ATTENTION:  If you are concerned because one or more Tree Swallow
nestlings is not growing well and is significantly smaller than its nestmates,
or if there has been a prolonged period of bad weather, or if you have
nestlings you believe may need human intervention in order to grow and
survive, please read and consider the possible options below:

Fostering Tree Swallow Nestlings
Hand-Feeding Tree Swallow Nestlings in the Field
Temporary Care of Tree Swallow Nestlings

At the Project:
There are small nestlings in almost every box now.  When you check nests you may
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 both nestling and adult behavior.

Warning!  If you plan to have your nestlings banded your bander must wait until
they are
11-12 days old, not younger and not older!

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.  Between days 4 and 7, when
    growth rate is highest, they may gain 2.5 grams per day, and when their weight
    levels off by about day 12-13 they will have reached 15 times their hatching
    weight and may outweigh their parents.
  • However, nestling weight can swing up and down due to weather variations that
    affect food supplies.  Cold, wet and excessive wind may reduce weight gains.
  • Bad weather can also cause slower feather growth because if parents aren't
    able to supply enough food, 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 their bodies start to emerge at day 6 or 7.  
    By about day 15 or 16 their bodies will appear fully feathered.
  • "Flight feathers" of wings and tail, looking like quills at first, 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 the 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 about day five and day ten, eyes open from tiny slits to fully
    wide (see below).  Now nestlings can use sight as well as sound and touch to
    sense when adults are arriving with food.

  • Click here for a YouTube video showing many of the physical changes
    undergone by Francois Paquette''s Quebec Tree Swallow nestlings as they
    grow between day 1 and day 12 after hatching.

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 or green.
  • Wings and tails will be dark gray.
  • Under bodies will be duller white than adults'.
  • Some juveniles will show a very faint gray 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 as they compete with one another for food.
  • They still huddle together but spill out of the nest cup before long.
  • 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
  • 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 usually concentrates his feeding efforts at
    one nest.  The burden of feeding young at the other nest falls mostly or entirely
    on its female.
  • Feeding a full brood is normally too difficult for single adults, and smaller,
    weaker young in one-parent nests often starve.

Is the rate adults enter the boxes changing as young grow 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.  Their larger bodies
    require more food, and adults have to work harder and harder to satisfy them.
  • Seeking and capturing flying insects, and bringing this food to a brood of
    growing nestlings for 18-22 days straight is extremely energy-demanding for
  • The metabolic rates of female Tree Swallows provisioning young has been
    found to average five times higher than their basal metabolic rates!  It's no
    wonder females typically loose weight during the nestling period.  
  • Rates of feeding young 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?
  • Yes, young with fewer nestmates typically develop faster than those with many
    because they receive more food per hour than young in large broods.
  • Evidence that young in larger broods tend to beg more intensely supports the
    idea they are experiencing greater competition for food.

As the young swallows grow in size, and as their insulating feathers develop,
is heat from their mother's brooding still needed?
  • For at least a week after hatching the nestlings are not large enough either as
    individuals or huddled together in a ball to keep themselves sufficiently warm.
  • However, by about day 8 or 9 they are usually large enough and becoming
    feathered enough to maintain their own body heat, to thermoregulate.
  • By this time it also becomes impossible for the female adult to longer cover
    them all with her brood patch.
  • Daytime brooding normally ends by day 10, but females may continue to sleep
    overnight with their young for several days, especially during cold, wet spells.
  • Click here for a video showing how a female Tree Swallow in Francois
    Paquette's nest box, changes her brooding behavior as her nestlings grow.

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 commonly heard 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 other'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

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.
  • Click here to watch a cool in-box video of adult Tree Swallows disposing of
    fecal sacs after feeding their young.

Why are fecal sacs removed?
  • Sanitation is probably the main reason.  Harmful germs could grow in soiled
  • Predators could be attracted by smell of accumulated wastes.
  • Nestling feathers smeared by waste could lose their insulating value.

Questions for the next Topic:  Causes of Mortality.
  • Have you noted dead or missing nestlings when checking boxes?
  • What could kill nestling songbirds?
  • How have you tried to prevent swallow mortality?

Tree Swallow Nestling Growth
and Development to 12 Days
Learn About Birds at Tree Swallow Nest Box Projects
Hatch                        3 Days                       6 Days
9 Days                                      12 Days