At the Project:
In most boxes incubation is in full swing, with females staying inside boxes warming
their eggs for ten to fifteen minutes or so at a stretch.  You might expect incubation
would be a relatively quiet time in the Tree Swallow nesting cycle, and perhaps it is at
most nests.  However, it may surprise you to learn that intense competition for some
boxes and the danger of nest takeovers, especially by females, still exists, and fights to
the death over possession of those crucial nest sites are possible.  In the picture below
an after-second-year (ASY) female resident (blue) struggles to defend her box from an
intruding second-year (SY) female (brown).

Why do "floaters" continue to intrude at your project?
  • Floaters, both males and females, that failed to obtain a nest site earlier still
    "want" to pass on their genes, to leave descendants this breeding season, so
    they continue to intrude upon nesting pairs "looking" for chances to reproduce.
  • Intruding males may try to oust resident males, but many seem to simply seek
    copulations with receptive resident females that haven't finished laying their
    eggs, and are still fertile.
  • Intruding females, on the other hand, require a nest site in order to reproduce.
  • Female floaters may replace a resident female that has died or deserted, but if
    there are no opportunities of this kind, their only option is to take over other
    females' nest sites by force.

Why are resident females especially vulnerable to takeover attempts during
  • Competing for nest sites, building nests, and laying eggs has placed heavy
    demands on these females' energy and body reserves.  And now they must
    spend most of their time finding food and incubating.
  • While resident females are inside boxes incubating intruding females can
    approach resident males, who seldom chase any female away.  In fact males
    often sing and display, "inviting" floater females to approach.
  • Courtship behavior between resident males and floater females may proceed
    without resident females being able to intervene.
  • Floating females may be able to identify resident females that are particularly

How can you tell if a female takeover attempt is in progress?
  • You may notice an intruder persistently circling over a particular box, or fluttering
    in front of the hole and trying to enter.  
  • Intruding females may continue this circling for many hours or even days.  The
    resident female must divert her energy from incubating and self maintenance to
    try to drive the intruder away.

  • Even more obvious signs that takeovers are under way are prolonged, no holds
    barred, fights.
  • Battling birds may grapple in flight, tumble to the ground, and continue to fight
    there.  Click Here to view Aaron Riding's YouTube video of fighting swallows.
  • If a resident female has been sufficiently worn down by all the tasks she's
    undertaken she may be driven from her nest and eggs by the floater, and
    perhaps even killed.
  • On the other hand, it could be the intruding female that is badly beaten or killed.
  • You may discover dead or wounded swallows inside boxes or on the ground
    below.  If you do, examine the bird for wounds to the skull, where most lethal
    damage is inflicted, as seen on the dead SY female below.

Don't resident males help their mates resist takeovers?
  • A few males do intercede to help prevent female nest takeovers (see below), but
    experiments with models have shown this to be quite rare.
  • Males usually remain bystanders and allow the females to fight it out.

What does a successful takeover female do with the original female's nest
and eggs?
  • Females who succeed in taking other females' nest sites usually cover the
    original owners' eggs quickly with a layer of vegetation and lay their own clutch
    on top within a few days.  
  • Normally, the original clutch of eggs is completely hidden under the new nest
    material, but sometimes the old eggs are partially visible beneath the takeover
    female's new clutch.
  • The covered eggs of the original clutch no longer receive proper incubation and
    the embryos inside die.
  • In the picture below a takeover female's nest was lifted after her young had
    fledged, revealing the dead eggs of the original resident female buried below.

Is there anything you can do to prevent strife and potential loss of life from
  • Your best option is to reserve a box or two, to be put up at least 100' from all
    other boxes, in case you notice a serious takeover attempt taking place.  
  • This new box may be accepted by the intruding female, provided a male claims it
    as well.
  • Regardless of our feelings about nest takeovers we need to realize that
    competition for opportunities to reproduce is the rule among songbirds.  
  • Replacements and displacements go on around us all the time with other bird
    species, mostly hidden from our view.  But as you have learned, with Tree
    Swallows you will see what's really going on.

Be on the lookout for takeovers, especially during incubation.  They can
happen very fast!

Questions for the next Topic:  Feather Care and Function.
  • How many uses can you think of for bird feathers?
  • How do your Tree Swallows care for their feathers?
  • What happens if a bird's feathers wear out?

Nest Takeovers
by Female Tree Swallows
Learn About Birds at Tree Swallow Nest Box Projects