enough from the nest box area so the odor of
    decay won't attract predators to the nest.

Questions for the next Topic:  Parental Care of Small Nestlings.
  • How do adults' lives change after eggs hatch?
  • What do nestlings need if they are to live to fledging?

Hatching, Altricial Young,
Begging and Brood Reduction
Learn About Birds at Tree Swallow Nest Box Projects