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















Concepts:
How do nestling songbirds let adults know they're hungry?
  • They beg vigorously, raising their heads, opening their bills to expose their
    colorful mouth linings, waving their wings and heads, and making begging calls.
  • Begging displays are critical because they give adults information on nestling
    hunger.  The hungrier nestlings become the harder they beg.  
  • Parents use begging behavior of their young to determine how often they
    bring food.  If nestling begging rate, duration, and loudness increase, parents
    increase food delivery accordingly.
  • Begging involves both cooperation and competition among nestlings.  Their
    combined displays stimulate parents to bring food to the brood.  Their
    individual displays determine the particular one nestling a parent actually gives
    food to on any given feeding visit.
  • Feeding efforts of both adults are normally necessary if most or all young are to
    survive the nestling period.

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, and an average
    of 8,000 prey fed per brood per day.
  • The entire bolus goes to one begging young, so 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 those 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.  Fewer young means lower reproductive output.
  • 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 cup is shaped and 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 "friends".  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 are large
    enough to thermoregulate effectively, using a combination of their individual
    metabolic heat, 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.  
  • It also becomes harder for females to cover their young with their bodies
    effectively as the young grow larger.
  • By day six or seven after hatching most females will have stopped daytime
    brooding.
  • Overnight brooding continues for a few additional days.
  • Once brooding has stopped neither adult spends much time inside the box.
  • After she stops brooding her nestlings the blood vessels in a female's brood
    patch begin to shrink, and her breast feathers begin to grow back.

Question 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