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 usually unavailable during northern winters.
  • So Tree Swallows concentrate at buggy southern wetlands in winter,
    sometimes in huge flocks.  
  • Unfortunately, many wetlands on the wintering grounds have been lost or
    degraded by development, agriculture and aquaculture.

But why do Tree Swallows return to the breeding grounds so very early?  Isn't this
dangerous?  After all, 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 nest sites so they can have a chance to reproduce.  
    The next 80-90 days of their lives will center on nest sites and their contents.
  • Tree Swallows caught in early-season cold weather must struggle to stay alive.
  • Note how the swallows in Colin Haigh's photo below have huddled together to
    conserve body heat.  Some have actually perched on the backs of others!  
    They have also fluffed up their feathers to trap more air underneath, which
    increases its insulation value.
















  • Another form of huddling to conserve heat is "communal roosting", in which a
    group of birds crowds into a cavity or nest box overnight.  This behavior
    probably saves the lives of many Tree Swallows each year, but in spite of these
    efforts birds weakened by days with little heat or food may die.  
  • Be warned.  You may find dead adult Tree Swallows inside your boxes in spring,
    killed during cold snaps.  The seven below were removed from a single box.













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 of some sort.

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 cavities
    made by other creatures or natural tree holes for nest sites.
  • This shortage of nest cavities is one factor limiting the population size of the
    Tree Swallow species.

So who does make the nest cavities?
  • Tree Swallows usually use old woodpecker holes or naturally occurring holes in
    trees.

Are there advantages in cavity nesting?
  • Cavities may give protection 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 suitable nest holes around?
  • No, there usually aren't enough cavities to go around.

Do Tree Swallows compete with other bird species for nest sites?
  • Yes, there is lots of potential for "interspecific competition".
  • Tree Swallows may compete with bluebirds (see below), 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, because they have identical requirements.
  • "Intraspecific competition" can be intense for many songbirds.















So that's why Tree Swallows return so early.
  • Male Tree Swallows usually arrive first.  They try to find and claim nest cavities
    as soon as possible, to beat potential competitors, especially other male Tree
    Swallows, to the sites.
  • Females begin to arrive a few days to a week after males.
  • Intense competition for nest sites is one of a Tree Swallow's major life issues.  
    This goes for both males and females.
  • 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 father young, as we'll see
    later.

Did you notice intraspecific competition among the swallows for nest sites, or were
boxes being claimed without a lot of commotion and squabbling?
  • You should see a great deal of intraspecific competition.  
  • Most of the behaviors described below are given by box claimers when they
    are stimulated by the approach of intruders to defend or advertise their boxes.
  • Whether a box claimer uses calls, songs, physical displays, chases, or actual
    fighting (see below in the photo by Elinor Schindel) depends on the sex of the
    intruder and how close and determined its approach is.















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 gaping open-mouthed.















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.

What is a territory?
  • A territory is a place or area that one or more animals defend.
  • Songbirds territories have three main uses: courtship, nesting, or feeding.
  • A single territory can serve more than one of these three functions.

Are Tree Swallows territorial and, if so, how large is their territory and what does
it include?
  • Tree Swallows are territorial but they only defend their nest cavity plus a small
    right area around it.

Some songbirds have much larger territories.  Why are Tree Swallow territories so
small?
  • Tree Swallow territories are used just for courtship and nesting.  They don't
    defend feeding territories around their nests then way many birds do.  
  • This fact allows us to space our swallow boxes just 100' apart.
  • In contrast, species like bluebirds must defend enough territory around their
    nests to supply themselves and their young with food.  This is why bluebird
    boxes should be spaced at least 300' apart.

Many pairs of Tree Swallows can nest in a relatively 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 territorial
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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