At the Project:
Nest building is proceeding nicely.  Listen now for the rapid "tic-tic-tic" of the Ticking
that male Tree Swallows give when they want to copulate.  You should see both
successful and unsuccessful mating attempts over the next couple of weeks.  Watch
closely to see if you can tell which sex, male or female, controls whether a copulation
attempt is successful.  Photo below by Ron Grant.

Also, when checking boxes look in the nest cup, or if you can't see through the
feathers feel very gently in the cup.  Reason: when copulations begin, egg laying is
soon to follow.

Mating is an extremely important event in a bird's life, because it's the way
his or her genes can pass into the next generation.

There appear to be two different situations in which Tree Swallows mate.  The
first is highly visible, and is the one you are almost certain to witness.  
  • This mode often starts when a male flies toward a female perched out in the
    open.  As he approaches he makes rapid Tick Calls that signal his intend to
    attempt copulation.
  • The female perches with her back and tail held horizontal.

  • The male lands on the female's flattened back and, using his wings for balance,
    he grabs her head feathers in his bill.

  • The male pivots his tail under the female's so his "cloaca" contacts hers, and
    his sperm is transferred to her.
  • The actual cloaca to cloaca contact is quick.  It only takes a few seconds to
    transfer the male's sperm.
  • A successful copulation usually involves multiple cloacal contacts.

  • The male may fly off briefly, then land again for additional copulations.

  • Click here to view a YouTube video of Tree Swallows mating successfully  
    Notice how many cloacal contacts the pair in the video make.

If a female does not want to mate at this time, what can she do?
  • She can easily prevent a male from copulating by not leveling her body, or by
    flipping her tail up when he tries to land, as the female below is doing.

  • She can also twist her head around and threaten him with her bill open.
  • So a female Tree Swallow's behavior can determine if a male's copulation
    attempts are successful or not.  If she doesn't want to, it won't happen.
  • Click here to view a YouTube video of a female Tree Swallow rejecting a male's
    attempts to copulate.

What is this "cloaca"?
  • Birds have only one opening for their digestive, excretory and reproductive
    systems.  This cloaca ("sewer" in Latin) receives feces from the large intestine,
    uric acid from the kidneys, and either eggs or sperm from the gonads.  These
    are all released to the outside through a single "vent."

Does a male Tree Swallow have a penis?
  • No, but male songbirds develop a "cloacal protuberance" during the nesting
    season.  The "CP" is a swelling of lower portions of the ducts that carry sperm
    from a male's testes to his cloaca.
  • The cloacal protuberance acts as a temporary holding chamber where a male's
    sperm can complete their development and be kept cool before their transfer to
    a female during mating.
  • The cloacal protuberance below is typical of a breeding male swallow.

  • The "CP" sticks out slightly and during the act of mating it is everted farther,
    which helps the male transfer his sperm into the female's cloaca.
  • You can clearly see a male's everted "CP" in Euan Reid's photo below.

  • And in case you were wondering, a female's everted cloaca appears very
    similar, as you can see in the photo below by Juan G. Rodriguez.

  • Once a male's sperm has been transferred to a female swallow's body she can
    keep them alive for several days in sperm storage tubules located in her
    reproductive tract until they are needed to fertilize an egg.

What is the second mode or situation in which Tree Swallows mate?
  • Until the advent of in-box cameras we weren't aware that Tree Swallow
    copulation is sometimes attempted out of our sight inside nest cavities.
  • Recently we viewed videos taken by Francois Paquette of Quebec in which it
    appears within-box copulations may be forced on female swallows by males.
  • We encourage you to see what your opinion is after watching a YouTube video
    of this behavior by clicking here.
  • Regardless of whether this copulation is forced or not, we presume it must
    succeed often enough for the behavior to be retained in this species.

Extra-Pair Young are mentioned in the Forced Copulation video.  What does
the term Extra-Pair Young mean?
  • DNA studies of Tree Swallows show that resident females are the biological
    mothers of all the young in their nests.  In other words female Tree Swallows
    don't sneak eggs into other females' nests the way some bird species do.  
  • However, the same studies also revealed most Tree Swallow nests contain
    young fathered by more than one male through extra-pair copulations.  
  • In some nests most or even all young were fathered by other males, not the
    resident male!  Some nests had young fathered by 3 or more different males.
  • The brood of six young in the hat below all have the same mother but may well
    have two or more different fathers.

What's going on here?  Are females being forced to copulate by other males
or are they "cheating" on their resident partner?
  • We've seen that in one situation female Tree Swallows can easily reject males
    trying to copulate.  So in this situation at least "extra-pair" young aren't the
    result of males forcing mating.  Females must be willing or copulation won't
  • But perhaps there are times when a female may encourage males other than
    her resident male to mate with her.
  • Remember, both males and females "want" to pass on their genes to
    successful descendants, ones that are "good at being Tree Swallows."  

