At the Project:
There is still lots of commotion around boxes.  Second-year (SY) swallows, last
year's youngsters that migrate north two to four weeks after older birds, are
arriving now, adding to the chaos.

SY males can't be distinguished from older males.  But SY females, (at left in
the picture below), can be told by their brownish upper bodies and heads.   If
this is the first year of your project many swallows may be prospecting SYs.














Somehow during the commotion of site claiming courtship occurs, and Tree
Swallows pair up.

Concepts:
Tree Swallow pairs seem to form very quickly, perhaps because both male and
female need to secure a nest site as soon as possible.  Pair formation involves
physical and vocal displays that signal each bird's species, its sex, and its
willingness to nest.  Courtship displays also help potential pair members
overcome initial mutual hostility, bond, and stimulate later stages of breeding.

Why would males and females that need to find a mate be hostile to one
another?  
  • Many, if not most songbirds act strictly as individuals much 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 (usually by the male) or to flee from the other (usually by
    the female) when the other approaches or is present.
  • Courtship displays play an essential part in defusing these feelings.
  • Once the male and female have overcome the tension of mutual
    hostility and fear, they seem to "know" each other.  They have become
    used to the other's presence and tolerate it with a minimum of friction.

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 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 successfully.

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 males.
  • 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 few parasites.

How can males "recognize" good females?
  • Good females show they can keep nest sites by vigorous defence
    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 few 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:  Swallows of either sex 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 these behaviors may be involved in pair formation, and there are
undoubtedly other, more subtle ones that are hard for us to see.

Be aware that pairs can be broken.  One member can die or desert.  Resident
males or females can have their nest sites taken over by other swallows.  
Watch for desertions and takeover attempts.














Questions for the next Topic:  Nest Building.
  • 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?

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Pair Formation
Learn About Birds at Tree Swallow Nest Box Projects