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
gradually develop to the point where they can be used as the bird matures
physically.

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.  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, and 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.
  • 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 can fall to the ground or
    into water before reaching a solid perch.
  • Adults in the area will chase fledging young, pursuing them closely and calling
    loudly.  Some people believe this behavior stimulates young to keep flying
    until a perch is reached.  Other people consider it harassment.








Do adults care for fledged young?
  • Some juveniles appear to be completely independent at fledging, flying away
    and not looking back.
  • Others continue to beg and may receive food from parents for a few days
    after fledging, perhaps longer.








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.

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