At the Project:
Nest building is proceeding nicely.  Listen now for Ticking Calls, chittering "tick,
tick, tick"s 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.

Concepts:
Mating is the single most important event in a bird's life, because it's the way his or
her genes are passed into the next generation.

How do Tree Swallows mate?
  • The male makes rapid Tick Calls as he flies toward the perched female to
    attempt copulation.
  • The female perches with her back and tail held horizontal.











  • The male lands on the female's back and, using his wings for balance, 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 more copulation.











If a female is not receptive, what can she do?
  • She can prevent a male from landing by not leveling her body, or by flipping her
    tail up when he tries to land.
  • She can twist her head around and threaten him with her bill open.
  • So female Tree Swallows' behavior determines whether males' copulation
    attempts are successful or not.

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 released to the outside through the "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.
  • 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 partially everted "CP" in Euan Reid's photo below.

























Extra-pair Young:
  • DNA studies of Tree Swallows show that nest females are the biological
    mothers of all the young in their nests.  In other words female swallows don't
    sneak eggs into other females' nests like some bird species do.  
  • But the same studies revealed most nests contain young fathered by more
    than one male.  In some nests most or even all young were fathered by other
    males.  Many nests had young fathered by 3 to 6 different males.

What's going on here?  Are females being forced to copulate by other males or are
they "cheating"?
  • We've seen that female Tree Swallows can easily reject males trying to
    copulate.  So "extra-pair" young aren't the result of males forcing mating.  
    Females must be willing or copulation won't succeed.
  • But why would a female allow other males 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 a female Tree Swallow's point of view:
  • Maybe the only reason she paired with her nest male was because he possessed
     a nest cavity.
  • Maybe she "knows" other males that are "better" genetically, so she allows or
    seeks copulations with these males to produce the "fittest" descendants she
    can.
  • Maybe she "hedges her genetic bets" by having young with more than one
    male, giving her descendants with more genetic variety, so that some may
    have the best combination of genes for survival.

So male Tree Swallows, and other male songbirds, face a dilemma.  Males also want
to pass on their genes, to have their own successful descendants.  
  • For males time and energy spent raising young that aren't their own is totally
    wasted.
  • Is there anything males can do to assure they will be the biological fathers of
    the young in their own nests?
  • Male songbirds usually 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 the nest cavity but try to copulate
    with her as frequently as she allowed.  Then, even if she copulated with other
    males, his own sperm will be numerous in her oviducts, and there would be a
    good chance 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 copulations per box, even
    though just a few would be enough to fertilize a female's eggs.
  • Mate guarding is 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 descendants.

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 tiny most of the year, but during nesting they swell
    hundreds of times larger and can generate millions of sperm cells at very little
    cost in metabolic energy or materials to the male.
  • A male Tree Swallow's "goal" is to pass on his genes.  Since 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.  
  • 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 while female reproductive output is limited to the young she raises from
    eggs she lays, male reproductive output is potentially much higher provided
    enough females are receptive to him and his sperm win in the "sperm
    competition" with sperm of male rivals.

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:  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?










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Mating
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