At the Project:
There's still lots of commotion around boxes because, in addition to box claiming,
courtship and pair formation are also underway.  Plus, young, second-year swallows,
last year's nestlings that migrate north two to four weeks after older birds, are arriving
now, which is adding to the chaos.  The group in Ed Craft's photo below reached
Indiana in early May, well after the older Tree Swallows had returned.  

Note the plumage differences in these birds.  Second-year Tree Swallow males can't
be distinguished from older males;
all breeding age males have blue upper bodies.  
To complicate things even more, older, after-second-year (ASY)
females have the
same blue plumage as males!  But SY females can be told by their mostly brownish
upper bodies and heads.  So, of the 11 perched Tree Swallows in the photo above, 9
are brownish SY females with varying amounts of blue, and the 2 blue birds are either
SY males, ASY males, or ASY females, we can't tell which!  For more on the confusing
subject of determining sex and age of Tree Swallows click

If this is your project's first year a majority of your swallows may be SYs that have
never nested before.  In older, well-established projects you can expect a lower
percentage of females will be brownish SYs.

Although we may have trouble telling their sexes apart, the Tree Swallows have no
such difficulty, and somehow, during all the commotion of box claiming, courtship
occurs, and the swallows pair up.  And it happens fast, perhaps because both male
and female must find a nest site as soon as possible and defend it from all rivals.  

What is courtship and what are its purposes?
  • Courtship is the group of special vocal and physical displays a bird uses to
    signal others of that bird's species of its sex, its "quality," and that it is ready
    and willing to nest.  
  • Courtship displays are crucial in helping potential pair members overcome their
    normal feelings of mutual hostility and fear, so they can form pair bonds.
  • Courtship also helps synchronize the internal physical reproductive states of
    the pair, and stimulates later stages of nesting.

Why would males and females that need to find a mate be hostile to one
another or afraid of each other?
  • Aggression, fear, and sexual motivation to breed are all part of pair formation.
  • Many, if not most songbirds act strictly as individuals most of the year, but
    during the nesting season they must form partnerships with individuals of the
    opposite sex in order to reproduce.
  • To form this partnership the two birds must overcome instincts to act
    aggressively or to flee from the other when the other approaches or is present.

  • Courtship displays play an essential part in defusing these hostile or fearful
    feelings and actions.
  • Once the male and female get to "know" each other personally the tension of
    mutual hostility and fear gradually goes away.  They become used to the
    other's presence and tolerate it with a minimum of friction.

What is the typical sequence of events in Tree Swallow pair formation?
  • Males must successfully claim a nest cavity first, because females often won't
    pair with males unless they posses this most vital resource.
  • Males use displays and calls to point their cavity out to passing females.
  • Females that do approach may receive a hostile response from the males at
    first, the same type of response they would give to repel other males intruding
    into their space.
  • Females that are really interested must overcome their fear and return
    repeatedly, giving displays that identify themselves as females and gradually
    pacify the males.
  • Once the male comes to recognise a particular female as a non-threatening
    individual and a potential mate his hostility lessens, and the two can begin to
    use other displays to cement their pair bond.

How can you tell that a pair is established at a box?
  • You will see two swallows perched together at a box, defending it.
  • Males mainly defend against other intruding males, while females defend
    against intruding females.

What does a male Tree Swallow "want" or "look for" in a female?
  • A mate that is "good at being a Tree Swallow."  In other words a female that
    demonstrates by her behavior and physical attributes that she has the right
    combination of genes and experience for keeping herself alive and reproducing

What does a female Tree Swallow "want" or "look for" in a male?
  • The same that males want: a mate with genes and experience for keeping
    himself alive and reproducing successfully.

Therefore, Tree Swallows, and other birds, must be able to assess potential partners
and "recognize" genetically-based "quality."  

What happens if a "less fit" partner is accepted, one that isn't so good at
Tree Swallow life skills or at reproducing?
  • Swallows that choose inferior mates may leave fewer descendants.
  • Leaving descendants is the one and only reason for nesting.

How can females "recognize" good males?
  • Good males usually possess nest sites (but not always, as we will see).
  • Good males show they can keep nest sites by vigorous defense against other
  • Good males show strong, agile flight during chases.
  • Good males have healthy plumage.
  • Good males use proper songs, calls, and displays to attract females so females
    can gauge their qualities.
  • Good males show less evidence of parasites.

How can males "recognize" good females?
  • Good females vigorously defend nest sites against other females.
  • Good females show strong, agile flight during chases.
  • Good females have healthy plumage.
  • Good females respond properly to male courtship behaviors.
  • Good females initiate proper female courtship behaviors.
  • Good females show less evidence of parasites.

Some Tree Swallow nest claiming behaviors are also used in pair formation.  
Such dual purpose behaviors include:
  • Chatter Call:  Females will not pair with a male unless he possesses a nest
    site.  Chatter calls are given by males to encourage females to come look at
    their sites.  This call seems to proclaim  "I'm tough and strong.  I've claimed a
    nest site and defended it.  Come check it out."
  • Perching at Hole:  Males often perch at entrance holes, chattering and
    fluttering their wings, pointing the cavity out to passing females.
  • Chasing Another:  Males may chase females and vice versa, which allows
    potential pair members to display their flying abilities.
  • Gaping:  Both male and female swallows will gape, open-mouthed (see below),
    when the nearness of another swallow stimulates mixed feelings of hostility and
    fear.  Gaping may help defuse the tension caused by these feelings, allowing a
    male and female to tolerate each other's presence.

Some other behaviors seem just for pair formation.  While they aren't as
common as those listed above you may see:
  • Vertical Posture:  Male perches on box with body and head held steeply
    upright with bill pointed skyward.
  • Courtship Pounce:  Female response to male's Vertical Posture.  Female
    approaches perched male and displaces him, perching in his former spot.  Male
    may then Perch at Hole and Chatter.
  • Bobbing and Billing:  Perched pairs face each other, bobbing heads up and
    down, sometimes making contact with their bills.
All the above behaviors may be involved in courtship and pair formation, and
there are undoubtedly other, more subtle ones that are hard for us to see.

Be aware that pairs once formed may be broken:
  • One member can die or desert.  
  • Resident males or females can have their nest sites taken by other swallows.
  • Watch for desertions and takeover attempts.  They may occur quickly and
    without much warning.

Also be aware that about 5% of males in multi-box projects successfully claim
more than one nest box and pair with two females, one at each box.
  • This may be advantageous for males, who can potentially father more young.
  • However, if a male successfully claims two nest sites and has a female and
    young in both, he usually concentrates his feeding efforts at one nest only,
    especially as the young get older.  
  • The burden of feeding young at the other nest falls mostly or entirely on its
    female, and it has been found that fewer young are usually produced if just one
    parent brings food.

Questions for the next Topic:  Nest Building in Tree Swallows.
  • What is the purpose of a nest?  Why have one?
  • Have the Tree Swallows finished building their nests?  
  • If they haven't finished, or have barely started, what's taking them so long?

Courtship and Pair Formation
in Tree Swallows
Learn About Birds at Tree Swallow Nest Box Projects