ATTENTION:  If you have an older nestling that has tried unsuccessfully to
fledge or one that you believe may need human intervention for other
reasons click

At the Project:
It's finally happening.  Nestlings you've watched grow from eggs are fledging, leaving
the boxes they've grown up in.  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.

Warning!  If you see a large nestling perched halfway out the entrance hole, as in
the photo above, you know that
fledging, the act of flying from the nest for the first
time, is imminent.
 Please don't approach nestlings like these or you might
startle them into fledging before they are completely ready.  Swallow
nestlings that fledge prematurely can't fly well and may become grounded.  If
this happens they are usually abandoned by their parents and die.  (
Please be
aware that noted wildlife photographer Steve Byland, who took the above shot, takes
great care not to disturb any nestlings, including spending many hours in a blind if

The youngster in Steve McLaughlin's photo below was not so lucky.  It was
inadvertently startled from its box along a golf course.  Its puffy appearance and
closing eyes signal it is in deep trouble and probably won't survive.

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.
  • Some cavity-nesting species like Tree Swallows may stay in the nest longer.
  • Tree Swallows fledge as early as day 18 or 19 if good weather and feeding
    conditions have permitted rapid nestling development.
  • However, poor feeding conditions can slow swallow growth and development,
    delaying fledging until day 20-22 in some cases.
  • Nestling period length can also vary with brood number.  More young in a nest
    results in a longer nestling period, all other things being equal.
  • Nestling weight maximums, which often exceed their parents', decline as
    fledging nears, so at fledging nestlings weigh about the same as adults.  
  • Adults seem to reduce feeding rates as fledging nears, and once some of the
    nestlings begin to fledge parental visits to the nest decline dramatically.
  • At fledging nestling body feathers are fully grown.
  • However, nestling flight feathers have not completed their growth, being only
    about 80% to 85% of adult length at fledging, and some nestlings will 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 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's it like in a nest box the last day of two before fledging?
  • The best words we know to describe conditions are "hectic" and "bedlam."
  • Click here for a YouTube video of Francois Paquette's Tree Swallow nestlings
    that shows what we mean.

What happens at fledging?  
  • Older nestlings have been exercising their wings within the boxes for days.
  • Click here for an in-box YouTube video of 18 day old nestlings flapping.
  • At fledging young launch themselves from the entrance and try to fly.
  • Click here to watch six nestlings leave their box on YouTube.
  • 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 Tory Kallman.
  • Once some nestlings fledge, the parent swallows reduce their feeding visits to
    the nest, which may stimulate nestlings remaining inside to fledge also.

  • As their flying skills improve fledglings become able to accept food passed from
    adults while airborne.  Photo below by Gail West.

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
  • 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 of Tree Swallows.
  • How can you tell if there were parasites in a box?
  • What parasites do Tree Swallow nestlings have?

Tree Swallow Fledging
Learn About Birds at Tree Swallow Nest Box Projects