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

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 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