From the viewpoint of a female Tree Swallow "wanting" to pass on her genes:
  • Maybe the only reason she paired with her nest male in the first place was
    because he possessed a nest cavity, so she accepts him as her mate because
    she may not get another chance to produce offspring in her short life.
  • Perhaps she is aware of other males whose physical appearance and behavior
    have shown her they have "better genes" than the male at her nest, so she
    allows or seeks copulations with these males in order to produce the most "fit"
    descendants she can.  
  • In fact research has found that successful extra-pair males were usually older
    and experienced breeders, weighed more, had longer wings, had brighter
    plumage, and were in better condition generally than the resident male.  
  • Another possibility is that females seek to have young with more than one male
    because each male has a different combination of genes.  In this way she could
    raise descendants that differ widely in individual genetic makeup, increasing the
    chances that some might survive unpredictable future environmental conditions.

So resident male Tree Swallows face a dilemma.  Like the females, these
males want to pass on their genes, to have their own line of successful
  • For males time and energy spent raising young that aren't their own is totally
  • Is there anything resident males can do to assure they will be the biological
    fathers of the young in their own nests?
  • Most male songbirds must choose between two strategies for "paternity
    assurance," Mate Guarding or Frequent Copulation.

A male Tree Swallow's options are:
  • Mate Guarding:  He could follow his nest mate and guard her during the period
    when her eggs are ripening and they could be fertilized by another male's
    sperm.  He could prevent other males from approaching her, and prevent her
    from accepting or soliciting their copulations.  But if he followed her constantly
    the nest cavity he fought so hard for might be taken over by another male.
  • Frequent Copulation:  Instead of following his nest mate during her fertile
    period, he could concentrate on guarding his nest cavity but try to copulate with
    her as frequently as she allowed, both outside and inside the nest.  Then, even
    if she did copulate with other males, his own sperm would be numerous in her
    oviducts, increasing the chance that at least some young in his nest will be his.

So what tactic do male Tree Swallows actually use for paternity assurance?
Evidence for Mate Guarding could include:
  • Males that follow females to and from boxes.  
  • Most of the time either two or none are at the box.  Seldom is just one present.
Evidence for Frequent Copulation could include:
  • Pairs that are often apart during the female's fertile period.
  • When one of two leaves, the other stays at the box.
  • One is seen at the box alone frequently.
  • And, of course, frequent copulation.

Based on your observations do you think male Tree Swallows use mate
guarding or frequent copulation to assure their paternity?
  • You should be seeing more times when one of two left and the other stayed,
    rather than two leaving together or in close succession.
  • Because Tree Swallow males can't guard females and nest cavities
    simultaneously, they use frequent copulation as their primary paternity
    assurance tactic.  
  • Researchers found Tree Swallows averaged over 50 visible copulations per
    box, even though just a few would be enough to fertilize a female's eggs.
  • Mate guarding has been found to be weak or lacking in Tree Swallows, although
    it's the preferred tactic for many other songbird species.

There is another twist to this story.  Females must have a nest cavity to
reproduce, but male Tree Swallows have an alternate tactic for producing

Are eggs "costly" for females to make?
  • Yes, eggs are very large in relation to female body size, require a great deal of
    energy to produce, and must be packed with lots of nutrients.

Are sperm "costly" for males to make?
  • No, sperm are microscopic.  A male swallow's two testes, the glands that
    produces sperm, are very small most of the year, but during nesting they swell
    many times larger and can produce millions of sperm cells at very little cost to
    the male in terms of metabolic energy or materials.
  • A male Tree Swallow's "goal" is to pass on his genes.  Because he can make
    lots of sperm at little cost he should have lots to spare, so he may try to
    copulate with any willing female he can find.  
  • Males with nests can copulate with females from other nests.  In this way a male
    whose own mate "cheats" may father young in other nests.  
  • Even unpaired male "floaters," without nests of their own, have a chance to
    reproduce if they can copulate with and fertilize eggs of nesting females.
  • So, a female's reproductive output is limited to the young she raises from the
    eggs she lays, but a male's reproductive output could be much higher, if several
    females mate with him, and if his sperm are victorious in the "sperm wars" that
    take place against rival males' sperm within the females' reproductive tracts.

The lesson is that things aren't simple and clearcut in songbird reproduction,
and males and females can have different and possibly conflicting agendas.

Questions for the next Topic:  Eggs and Egg Laying.
  • What is an egg?  What's in it?
  • Why don't all females start laying on the same day?
  • Why don't all females lay the same number of eggs?

Mating in Tree Swallows
Learn About Birds at Tree Swallow Nest Box Projects