At the Project:
In spite of the thought, planning and care you put into your project there's bound to be
some mortality, due to things beyond your control.  Some eggs didn't hatch due to
embryo death or infertility, and you may have discovered dead young or adults.  Since
death is a constant possibility for songbirds, let's review common causes of mortality.  

Death comes in many ways for songbirds.  Life for most is short.  Tree Swallow
average lifespan has been calculated to be less than three years, similar to the
expectancy for many other familiar songbirds.  Most live through only a couple of
breeding seasons, if that.  Many die without leaving descendants.

Dangers to adults and young vary with species, season, and location.  The following
can cause of mortality of Tree Swallow adults, eggs and young during nesting.

Starvation and Hypothermia (exposure) of adults:
  • Prolonged periods of cold, wet, or windy weather that ground flying insects or
    make foraging difficult can starve birds that depend on them for food.
  • Adult Tree Swallows are potentially susceptible to cold, rainy or snowy spells
    because they migrate north so early.  
  • In bad weather many adults may crowd into one box to keep warm.  But some
    swallows in these "communal roosts" may die, as the seven emaciated, feces-
    smeared, barely recognizable swallows below did.  You may find others dead or
    dying on the ground below boxes.  

  • The swallow below was caught by cold spring weather and, unable to find food,
    burned its own tissues to keep alive.  Notice how the flight muscles that
    attached to its breastbone shrank to the point where flight was impossible.  
    Once its last resources were gone it died of starvation and hypothermia.

Chilling/Hypothermia and starvation of nestlings:
  • Nestling swallows are utterly dependent on their parents for food, and face
    disaster when most insects stop flying during cold, wet, windy weather.
  • Prolonged cold temperatures are most apt to kill because the young must go
    without energy from food just when they need it most to survive.
  • Chilled young need extra brooding by their mother, but this reduces her
    foraging time under conditions when adults already have difficulty finding food.
  • Research has shown nestlings 6-9 days old are most vulnerable, because they
    aren't fully able to generate enough of their own body heat and their feathers
    aren't developed enough to provide sufficient protection from heat loss.  
  • Starvation and chilling take the smallest and weakest first (see below: 3 dead at
    left, 2 survivors at right), but all may die if food intake is cut dramatically by
    prolonged bad weather.

Overheating/Hyperthermia of nestlings:
  • Tree Swallow nestlings can also die from too much heat (hyperthermia),
    especially if temperatures inside a box exceed 35 degrees C (95 degrees F).
  • Adding ventilation holes to boxes in hot locations can reduce the chance of
    overheating.  Vents that can be plugged during cold snaps are best.
  • Boxes with interiors at least 5" x 5", which allow young in a brood to spread out
    and cool off, also help prevent hyperthermia.  Please don't use small boxes.

Loss of a parent:
  • Since two adults need to work hard to feed both their nestlings and themselves,
    loss of one parent may doom some of the nestlings, unless the remaining
    parent has access to an exceptionally good food supply.
  • Females appear to be much better able than males to raise young on their own.
  • If you are certain, absolutely certain, that the female or both parents of a Tree
    Swallow brood have died or deserted, consider contacting a rehabilitator with
    experience raising and releasing insectivorous nestlings.
  • To find wildlife rehabilitators near you use either of these links:                  or

Nest site competition from other species (Interspecific Competition):
  • Other cavity nesting species may take a swallow's nest site, killing  adult
    swallows, eggs, or nestlings in the process.
  • House Wrens are major competitors for nest cavities in or near trees and
    shrubs.  Missing eggs, eggs with double holes in them, and broken eggs
    beneath boxes are typical of wren damage.  The swallow nest in the box below
    facing front is in grave danger from the lurking wren.  (You've placed your
    boxes out in open fields away from shrubs to eliminate wren competition).  For
    more on House Wren competition click here.

  • House (English) Sparrows (below left) are also major competitors for
    cavities.  They will kill adult Tree Swallows they trap in a box, and it's common to
    find dead swallows and bluebirds buried under House Sparrow nests.  In the
    picture below right by Dick Stauffer, several dead adult Tree Swallows are being
    covered by House Sparrow nest material.  (You've located your boxes as far as
    possible from houses and other buildings where House Sparrows congregate).  
    Please note: House Sparrows are not native, and not protected by law.  If you
    find House Sparrows using one of your boxes remove their nest and relocate
    the box and other nearby boxes not being used by swallows to safer places.  If
    the problem persists you may need to relocate your entire project next year.  To
    learn more about House Sparrow competition click here.

  • Bluebirds outweigh Tree Swallows (30 gm to 20 gm).  Determined bluebirds
    can oust swallows and destroy nest contents.  There are ways to minimize
    bluebird-swallow competition, such as hanging two boxes on one post or
    pairing boxes on separate poles.
  • Small entrances and field location discourage competition from large birds like
    Starlings, American Kestrels, Screech Owls, Flickers, and Grackles, but
    Tree Swallows nesting in natural cavities may be vulnerable to all these species.

Nest site competition from other Tree Swallows (Intraspecific Competition):
  • You've seen that Tree Swallows, both females and males, compete fiercely for
    limited nest sites.  Fight losers can die if trapped, pinned down, and struck
    repeatedly on the head by the victor's bill.  
  • Examine dead adults for head wounds.  They may be barely noticeable, like the
    puncture wounds behind the eye of the bird on the left, or obvious like those on
    the bird to the right.  Photo at right by Dick Stauffer.  (Note: House Sparrows
    also kill by this method and leave similar marks).  

