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 controls
whether a copulation attempt is successful.

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

Concepts:
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.
  • 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 determine whether males' copulation attempts
    are successful or not.

What is this "cloaca"?
  • Birds have just a single 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
    nesting.  This is a swelling of lower portions of the ducts carrying sperm
    from a male's testes to his cloaca.
  • The cloacal protuberance acts as a temporary storage chamber for
    sperm.  It sticks out slightly and helps males make cloacal contact with
    females sperm transfer.
  • The cloacal protuberance below is typical of a breeding male swallow.











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 your females
    don't sneak eggs into other females' nests like some bird species do.  
  • But the same studies revealed many nests contain young fathered by
    more than one male.  In some nests most or even all young were
    fathered by other males.  This is also found in many other bird species.

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:
  • 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.
  • 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 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.

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,
    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 size, and require lots of
    nutrients and energy to make.

Are sperm "costly" for males to make?
  • No, sperm are microscopic.  Males produce millions at little metabolic
    cost.

A male Tree Swallow's "goal" is to pass on his genes.  Since he can make lots of
sperm at little cost he may have some 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, and without having to spend time and energy
defending a nest and raising young.

So while female reproductive output is limited to young she raises from eggs
she lays, male output is potentially much higher provided enough females are
receptive to him and his sperm win in the "sperm wars".

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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