ATTENTION:  If you have a nestling that you believe is in trouble and needs
human intervention in order to survive, click

At the Project:
Hatching is complete in many boxes, and having small, completely dependent young
instead of eggs presents parent Tree Swallows with a new and different set of needs
they must meet.  For the next three weeks their lives will be focused on nestling care.

How do nestling songbirds let adults know they're hungry?
  • They beg, 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 accurate information on
    nestling hunger.  The hungrier nestlings become the harder they beg.  
  • A nestling's begging displays may also vary depending on its size,
    developmental stage, the number of nestmates it has, and how warm it is.
  • 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, if they can.
  • Begging involves both cooperation and competition among nestlings.  Their
    combined displays stimulate parents to bring food to the brood.  Their individual
    displays help determine which nestling a parent actually feeds on any given
    feeding visit.
  • Research has found that those nestlings that beg most intensely and most
    often, and those that position themselves in the center of the nest or directly
    under the entrance, consistently receive the most food.
  • Feeding efforts of both male and female adults are normally needed for most or
    all young in a brood to survive the nestling period.

What do Tree Swallows feed their young?
  • The same type of flying insects adults eat.
  • Insects of the Order Diptera (flies) are especially important foods, but parents
    also feed their young Homoptera (leafhoppers), Hymenoptera (ants, wasps,
    bees), Odonata (dragonflies) and Coleoptera (beetles) when available.
  • Nestling growth and survival depends heavily on the supply of flying insects
    adults can provide, which often depends in turn on the weather.  Young
    swallows can starve during extended cold, wet, and/or windy weather.
  • Research has also shown that survival is better when nestlings are fed insects
    from aquatic sources rather than terrestrial ones.
  • Parent Tree Swallows need to hunt for food almost non-stop, dawn to dusk,
    from the time their eggs hatch until their young leave the nest.

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).
  • Studies have found averages of between 19 and 34 prey per Tree Swallow
    bolus, with roughly 8,000 prey fed per brood per day.
  • Except when they are very small an entire bolus usually goes to one begging
    young, so only one young normally 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 receive more
    food on average, and 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 must have heat, lots of heat.  Since they have high ratios of
    surface skin area to internal volume and few insulating feathers to keep heat in,
    their bodies can lose heat very rapidly to the surrounding environment.  They
    aren't able to keep themselves warm, to "thermoregulate," yet.
  • The crucial job of keeping the small nestlings warm falls to the female parent,
    who "broods" them by covering them with the bare skin of her "brood patch",
    allowing heat from her own body to pass to her young.
  • In addition to keeping small nestlings from dying from hypothermia, brooding
    keeps nestling body temperatures high enough for the chemical processes of
    digestion and growth working well.
  • Some insulation for the nestlings also comes from the nest material and the way
    the nest cup is shaped and constructed.
  • Click Here to watch an in-box YouTube video showing how parent Tree
    Swallows care for their tiny, one-day-old nestlings.  You'll see the parents feed,
    remove waste, and brood the young, guard their box, and relieve each other.

Why do nestlings cluster tightly together?  Are they just being friendly?
  • Their "motive" is strictly instinctive.  They are not "friends."  They are in fact
    competitors for the food the adults bring.
  • Clustering helps each nestling reduce its heat loss by reducing the amount of
    each body's surface exposed directly to the air.  Cozying up to other warm
    bodies conserves one's own heat.
  • Clustering to retain heat is more effective in large broods.
  • Females of nests with large broods can reduce brooding earlier since the larger
    total 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
  • 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 seven or eight after hatching most females will have started reducing
    daytime brooding.
  • Click Here to watch a mother swallow's brooding change over time.
  • Overnight brooding usually continues for several additional days.
  • Once brooding has stopped neither adult spends much time inside the box.
  • As she ceases to brood her nestlings the blood vessels in a female's brood
    patch start to shrink, and her breast feathers begin to grow back.

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

Parental Care
f Small Nestlings
Learn About Birds at Tree Swallow Nest Box Projects