At the Project:
It's finally happening.  Nestlings you've watched grow from eggs are leaving the
boxes.  They're flying!

Contrary to what you might expect songbirds don't "learn" to fly, and their parents
don't "teach" them.  Young birds of flying species have inborn abilities to fly which
develop gradually as the birds' bodies mature.  When they reach the point where
they are physically ready for flight they will fly.




















If you see a large nestling perched halfway out the entrance hole, as in the photo
above by Steve Byland, you know that fledging, the act of flying from the nest for
the first time, is imminent.  Please don't approach nestlings like these to avoid
startling them into attempting fledging before they are completely ready.

Concepts:
How old are Tree Swallow nestlings when they leave the nest?
When do they "fledge"?
  • The key to Tree Swallow fledging is flying ability.  Tree Swallow nestlings must
    be able to fly well in order to fledge successfully.
  • Young of open-nesting songbirds often leave their nests as early as day 12-14.
  • In contrast, cavity-nesting species like Tree Swallows typically stay in the nest
    longer.
  • Tree Swallows fledge about day 18 or 19 if weather and feeding conditions
    have been good, permitting rapid nestling development.
  • However, poor feeding conditions can slow swallow development, delaying
    fledging until day 20-22.
  • Nestling period can also vary with brood number.  More young in a nest results
    in a longer nestling period, all other things being equal.
  • Nestling weight, which had exceeded adults', declines as fledging nears.  At
    fledging nestlings weigh about the same as adults.  Adults seem to reduce
    feeding rates as fledging nears.
  • At fledging nestling body feathers are fully grown.
  • However, flight feathers are only about 85% grown at fledging, and some
    nestlings attempt to fly with wings and tail shorter than this.

Warning!  Avoid disturbing nestlings known to be near fledging since ones that are
startled into fledging prematurely may not survive.  (We took the picture below of
18 day nestlings many years ago before we knew better.  We were very lucky they
didn't "explode" out of the box).















Note the dark gray upper body plumage of these near-fledging-age nestlings.  It's
quite different from the blue-green of adult males and ASY females, and from the
brown-green of SY females.

What happens at fledging?  
  • Older nestlings have been exercising their wings within the boxes for days.
  • At fledging young launch themselves from the entrance and try to fly.
  • Some fly strongly at once, and may stay airborne until far out of sight.
  • Less mature young may struggle to keep aloft, and may fall to the ground or
    into water before reaching a solid perch.
  • Adult Tree Swallows in the area will converge on and chase fledging young,
    pursuing them closely and calling loudly.  Some people believe this behavior
    stimulates young to keep flying until they reach a perch.  Others suggest the
    adults are harassing young that are not their own.  We favor the first idea.

Do adults care for young once they have fledged?
  • Some juveniles appear to be completely independent at fledging, flying away
    without seeming to look back.
  • Others continue to beg noisily, and may receive food from their parents for a
    few days after fledging, perhaps longer.  (Photo below by Jim Martin).

















Do fledglings return to their nests?
  • Fledged juveniles often flutter about, perch on, and even enter boxes.  They
    seem attracted to ones still containing nestlings.
  • But any returns to the nest they fledged from are probably accidental.  
  • Fledglings don't return to nests to roost, and they don't help adults feed
    nestlings.  In fact there is evidence juveniles may enter boxes containing
    nestlings and out-beg them for food.
  • For more on these subjects see Juveniles.

Have you noticed anything different in the appearance of adults lately?
  • At about the time Tree Swallow nestlings begin to fledge the adults are
    starting their annual molt.
  • Tree Swallow molt doesn't happen all at once but proceeds gradually over
    several months, during which all their feathers are replaced.
  • Their molt, that starts on the breeding grounds in late June or early July, will
    continue as the swallows migrate south, and won't be finished until October
    or November.
  • Look for notches in the wings and tails of adults where flight feathers are
    being lost and replaced one by one.   









Questions for next Topic:  Ectoparasites.
  • How can you tell if there were parasites in a box?
  • What parasites do Tree Swallow nestlings have?









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Fledging
Learn About Birds at Tree Swallow Nest Box Projects