Mortality Factors

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, 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 especially 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.

Nestling swallows, being utterly dependent, face disaster when food supplies
become inadequate or interrupted by cold, wet, windy weather. Chilled
young need extra brooding, but this reduces foraging time under conditions
when adults already have difficulty finding food. 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 if bad weather is prolonged 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 exceptionally good 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. 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).

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, bluebirds, or wrens buried under House Sparrow nests. In the
picture below right 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 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 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. (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 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.

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 probably been
"mobbed" and dived on. These are pretend attacks, 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.
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
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 warty growths on the foot and bill base of the SY female below
are suggestive of Avian Pox, a viral disease. (Photo by Tim van Nus).

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.

Disease tends to hit individuals with stressed immune systems the hardest,
like those already weakened by starvation, cold, injuries, or heavy parasite
loads.

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
little dots on the mud rim of the Barn Swallow nest below are 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 when nest sites are located near roads
and after nesting when swallows sun themselves on road surfaces.

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

Questions for the next Topic: Older Nestlings.