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 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 they've
    reached 15 times their hatching weight, and often outweigh adults!
  • Strips on their bodies have darkened where tracts of feather follicles
    are growing (see below).
  • From left: hatch, 3 day, 6 day, 9 day, and 12 day-old-nestlings.






  • Contour feathers, that will cover and shape the body, start to emerge
    at day six to seven.  By about day 15-16 bodies will be fully feathered.
  • Flight feather sheaths, looking like quills, begin poking through the skin
    on wings and tails by about day 7- 8 (see below).

















  • By about day 10-12 partially erupted wing and tail feathers resemble
    little paintbrushes (see the tail feathers below).  These flight feathers
    should be completely erupted from their sheaths by day 16 or so, and
    will continue to elongate through fledging and beyond.









  • Gradually, between day six and ten, nestling 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.
  • Begging peeps are becoming louder and more persistent.
  • 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 wings inside the box.

Are all young in a nest about the same size and state of development?
  • In some nests they are.
  • In others one or sometimes two may be noticeably smaller.

Where do you think lagging nestlings were in the clutch's laying and hatching
order?
  • If they were last laid and last hatched by a day, they were at a size
    disadvantage to begin with and may not be able to compete well for
    food with their older, larger nestmates.
  • Individual nestlings can also lag due to disease, injury, or genetic
    weakness.

Who feeds the nestlings; males, females, or both?
  • Normally both feed nestlings, and at about the same rate, 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 only feeds young at one
    nest, leaving the burden of feeding young at the secondary nest
    entirely to its female.
  • Feeding a full brood is normally too difficult for single adults, and the
    smaller, weaker young usually starve.

Is the rate adults enter the boxes, presumably to feed nestlings, changing as
young gain in size?
  • Feeding rates increase as nestlings grow.  Larger bodies require more
    food, and adults have to work hard to satisfy them.
  • Boxes with more young show higher feeding rates than boxes with few
    young, but adults seldom are able to increase feeding in proportion to
    number of young.  Therefore, nestlings in large broods 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.  This call seems to alert an adult at the box
    to the second's approach, so that relieving occurs efficiently.
  • Adults aren't together much, especially after females stop daytime
    brooding.

Have you noticed adults carrying out "fecal sacs"?
  • Fecal sacs are tough, flexible membranes enclosing nestlings' feces.
  • They make it easier for adults to remove wastes from nests.
  • Nestlings usually produce a fecal sac when fed; in one end and out the
    other!
  • Adults may actually eat the fecal sacs of small nestlings.  However, as
    nestlings grow adults remove sacs in their bills and drop them away
    from the nest.
  • You should see adults carrying 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 insulating value.

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

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Nestling Growth
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