At the Project:
Still more Tree Swallows have arrived.  Activity may seem very frenzied and confusing
to you as individuals and groups of swallows fly from box to box displaying, chasing
and calling excitedly.  Everything appears unsettled because in fact they aren't yet
settled.  The crucial competition to claim boxes for nest sites is underway.

Birds show tremendous diversity in nest 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 a major factor limiting the population size of the
    Tree Swallow species.

So who does make the nest cavities?
  • Under natural conditions Tree Swallows usually use old woodpecker holes or
    holes in trees where wood has rotted away.

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

Are there lots of suitable nest holes around?
  • No, there usually aren't enough cavities to go around.  This is why Tree
    Swallows sometimes nest in what may seem to us very odd places, such as
    inside the gas supply equipment in Karl Overman's photo below.

Do Tree Swallows compete with other bird species for nest sites?
  • Yes, "interspecific competition" for nest cavities is very common.
  • Tree Swallows may compete with bluebirds (see below), wrens, chickadees,
    martins, 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 occur.

Who are a Tree Swallow's biggest competitors apt to be?
  • Other Tree Swallows, because they have identical nesting requirements.
  • "Intraspecific competition", competition with members of one's own species,
    can be intense for many birds.

Nest site competition is the main reason Tree Swallows migrate north so early.
  • Male Tree Swallows usually arrive first.  They try to find and claim nest cavities
    as soon as possible on order to beat potential competitors, especially other
    male Tree Swallows, to the sites.
  • Females begin to arrive a few days to a week after the first males.

Intense competition for nest sites is one of a Tree Swallow's major life issues,
for both males and females.
  • If a female doesn't obtain a nest cavity she can't reproduce.  For these short-
    lived birds missing even one nesting season is a serious failure, so females
    often compete very aggressively for nest sites.
  • Males compete for sites also, but as we'll see later, males that don't possess
    nest cavities may still be able to father young.

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 to repel intruders varies with individuals and by how close an intruder
    approaches and how determined it is.  The photo below is by Elinor Schindel.

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 the Entrance Hole: guarding or advertising the cavity entrance,
    often with chattering and wing fluttering.
  • 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 (see below).

  • Chasing Intruders:  Rapid flying pursuit of other swallows, attempting to drive
    them away from a box.  
  • Fighting:  Competition for boxes may escalate.  Mid-air battles may occur with
    leg grappling, wing beating, gaping and biting.  Combatants sometimes fall to
    the ground together and continue the struggle there.  Photo below from Rick
    Brown,, of two males fighting.   

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, and/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 usually defend only their nest cavity plus a
    relatively small area around it.  Photo below from Tim van Nus shows a Tree
    Swallow on its territory on treeless Middleton Island, Alaska.

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 the way many other 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 do not form closely-spaced
    clusters of nests the way Bank Swallows, Cliff Swallows, and Purple Martins do.

Who do you think is chasing and fighting during nest site claiming and
territorial defense, male swallows, female swallows, or both?
  • Both.  Males fight other males and females fight other females.  
  • Remember females can't reproduce if they don't successfully defend their nest
    cavities from other females.
  • Females often fight hard, sometimes even to the death, for chances to nest!
  • In the photo below one female has driven another female to the ground and is
    giving it a severe beating as they struggle for possession of a nest box.
  • Contests between female swallows may also occur inside nest boxes.  
  • Click here to view a YouTube video from Francois Paquette showing one
    female making an intense defense of its box against an intruding female.
  • Male Tree Swallows seldom interfere when females battle over nest sites.  

Questions for the next Topic:  Courtship and Pair Formation in Tree Swallows.
  • 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?

Nest Site Competition and Box Claiming
ATTENTION: As of 3/31/2021, websites using Yahoo SiteBuilder, as one this does, will no longer be supported.  
We have used WordPress to create a replacement,  This new website has a different
appearance but the same content.  We apologize for any inconvenience this change may cause you.