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 find that serious and intense
competition, especially between females, still exists for possession of particular
boxes.  In the picture below an after-second-year female resident (blue) struggles to
defend her box from an intruding second-year female (brown).

















Concepts:
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
incubation?
  • 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 rarely 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
    vulnerable.

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.
  • 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 on the ground beneath boxes.

Don't resident males help their mates resist takeovers?
  • Some males do intercede to help prevent female nest takeovers (see below),
    but others don't appear to take sides.











What does a successful takeover female do with the original female's nest and eggs?
  • Females who succeed in taking other females' nest sites quickly cover the
    original owners' eggs with a layer of vegetation and lay their own clutch on top.
  • Sometimes the original clutch of eggs is completely covered over with new nest
    material, but often the old eggs are visible beneath the takeover female's new
    clutch.
  • In the picture below a takeover female's nest was lifted up after her young had
    fledged to reveal dead eggs in the nest of the original female that the takeover
    female had covered over.












  • Late-nesting takeover females begin laying in a very short time, as soon as four
    or five days after the takeover, as though they are in a real hurry compared to
    the early-nesting females they supplanted.
  • The covered-up eggs of the original clutch no longer receive proper incubation
    and the embryos inside die.

Is there anything you can do to prevent strife and potential loss of life from
takeovers?
  • 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 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
    species, mostly hidden from our view, but as you have learned, with Tree
    Swallows you really see what's going on.














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

Questions for the next Topic:  Feather Care.
  • 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?









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Takeovers
Learn About Birds at Tree Swallow Nest Box Projects