At the Project:
Hatching's complete in many boxes, and nestlings are growing by leaps and
bounds.  Having young instead of eggs has changed adults' lives dramatically.  
Nestlings have a different set of needs their parents must meet.

Concepts:
How do nestlings let adults know they're hungry?
  • They beg, raising their heads, opening their mouths, waving their
    wings and heads, and once they're a few days old they begin to make
    begging calls.
  • Begging displays are critical because they give adults information on
    nestling hunger.  Nestlings that don't beg won't be fed.
  • Gaping, begging behavior of young stimulate males and females to
    gather food and feed their nestlings.
  • Feeding efforts of both adults are normally necessary if all young are to
    survive.









What do Tree Swallows feed their young?
  • The same type of flying insects adults eat.
  • Insects of the Orders Diptera (flies), Homoptera (leafhoppers),
    Hymenoptera (ants, wasps, bees), and Coleoptera (beetles) are their
    primary foods.

Do Tree Swallows feed their young one insect at a time?
  • No.  Adults catch many prey per foraging trip, holding them in a tight
    mass or "bolus" in their bill and throat (see below).
  • One study found an average of 19 prey per Tree Swallow bolus, with an
    average of 8,000 prey fed per brood per day.
  • The entire bolus goes to one begging young.  Only one young gets fed
    per adult feeding visit.















Wouldn't it be easier for the adults if they only had a few young to feed?
  • Yes, and in nests with only a few young, nestlings tend to grow faster
    and fledge sooner.
  • However, adults with small broods may pass on fewer copies of their
    genes to future generations.  Their reproductive output is lower.
  • More young means more work for adults, but their reproductive output
    is potentially higher.

Besides fuel and raw material supplied by adults through food, what else do
the small swallow nestlings need?
  • Small young have high ratios of surface skin area to internal volume and
    little in the way of insulating feathers.  They can lose heat very rapidly
    to the surrounding environment.  They aren't able to keep themselves
    warm, to "thermoregulate", yet.
  • As she did when they were in the eggs, the female swallow covers her
    small nestlings and warms them, conducting her body's heat to them
    through her "brood patch".
  • In addition to keeping the nestlings from dying of hypothermia,
    brooding keeps nestling body temperatures high enough for digestive
    and growth enzymes to work well.
  • Some insulation for the nestlings comes from the nest material and the
    way the nest is constructed.
  • You can just see some small nestlings under the brooding female below.








Why do nestlings cluster tightly together?  Are they just being friendly?
  • Their "motive" is strictly instinctive.  They are not being friendly.  They
    are in fact deadly rivals for the food the adults bring.
  • Clustering helps each nestling reduce conductive heat loss by reducing
    the amount of each body's surface area exposed directly to the air.  
    Cozying up to other warm bodies conserves their own heat.
  • Clustering to retain heat is more effective in large broods.
  • Females of nests with large broods can stop brooding earlier since the
    larger clustering mass of the nestlings allows them to thermoregulate
    effectively, to maintain their own heat level, at a younger age and
    smaller individual size.











What happens at night?
  • Females remain in the nest overnight brooding small young until they
    can thermoregulate effectively through a combination of their heat of
    metabolism, feather insulation, and conductive warming by clustering
    with their nestmates.

When does the female stop brooding?
  • It depends on the weather, how many young are in the nest, and how
    fast they are growing.  She can't stop brooding until her nestlings can
    thermoregulate effectively.
  • Cooler weather requires more intensive brooding.
  • As nestlings grow their surface to volume ratio decreases, allowing
    better heat retention, plus their developing feathers start to have real
    insulating value.
  • By day five or six after hatching most females will have stopped
    daytime brooding.
  • Overnight brooding continues for a few additional days.
  • Once daytime brooding has stopped neither adult spends much time
    inside the box.
  • Once she stops brooding her nestlings the blood vessels in a female's
    brood patch begin to shrink, and her breast feathers start to grow back.

Questions for the next Topic:  Sexing and Aging.
  • How can you tell the sex and age of the parent swallows?

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Nestling Care
Learn About Birds at Tree Swallow Nest Box Projects