Egg Laying

At the Project:
For weeks you've probably been wondering: when are these birds finally going
to lay eggs? Well finally they have! Checking a box and seeing or feeling the
first egg of the season is an exciting moment. Eggs are really pretty amazing
things. Pick one up carefully and look at it. With a lot of work by the adults,
your good management, and some luck the contents of that little egg will
develop into a bird that will fly away in five weeks or so!

Concepts:
A fertilized bird egg is a container in which genes from the mother and father
"program" the building of an organism, an embryonic bird. Chemical and
physical processes of development change the embryo from a single cell to
the partially grown bird we will see at hatching.

During the egg laying period a female songbird
releases one ripened egg cell per day from her
ovary. Each egg cell or "ovum" consists of the
"yolk" that contains much of the future egg's
raw materials and fuel supply, plus the female's
genetic material that floats on top of the yolk
in a tiny island of cytoplasm. The ovum is
fertilized by a male's sperm while it's in or near
the upper opening of the female's oviduct. As
the fertilized ovum passes down the oviduct
toward the female's cloaca it is covered by the
rest of the egg's components: the layers of
albumen ("white"), shell membranes, and finally
the shell. The whole process, from ovulation of
the ovum to laying the completed egg, takes
about 24 hours.

A fertilized egg contains:

Genetic instructions from both its parents, encoded in DNA, for
"building" a bird. In a fertilized egg the earliest stages of embryonic
development take place within the "germinal spot", a light-colored area
on the yolk's surface.
Raw material and fuel in the yolk and albumen. Eggs must be packed
with all the proteins, fats, carbohydrates, minerals and water necessary
for embryonic development until hatching.
Shell membranes that help seal moisture in and through which oxygen
enters the egg and carbon dioxide and other waste gases produced by
embryonic metabolism exit.
A hard outer shell that protects the embryo, is a source of minerals and
is involved in gas exchange between the egg and its environment.

At home, crack a raw (unfertilized) chicken egg into a bowl and examine
both egg and shell. Look for:

Outer and inner shell membranes.
Albumen: the egg white.
Yolk and the thin membranes surrounding the yolk.
Germinal Spot: a small white disc on the yolk surface where the
embryo would have started to develop.
Chalazae: white curly structures in the albumen that connect to the
yolk and which twist as the egg is moved, keeping the embryo upright.

Also at home, dissect a hard-boiled chicken egg. Look for:

Outer and inner shell membranes.
Air space at large end.
Albumen.
Yolk and yolk membranes.
Germinal spot.

Tree Swallows have been back for six weeks or more. Why did it take so
long to start laying eggs, and why did they start laying now?

Tree Swallow eggs are small, about 1.9 gm, but female swallows are
small birds, averaging only 20 gm. Each egg weighs about one-tenth
the weight of the bird laying it, and females lay between four and
seven eggs, one per day.
Producing eggs takes lots of energy and raw materials, which comes
ultimately from food females must catch.
Weather must warm up enough to allow a good supply of flying insect
food in order for females to build internal resources necessary for egg
production.
When females finally pass a hormonal and physical threshold their
ovaries begin ripening eggs.

Why don't all Tree Swallows in an area start laying the same day?

Genetic differences, and variation in foraging success, physical
condition, and age, all effect individual laying thresholds.

How synchronized is laying?

It varies from place to place, and from year to year, but a majority of
females at a project usually begin laying within a 10 day period. They
are rather well-synchronized.
There are always a few that lay outside this peak period.
There may also be late clutches following female replacement due to a
death, desertion, or takeover.
Second-year (SY) females, laying for the first time in their lives, usually
lay later.

Why do females lay different numbers of eggs?

Genetic differences, variation in foraging success, physical condition,
and age can all effect egg production.
Weather changes during laying may effect a female's ability to maintain
her egg producing condition.
Resisting nest takeover attempts can drain reserves and reduce a
female's ability to produce eggs.
SY females average fewer eggs than older females.

Why do female Tree Swallows lay 4-7 eggs? Why not 1 or 2, or 10 or 12?
Other bird species do have clutches these sizes.

Eggs become young that need care and feeding. There seems to be a
genetically-based "best number" of young for each species, one that
over thousands of years has allowed adults to leave the most
descendants.
Reasons for a species' average "clutch size" have been subject of much
research, but there is still debate over ultimate causes.

What time of day are Tree Swallow eggs laid?

Usually about dawn or shortly after.

Warning!!! During laying do not make box checks before 9:00 A.M.
Females disturbed in the act of laying could desert!

How often are eggs laid?

Female Tree Swallows lay one egg per day, never more.
Bad weather may cause a female to skip a day or two.

What are Tree Swallow eggs like?

Small, averaging just under 2 gm in weight.
Pure white. May appear pinkish briefly after laying.
Air space in large end.
Eggs from different females can differ in shape.

Many bird eggs have camouflage markings. Why don't these?

We don't know for certain why many cavity nesters like Tree Swallows
have unmarked eggs.
Tree Swallow eggs are hidden inside a dark cavity so perhaps producing
eggs with cryptic markings would waste energy since predators aren't
apt to see them.
On the other hand, perhaps white eggs actually are camouflaged
against the background of white feathers.
Perhaps white eggs are easier for adults to see in the cavity.

When do we consider that a female swallow has completed her clutch?

For our records we consider a clutch complete if there have been no
new eggs for three days.

If something happens to a female's eggs can she lay a second clutch?

Female Tree Swallows are "single brooded", that is they normally
produce only one set of eggs per nesting season.
Females are usually capable of producing a second clutch to replace one
that's been lost, but it may have fewer eggs.
If a female's first nest is predated, she usually deserts that nest site.

Questions for the next Topic: Incubation.
Eggs have the plan and the raw materials to produce embryos, but they need
something more before embryonic development can proceed.