At the Project:
Your Control Sheet shows the earliest nests have reached incubation day 14,
when eggs normally start hatching. Carefully opening these boxes you 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 reflex.

And don't worry about the adults flying around screaming and diving 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.

Note: the first four or five days after hatching is the best time to band adults.

Concepts:
How do bird eggs hatch?
  • Bird embryos use an "egg tooth", a small temporary structure on the
    top of their bills, to cut their shells in half from the inside.
  • The egg tooth is rubbed against the shell, which has been weakened as
    the growing embryo absorbed calcium from it.
  • The embryo turns as it rubs with the egg tooth, creating a circular cut
    around the egg's large end.
  • When the cut is complete the hatching bird pushes the shell halves and
    works itself free of shell membranes.
  • Hatching takes small songbirds up to five, very tiring hours.









You may see "pipping" in eggs about to hatch.
  • A pipped egg shows a thin line in the shell where the embryo has
    begun cutting.

Warning!!!  Do not touch pipping eggs!  Damaging them 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 female birds eat the shells, probably to regain minerals
    their bodies had given up when the shell was produced.
  • Other species, including Tree Swallows, carry shells away and discard
    them.

You'll find that not all eggs hatch.  Why is this?  
Do ones that didn't hatch appear different?
  • Some eggs may not have been fertilized.
  • In other eggs embryos may have died from genetic defects or from
    exposure to cold.
  • Some unhatched eggs will look dark and discolored.  
  • Others may seem clear and watery inside.
  • Still others may be light in weight because air has leaked in and dried
    the contents.

What are young Tree Swallows like when they hatch?
  • They are tiny, weighing less than 1.5 gm.
  • They are mostly naked, with little whiffs of down on back and head.
  • Internal organs and the last of the yolk are visible through the thin skin
    of the abdomen.
  • Their eyes are completely closed.
  • Their beaks are large with fleshy yellow edges.







Can they move?
  • 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 behavior, and is intended to make adults feed them.
  • Although they can't see, newly hatched young will beg when stimulated
    by sounds, touch, sudden shade, or nest movement.  All these could
    mean an adult has arrived with food.
  • Young nestlings sleep most of the time, begging when they think an
    adult has arrived.
  • Newly hatched Tree Swallows don't vocalize, but within days they will
    begin peeping as they beg.

What are the yellow, fleshy borders around their mouths for?
  • These borders are called "oral flanges".  They are sensitive, and if
    touched stimulate the nestling to beg.  Try it.
  • The sight of a nestling's gaping mouth with its colorful flange acts
    stimulates adults to feed the nestling.
  • The gape and flange form a target for food placement in the dim light
    of the nest cavity.

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








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 can't do much more than beg,
    eat, digest, and defecate.  They must develop much more 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 set number of days of incubation?
  • Each species' embryos hatch after they develop as much as 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 reach 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
    and/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.
  • If food is abundant adults can supply enough food so all young fledge.
  • However, when food is short, 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.

So why do some female Tree Swallows "choose" to 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 increases the odds
    that 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 siblings.

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:  Nestling Care.
  • How do adults' lives change after eggs hatch?
  • What do nestlings need if they are to live to fledging?

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Hatching
Learn About Birds at Tree Swallow Nest Box Projects