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.  















Concepts:
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 expectancy for many 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 are common causes of mortality of Tree Swallows during nesting.

Starvation and Hypothermia (exposure):
  • Prolonged periods of cold and/or wet weather that ground flying insects
     can starve birds that depend on them.
  • Adult Tree Swallows are especially susceptible to cold, rainy or snowy
    spells because they migrate north so early.  In bad weather many adults
    may crowd into one box, where some may die, as the seven emaciated
    swallows below did.  You may find others dead or dying below boxes.  
    Still others die elsewhere out of our view.








  • Nestling swallows, being utterly dependent, are also vulnerable when
    food supply is inadequate or interrupted.  Starvation and exposure
    take the smallest and weakest first (see below: 3 dead at left, 2
    survivors at right), but all may die if food is cut off by bad weather for
    several days.







Loss of a parent:
  • Since two adults need to work hard to feed both their nestlings and
    themselves, loss of one parent normally dooms some of the nestlings,
    unless the remaining parent has access to an exceptional food supply.

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.  (You've placed your
    boxes out in open fields to eliminate wren competition).
  • House (English) Sparrows 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, bluebirds, or wrens buried under House Sparrow nests.  
    In the picture below by Dick Stauffer, several dead 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 any other
    nearby boxes not being used by swallows to a safer place.  If the
    problem persists you may need to relocate your project next year.







  • 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.
  • 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.  (Picture to right by Dick
    Stauffer).  (Note: House Sparrows also kill by this method and leave
    similar marks).  






Infanticide:
  • 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 reproduce.
  • 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 sites.
  • Be aware of the role infanticide may take in takeover attempts.  
    Discovering small young with wounds like head gash on 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.









Predators:
  • Many animals search for songbird 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 below.
  • Nests where all nestlings disappear 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 been
    "mobbed" and dived on.  These are anti-predator behaviors, but there
    really isn't much Tree Swallows can do to stop determined predators
    from reaching nests.
  • You've tried 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 nestlings.
  • Even such superb fliers as Tree Swallows can be caught by raptors like
    Sharp-shinned Hawks, Cooper's Hawks, Merlins, and Kestrels.   
    Inexperienced, slower-flying juvenile swallows are most vulnerable.  
    Placing boxes well out in open fields allows your swallows to spy
    incoming raptors at a distance.

Disease:
  • Like all birds, Tree Swallows are susceptible to viral, bacterial, and
    protozoan diseases.  The swallow below appears to have died from
    complications of an "eye pox".
  • Disease tends to hit individuals with stressed immune systems hardest,
    like those already weakened by starvation and cold.






Ectoparasites:
  • Tree Swallows and their nests often harbor ectoparasites, such as
    blowfly larvae, fleas, and mites, that suck body fluids of nestlings and
    adults.  The red dots on the flange of the nestling below are
    bloodsucking mites.






  • Very few ectoparasites are proven to kill nestlings directly.
  • However, ectoparasites can definitely weaken nestlings, making them
    vulnerable to disease and less apt to survive after fledging.
  • Songbirds like swallows also host a variety of internal, or endoparasites.
  • See Parasites for more information on ectoparasites and their effects.

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. (Picture by Dick Stauffer).






Human activity:
  • Two other sources of swallow mortality are vandalism and desertion of
    nests disturbed by over-curious people.
  • Contamination of environment and food supply by pesticides,
    herbicides, and industrial pollutants are also threats.
  • Collision with vehicles can take a toll, especially where nest sites are
    located near roads.






An additional, very comprehensive discussion of Tree Swallow mortality can
be found at www.sialis.org/tresdead.

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

                                                     Top
Mortality
Learn About Birds at Tree Swallow Nest Box Projects