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.
- A fertilized bird egg contains genes from the mother and father which will
"program" the building of an organism, an embryonic bird. Chemical and
physical processes of development will change the embryo from a single cell to
the partially grown bird we will see at hatching.
- Most modern-day birds have just one ovary and one oviduct, and lay only one
egg at a time. This may help keep body weight low for flight, and also avoid the
problem of having two large delicate eggs developing at the same time.
- During her egg laying period a female songbird releases one ripened egg cell or
ovum from her ovary per day. Each ovum consists of a yolk that contains
much of the future egg's raw materials and fuel supply, plus the female's
genetic material which floats like a tiny island on the surface of the yolk.
- An ovum is fertilized by a male's sperm just minutes after ovulation, while the
ovum is still near the upper opening of the female's oviduct. At this point the
egg has no "white" or shell.
- After fertilization the ovum passes down the tube-like oviduct toward the
- As the ovum moves through the oviduct it is covered step by step with the rest
of the egg's components: first by several layers of albumen (white), then the
shell membranes, and finally by the hard shell.
- Finally the completed egg passes through the cloaca and is expelled from the
female's body into the nest in the act of egg laying.
- This process, from ovulation to egg laying, takes about 24 hours.
- In songbirds ovulation of the next ripe ovum occurs 15-75 minutes after an egg
A fertilized songbird's egg possesses:
- 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 surface of the yolk.
- A yellow yolk packed with fats, proteins, carbohydrates, vitamins, and minerals,
which will supply raw materials and energy needed for embryonic development.
Yolks also contain immune system compounds that will help protect the embryo
from disease and other trauma.
- A clear or milky-colored albumen or "white," that supplies water and some
nutrients to the embryo. It also cushions the yolk and embryo and keeps them
from drying out.
- Shell membranes that help seal moisture in, and through which oxygen enters
the egg and carbon dioxide and other waste gases produced by embryonic
- A hard but porous shell that helps protect the embryo from environmental
threats, is a source of minerals during embryo growth, and is involved in gas
exchange between the egg and its environment.
- Anti-germ protection for the embryo furnished by the shell and membrane
barriers, and by anti-microbial proteins in the albumen.
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.
- Yolk and yolk membranes.
- Germinal spot.
Tree Swallows have been back for six weeks or more. Why did it take so long
for females to start laying eggs, and why did they start laying now?
- Tree Swallow eggs are small, averaging about 1.8 gm at laying, but female
swallows are also 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 grid usually begin laying within a 10-15 day period. They are rather
- There are usually 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
- 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 egg clutches these sizes.
- Eggs become young that need care and feeding.
- For each species there seems to be a genetically-based "best number" of
young, one that over time 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. because
females disturbed in the act of laying could desert their nests.
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.
Is it hard for a female swallow to lay an egg?
- You be the judge. Click Here to watch Kathryn Leonardo's YouTube video and
count a Virginia Tree Swallow's contractions as she tries to lay an egg.
- Can you tell when she's done and what she needs to do when she's finished?
What are Tree Swallow eggs like?
- Small, averaging only about 1.8 g in weight when first laid.
- Pure white. May appear pinkish briefly after laying.
- Air space visible 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
- Tree Swallow eggs are hidden inside a dark cavity so perhaps producing eggs
with cryptic markings would be a waste of energy since predators aren't apt to
see them anyway.
- On the other hand, perhaps white eggs actually are camouflaged against the
background of white feathers of Tree Swallow nests.
- 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 almost always "single brooded," that is they normally
produce only one set of eggs per nesting season.
- There is recent evidence that a small proportion of females in the most
southerly parts of the swallows' breeding range are "double brooded," which
means they are nesting a second time after successfully raising a first brood.
- Females are usually capable of producing a second clutch to replace one that's
been lost, but second clutches 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.
- What else do eggs need?
- Where will it come from?
Learn About Birds at Tree Swallow Nest Box Projects
|Nesting Guide, Spring Return, Songbird Behavior, Song and Calls, Nest Site Competition,
Pairing Up, Nest Building, Bird Flight, Mating, Eggs and Egg Laying, Incubation,
Takeovers, Feather Care, Hatching, Nestling Care, Sexing and Aging,
Nestling Growth, Mortality, Older Nestlings, Fledging, Ectoparasites, Juveniles,
Flocks, Migration, Molt, and Winter, Box Care and Project Assessment