  • Infanticide is a special type of intraspecific competition.
  • Nestlings are sometimes killed by floating adults that want to take over a nest
    site, demonstrating again how intensely these birds compete for a chance to
  • A takeover male may throw out the nest male's young if they are small enough
    to carry, or peck older young to death.  This infanticide may induce the resident
    female to re-mate with the new male and produce a new set of young that are
    his own descendants.
  • Floater females may kill small young to force resident females to yield their nest
  • Be aware of the role infanticide may take in takeover attempts.  Discovering
    small young with wounds like the bloody gash on the head of the nestling below,
    or that small nestlings have disappeared from a box, may signal that a takeover
    attempt is in progress.  But keep in mind resident adults may remove small
    nestlings that die of other causes.
  • If you find a nestling with a wound such as that below, daily application of an
    anti-bacterial ointment such as Neosporin may aid healing.

  • Many animals search for bird nests and eat eggs, nestlings and adults they find.
  • Mammals preying on Tree Swallow nests include raccoons, cats, weasels,
    squirrels, chipmunks and possums.  Mammalian predators may tear up nests,
    pull nest material out the hole, or leave uneaten wings and feet on the ground
  • Nests where all nestlings disappear at once but the nest is undisturbed may
    have been predated by a constricting snake able to climb to the cavity.
  • You've heard Tree Swallows give "Alarm Calls," and you've probably been
    "mobbed" and dived on.  These behaviors are meant to distract or scare
    predators, but in reality there isn't much Tree Swallows can do to stop
    determined predators from reaching their nests.
  • Some large birds, including Crows, Jays and Grackles, raid nests of other
    birds.  Tree Swallows use calls, dive-bombing, mobbing and even physical
    attacks to drive avian predators away, and sometimes they succeed.  The
    swallow in the photo below from Kay and Bert Grant has actually grabbed the
    back of a nest-raiding Magpie with its feet and is pecking its head.

  • You've done your best to prevent nest predation by locating boxes away from
    predator lanes, and by using pole guards and grease bands.
  • Grease bands also deter ants, which can infest nests, and torment or kill
  • Even such superb fliers as Tree Swallows can be caught by raptors like Sharp-
    shinned Hawks, Cooper's Hawks, Merlins, and Kestrels.  Look carefully and
    you'll see that the Merlin in the photo below from Tony Leukering 2011 is
    carrying a Tree Swallow in its talons.  Inexperienced, slower-flying juvenile
    swallows are most vulnerable.  Placing boxes well out in open fields allows your
    swallows to spot incoming raptors at a distance.  

  • Like all birds, Tree Swallow adults and nestlings are susceptible to viral,
    bacterial, fungal and protozoan diseases.  The growth distorting the bird's bill
    bird below is suggestive of Avian Pox, a viral disease.  Photo by Dick Stauffer.

  • Note the eye of the dead adult below.  It may have died from complications of
    Avian Pox or perhaps from Avian Conjunctivitis, a bacterial disease.

  • Diseases tend to hit individuals with stressed immune systems the hardest, like
    those already weakened by starvation, cold, injuries, or parasites.

  • Tree Swallows and their nests often harbor ectoparasites, such as blowfly
    larvae, fleas, and mites, that suck body fluids of nestlings and adults.  The little
    dots on the mud rim of the Barn Swallow nest below are mites.

  • Very few ectoparasites have been proven to kill swallow nestlings directly.
  • However, ectoparasites can definitely stunt nestling growth, weaken them, make
    them more vulnerable to disease, and less apt to survive after fledging.
  • Parent swallows may refuse to enter boxes having heavy infestations of mites,
    abandoning their nestlings to starve.  Click here for control of nest mites.
  • Songbirds like swallows also host a variety of internal, or endoparasites.
  • Click here to view the Purple Martin Conservation Association's outstanding
    page on ectoparasites and their effects.  Purple Martins are large swallows that
    host many of the same type of parasites as Tree Swallows.
  • Also see Parasites for post-nesting checks for ectoparasites.

Developmental Abnormalities:
  • Some Tree Swallow embryos fail to develop normally for a variety of causes.
  • The result may be lethal to embryos in the egg or later to young swallows during
    nestling stages or even after fledging.
  • As long as the nestling below was being cared for by its parents it may have
    survived, but its abnormal foot would have jeopardized its chances as an
    independent bird. Photo below by Dick Stauffer.

Human activity:
  • Swallow mortality can be caused by deliberate vandalism and desertion of nests
    with death of eggs and nestlings may follow disturbance by curious people.
  • Contamination of local environments and food supplies by toxic pesticides,
    herbicides, and industrial pollutants may also threaten swallow survival through
    disruption of their bodies' internal metabolic processes.
  • Collision with vehicles can take a toll when nest sites are located near roads
    and after nesting when swallows sun themselves on road surfaces.  Photo  
    below of a road-killed juvenile Tree Swallow by Laura Erickson.

Questions for the next Topic:  Older Nestling Development 13 Days to
  • Why must box checks stop after nestling day 12?
  • What behaviors indicate nestlings will soon be leaving their nests

Causes of Mortality
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