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 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).
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 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
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
- 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
- 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.
- Usually, the original clutch of eggs is completely covered over with new nest
material, but sometimes the old eggs are visible beneath the takeover female's
- In the picture below a takeover female's nest was lifted after her young had
fledged, revealing dead eggs laid by the original resident female.
- 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 hurry compared to the
early-nesting females they supplanted.
- The covered 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
- 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
- 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 bird
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?
Learn About Birds at Tree Swallow Nest Box Projects
|Nesting Guide, Spring Return, Songbird Behavior, Song and Calls, Nest Site Competition,
Pairing Up, Nest Building, Bird Flight, Mating, Eggs and Egg Laying, Incubation,
Takeovers, Feather Care, Hatching, Nestling Care, Sexing and Aging,
Nestling Growth, Mortality, Older Nestlings, Fledging, Ectoparasites, Juveniles,
Flocks, Migration, Molt, and Winter, Box Care and Project Assessment