At Your Boxes:
In most boxes incubation is in full swing, with females staying inside boxes warming their eggs for ten to fifteen minutes or more 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 female Tree Swallows, still exists. In fact, fights to the death over possession of those crucial nest sites are even possible.
In the picture above an after-second-year (ASY) female resident (blue) struggles to defend her box from an intruding second-year (SY) female (brown).
Concepts:
Why do “floaters” continue to intrude on nest boxes?
- 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. For this reason they continue to intrude upon nesting pairs “looking” for chances to reproduce.
- Intruding males may try to oust resident males. But some simply seek to copulate with receptive resident females that haven’t finished laying their eggs and are still fertile.
- Intruding females, on the other hand, are looking for the nest site they require 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 try to take over other females’ nest sites by force.
Why are resident females especially vulnerable to nest takeovers during incubation?
- Competing for nest sites, building nests, and laying eggs has placed heavy demands on these resident females’ energy and body reserves. And now they must spend most of their time either finding food or 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 leave incubating to intervene.
- Floating females may be able to identify resident females that are particularly weakened and vulnerable.
How can you tell when female nest takeover attempts are 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 eventually 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 between females.
- Battling birds may grapple in flight, tumble to the ground, and continue to fight there. Watch an example in Aaron Riding’s YouTube video of fighting Tree 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, the resident female could beat or kill the intruding female.
- 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. This is where the battling females inflict the most lethal damage, 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 let the females 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 begin laying 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 we lifted a takeover female’s nest 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 nest takeovers?
- Dealing with nest takeover attempts by female Tree Swallows is a difficult issue in Tree Swallow project management.
- Your best option is to keep a box or two in reserve. Then, if you notice a serious takeover attempt taking place, put a new one up about 100′ from the disputed box.
- Maybe the intruding female will accept this new box, provided a male claims it as well.
- Regardless of our feelings we must realize that competition for opportunities to reproduce is the rule among songbirds.
- There is more on the subject of Tree Swallow competition on this page: Nest Site Competition and Box Claiming.
- Replacements and displacements go on around us all the time with other bird species. We usually don’t witness it, but as you have learned, with Tree Swallows you will see what’s really going on.
- Below, the winner in a nest takeover by a female Tree Swallow.
Be on the lookout for attempted nest takeovers by female Tree Swallows, especially during incubation. They can happen very fast!
Questions for the next Topic: Feather Care in Tree Swallows
- How many ways can you think of that birds “use” feathers?
- How do your Tree Swallows care for their feathers?
- What happens if a bird’s feathers wear out?
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Creating Tree Swallow Nest Box Projects
Spring Return
Nesting Season Behavior
Song and Calls
Nest Site Claiming
Pair Formation
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Diary of One Season at Salmon Creek
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Banding Your Tree Swallows Banding Adults Banding Nestlings
Tree Swallows in Research Research Bibliography Glossary of Terms