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 nest females are inside boxes incubating intruding females can
    approach nest 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,
    and fluttering in front of the hole, or trying to enter.  
  • Intruding females may continue this circling for many hours or even
    days.  The nest 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 six original eggs she 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 eggs in the original clutch will no longer receive proper incubation
    and the embryos will 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 as a diversion
    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