Nestling Growth

At the Project:
There are small nestlings in almost every box now. When you make nest checks you
have to take them out to get an accurate count, so you might as well examine
them. Slip your fingers gently under each and lift. Sure, you'll be defecated on, but
don't miss opportunities for close-up looks at nestling growth and development.
They won't be small for long.

Also, watch for changes in nestling and adult behavior.

Warning! If you plan to have your nestlings banded your bander must wait until
they are 11-12 days old, no younger and no older!

Concepts:
How are the nestlings changing?

They are growing very rapidly!
Their body proportions are changing. Their wings are becoming larger and
abdomens smaller relative to the rest of their bodies.
They are gaining weight at an explosive rate. By day 12-15, if they are
receiving sufficient food, they will reach 15 times their hatching weight and
outweigh their parents!
However, nestling weight can swing rapidly up and down due to variations in
the weather that affect food supplies.
Again, bad weather can result in slower feather growth because parents aren't
able to supply enough food and more of each nestling's energy must be
diverted from growth to thermoregulation.
In the pictures below observe the changes as nestlings grow.

Note how strips on their bodies darken between hatching and day 6. These
are "feather tracts" where feather follicles are rapidly growing.
"Contour feathers", that will cover and shape their bodies, start to emerge at
day 6 or 7. By about day 15 or 16 their bodies will be fully feathered.
"Flight feathers" of wings and tail, looking like quills, begin poking through the
skin of wings and tails by about day 6 or 7. At this age these feathers are
surrounded by waxy sheaths that protect them as they grow out from their
pit-like follicles.

By about day 10 to12 wing and tail feathers that have partially erupted from
their sheaths resemble little paintbrushes (see the tail feathers below).
These flight feathers should be completely erupted by day 16 or so, and will
continue to elongate through fledging and beyond.

Gradually, between day six and ten, eyes open from tiny slits to fully wide.

Are nestlings' feathers the same color as adults'?

No, nestlings are acquiring a unique "juvenile plumage", which they only have
for the first few months of their lives.
Juvenile upper bodies will be sooty gray with no iridescent blue-green.
Wings and tails will be dark gray.
Under bodies will be duller white than adults'.
Some juveniles will show a very faint sooty chest band.

How is nestling behavior changing?

They are becoming more and more active, shifting position often.
They are more squirmy when handled.
The simple begging peeps they once gave have become much louder, more
persistent, and complex.
They still huddle together but overflow the cup by day eight or so.
As they get larger they begin to preen erupting feathers, and stretch their
growing wings.

Are all young in a nest about the same size and state of development?

In some nests they are, but in others one or more nestlings may be lagging
behind.
The nestling at right in the picture below is noticeably smaller.

Where do you think a nestling that is smaller than its nestmates may have been in
the clutch's egg laying and hatching order?

If it was from the last egg laid and was the last hatched by a day, it started
with a size disadvantage. It may not be able to compete well for food with its
older, larger nestmates, and if food supplies become short it may die.
Individual nestlings can also lag due to disease, injury, or genetic weakness.

Who feeds the nestlings; males, females, or both?

Normally both feed nestlings, with females tending to bring food a bit more
often, although individual rates can vary.
However, if one parent dies or deserts there will be less food for the young at
the nest.
Also, if a male successfully claims two nest sites and has a female and young in
both (this happens occasionally), he concentrates his feeding efforts at one
nest. The burden of feeding young at the other nest mostly or entirely on its
female.
Feeding a full brood is normally too difficult for single adults, and smaller,
weaker young in these nests often starve.

Is the rate adults enter the boxes changing as young gain in size?

Research has shown that 95% of adult visits to boxes containing nestlings
involve feeding the young.
Feeding rates increase as small nestlings grow rapidly. Larger bodies require
more food, and adults have to work harder to satisfy them.
Feeding rates reach a peak by about day 8 and then remain at this level until
late in the nestling period when feeding rates taper off.
Nests with more young show higher feeding rates by both males and females
than nests with few young, but adults seldom are able to increase feeding in
direct proportion to number of young. So nestlings in large broods tend to
get fed fewer total food items per hour.

Does brood number effect rate of growth and development?

Young with few nestmates typically develop faster than those with many
because they are fed more often than young in large broods.

Have you noticed changes in how adults approach and enter boxes?

As nestlings grow the need for speedy, efficient transfer of food from adult to
young increases. Time lost is time adults can't forage.
Females now approach and enter boxes quickly and directly, swooping to hole
and ducking inside in a single fluid motion.

Males may hesitate, often perching at the hole briefly before entering.

What other changes do you notice in the behavior of adults?

As more efforts focus on feeding growing nestlings, adults have less time for
other activities.
There is less perching and body maintenance, less box guarding, and fewer
songs and calls.
One call that is becoming more common is the "Gurgle" or Contact Call, given
by adults as they relieve each other at the nest or as they near the hole
before entering. The Gurgle seems to alert an adult at the box to the
second's approach, so that relieving occurs efficiently. It also informs
nestlings inside the box that adult with food is arriving.
Adults aren't together much now, especially after females stop daytime
brooding.

Have you noticed adults carrying out "fecal sacs" in their mouths?

"Fecal sacs" are tough, flexible, gelatinous membranes enclosing a songbird
nestlings' feces.
They seem to make it easier for adults to remove wastes from nests.
Nestlings usually produce a fecal sac immediately after being fed, almost as a
reflex; in one end and out the other!
The nestling that was fed usually turns around, backs up, and ejects a fecal
sac, which is grabbed at once by the adult for removal.
Adult songbirds, including Tree Swallows, may actually eat the fecal sacs of
small nestlings. However, sacs of larger nestlings are usually removed and
dropped away from the nest (see below in the photo by Brett Burleigh).
You should be able to see adult swallows carrying white fecal sacs suddenly
dip their heads in flight, opening their bills to drop the sac.

Why are fecal sacs removed?

Sanitation is probably the main reason. Harmful germs could grow in soiled
nests.
Predators could be attracted by smell of accumulated wastes.
Nestling feathers smeared by waste could lose their insulating value.