At the Project:
Still more Tree Swallows have arrived.  Activity really seems frenzied and
confusing as individuals and groups of swallows fly from box to box displaying,
chasing and calling excitedly.  Everything seems unsettled because in fact
they aren't yet settled.  The crucial process of nest site claiming is underway.










Concepts:
Where did these Tree Swallows spend the winter months?
  • Many Tree Swallows winter in the southern U.S., especially Florida and
    Louisiana.  Others winter farther south in Mexico, Central America and
    the Caribbean islands.

What's the advantage of wintering in the south?
  • Tree Swallows' primary food is insects.
  • Flying insects are rare during northern winters.
  • Tree Swallows concentrate at buggy southern wetlands in winter,
    sometimes in huge flocks.  Unfortunately many wetlands have been lost
    or degraded by development, agriculture and aquaculture.

But why did Tree Swallows return so early to the breeding grounds?  The
north isn't very buggy in March and April.
  • The swallows return early, risking death from starvation and
    hypothermia, for one reason, to try to get a nest site.
  • Their next 70-80 days will center on nest sites and their contents.

Birds show tremendous diversity in nests placement and construction.
Your Tree Swallows will nest in boxes, but where would they nest if there
weren't any boxes?
  • In tree cavities.  They always nest in cavities.

Do Tree Swallows make their own nest cavities?
  • No, Tree Swallows are "obligate secondary cavity nesters".  They must
    nest in cavities but can't make their own.  They are totally dependent
    on other creatures for nest sites.
  • One factor limiting their population size is a shortage of nest cavities.

So who makes the nest cavities?
  • Tree Swallows usually use old woodpecker holes or natural tree cavities.

Are there advantages in cavity nesting?
  • Cavities may protect from some predators.
  • Cavities may be warmer and drier for eggs and young.

Are there disadvantages in cavity nesting?
  • Predators can trap birds inside.
  • Secondary cavity nesters lose flexibility, depending on others to
    furnish a vital resource.

Are there lots of potential nest holes around?
  • No, there usually aren't enough cavities to go around.

Do Tree Swallows compete with other birds for nest sites?
  • Yes, there is lots of interspecific competition.
  • Tree Swallows may compete with bluebirds, wrens, chickadees, owls,
    kestrels, and some ducks and flycatchers.
  • Non-native House (English) Sparrows and Starlings can be severe
    competitors.
  • You've designed and placed your boxes to minimize interspecific
    competition, but you may still see some.

What are Tree Swallows' biggest competitors apt to be?
  • Other Tree Swallows.  They have exactly the same requirements.
  • Intraspecific competition can be intense for many songbirds.








So that's why Tree Swallows return so early.
  • They try to find and claim nest cavities as soon as possible, to beat
    competitors, especially other Tree Swallows, to the sites.
  • Intense competition for nest sites is one of a Tree Swallow's major life
    issues.
  • Females that don't get nest cavities can't reproduce.  For short-lived
    birds missing a nesting season is a serious failure.
  • Females won't form a pair with a male unless he possesses a nest site.
  • Males that don't get nest cavities may still be able to breed, as we'll see
    later.

Did you notice intraspecific competition among Tree Swallows for nest sites,
or were boxes being claimed without a lot of commotion and squabbling?

You should see lots of intraspecific competition.  Most of the following
behaviors are given by box claimers when other swallows approach.  Box
claimers are stimulated to defend or advertise their boxes.









Behaviors you should see during nest site claiming include:
  • Chatter Call (also called "nest site advertising call"): A loud, rapid
    monotone chatter.  The body is often flattened with wings fluttering
    and head aimed at another swallow as the call is made.  The chatter is
    normally given by males to other males, and seems to mean "this site is
    mine, keep away".  When given to females it can mean "this site is
    mine, come and see".
  • Perching at Hole: guarding or advertising the cavity entrance, often
    with chattering and wing fluttering.
  • Chasing Intruders:  Rapid flying pursuit of other swallows, driving them
    away from a box.  Mid-air fighting may occur.
  • Forcing Intruders Away from boxes:  Not outright chasing, but
    preventing close approach or landing of other swallows by use of song,
    call, or threat displays such as bill snapping and open-mouthed gaping.

So how are boxes actually claimed?
  • It's confusing to us.  It seems like a lot of commotion, but the swallows
    know just what's happening.
  • Somehow through a combination of perching at a site, entering and
    examining the cavity, advertising it with song and physical display, and
    chasing and fighting persistent competitors, nest site ownership is
    established.

Are Tree Swallows territorial?  
If so, how large is their territory and what does it include?
  • Tree Swallows only defend a cavity and a small right area around it.

Why are their territories so small?
  • Tree Swallows don't defend a feeding territory around their nest the
    way many birds do.  That allows us to space boxes 100' apart.
  • In contrast, species like bluebirds must defend enough territory to
    supply themselves and their young with food.  Bluebird boxes should be
    at least 300' apart.

Many pairs of Tree Swallows can nest in a small area.  Does this mean they
are colonial?
  • Although some people consider them "semi-colonial", in nature most
    Tree Swallow nests are not tightly grouped.  They are not colonial in
    the way Bank Swallows, Cliff Swallows, or Purple Martins are.

Who do you think is chasing and fighting during nest site claiming and
defense, male swallows, female swallows, or both?
  • Both.  It's not just males.  Remember females can't reproduce if they
    don't successfully defend cavities from other females.
  • Females often fight hard, sometimes to the death, for chances to nest!








Questions for the next Topic:  Pair Formation.
  • What does a male Tree Swallow "want" in a female mate?
  • What does a female Tree Swallow "want" in a male mate?
  • What are the consequences if they make the wrong selection?

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Nest Site Claiming
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