Incubation

At the Project:
Egg laying is almost over. Most clutches are complete. When you check
boxes open them just a crack to make sure certain no adult is inside. If there
is, gently close the door, then move away and wait for the bird to exit. This
may take awhile so you might want to check elsewhere, then return. Never
force an adult off its nest.

Concepts:
A fertilized bird egg starts as a single cell with the genetic instructions for
building a bird, plus sources of fuel and raw materials. But if you took an egg
home and put it in a cup on a shelf what would happen?

Embryonic growth would not occur. 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.
Cells must divide rapidly, migrate around, differentiate into more
specialized cells, and 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.
Bird eggs require temperatures of about 100 degrees F for embryonic
development. This is much higher than normal air temperature.
So Tree Swallows must add heat to get their eggs to develop.

How do Tree Swallows add heat to their eggs?

Females cover eggs with their bodies, conducting their own body heat
to their eggs.
This form of temperature regulation is called incubation.

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 females.

What happens if eggs are knocked out of the cup or pushed down into the
nest material?

Some songbirds retrieve dislodged eggs within reach. If they didn't the
egg would die.
Other species, including Tree Swallows, ignore partially buried eggs or
eggs displaced out of the cup, even if in plain sight.

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. A large area of breast and belly skin loses its
feathers and becomes swollen through water retention and expansion
of blood vessels. The brood patch below is typical.
Incubating females press this bare skin directly onto eggs, allowing her
body heat to transfer to the eggs.

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 days.
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 stay inside boxes many minutes. 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.
In songbirds like Tree Swallows, where only females incubate, females
average of 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 for embryonic development.

Here's a way to tell 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 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.
Why?

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.

Incubating songbirds turn eggs over periodically with their bills. Why?

Turning keeps membranes and embryos from sticking to interior shell
surfaces, which could cause embryo death.

What happens to the eggs at night?

Before incubation neither male or female swallows stay in boxes at
night unless it's extremely cold. They usually roost in nearby trees,
shrubs or marsh vegetation.
However, once incubation starts females stay in boxes overnight.
Staying in cavities overnight is dangerous. Females can be trapped by
nocturnal predators like raccoons, possums, or cats in boxes without
predator guards.

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 guard boxes while females leave to feed.
Although males don't incubate, they may perch at holes and peek in, or
enter, perhaps to "inspect" things.
More often males perch on boxes or poles, preening and giving
occasional Up and Down Songs to passing swallows. Chatter Calls aren't
heard much now, except if floater females approach and males want to
attract them.
Nest females usually fly directly to the hole and duck right into boxes.
Returning swallows often give Gurgles (also called Contact Calls). At
this signal the other usually 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.

Questions for the next Topic: Takeovers.

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?

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Incubation

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