At the Project:
The Tree Swallow nesting season is over.  Your swallows have gone now, but as you
go about your days you may find yourself wondering where they are at times and
what's next in their lives?  Hint: after nesting is completed Tree Swallows become
social!   The photo below by Ken Schneider shows a small part of a
much larger flock.

What do Tree Swallows do when not nesting?
  • For almost nine months of each year Tree Swallows are not nesting.  But
    everything they do during this time is geared to staying alive so they can try to
    nest next season.  That's really what a songbird's life is about.
  • The off season is a vital time.  In the interval between nesting seasons many
    songbirds molt one or more times, replacing all or most of their feathers.  And
    while a few species stay where they nested, many others migrate away to spend
    winter where they have a better chance of survival.

Why do Tree Swallows flock together when not nesting?
  • Most Tree Swallows move to large marshes soon after nesting.  The attraction
    seems to be abundant food and beds of cattails or reeds for roosting at night.
  • At these stopover or staging places juveniles can become adept at finding and
    capturing food, and both juveniles and adults can build their energy reserves.  
  • Now, instead of isolated territorial breeding pairs you are more apt to find Tree
    Swallows in large flocks of from hundreds up to hundreds of thousands!  
  • During these summer months various swallow species often flock together.  The
    photo below shows a mixed group of Bank and Tree Swallows.   
  • Since many birds form flocks there must be advantages.  But please note that
    the benefits of flocking can vary from species to species, and the reasons Tree
    Swallows flock may be different from those of some other songbirds.

One reason birds flock: more eyes and ears can detect predators easier.
  • If a group has many members some will inevitably be looking and listening for
    danger at any given moment, so the rest should be able to spend more time
    searching for food and eating.  
  • Flocking may also make it harder for predators to single out prey from among
    the confusion of many moving birds.   
  • However, flocking can have disadvantages.  Conspicuous flocks may attract
    predators like the Merlin below, which has just captured a Tree Swallow and
    appears to be severing its spine.  Photo by Catherine Hamilton of Birdspot.

Another benefit of flocking: more eyes can discover food more efficiently.  
  • Bird species have been found to form feeding flocks if their food supply is found
    in scattered patches, especially if the food is only temporarily abundant.  
  • Species of birds that forage on irregular insect swarms, moving schools of fish,
    or seasonal crops of seeds, fruits, and berries, typically feed in flocks.  
  • A Tree Swallow's food is primarily flying insects, but unlike all other swallows
    Tree Swallows also eat certain small berries and seeds.  
  • Bird guides state Tree Swallows resort to berries in bad weather, but in truth
    they consume their favorite plant foods whenever they find them, regardless of
    the weather or season.  
  • We've often watched post-breeding flocks in western NY eating fruits of
    bayberry, shrub dogwood, red cedar, and arrowwood viburnum on hot, buggy
    August days.  The swallows grasp the berries in their mouths, pluck them with a
    twisting motion of their heads, and gulp them down whole.
  • The chart below shows how an average Tree Swallow's percent of plant food
    varies over a year.  You can see how important plants become after nesting.

  • So, since they prefer both flying insects and seasonally abundant plant foods
    it's no wonder that after nesting Tree Swallows flock up to search for food.
  • Tree Swallows are especially fond of fruits of waxmyrtle and bayberry bushes
    that grow in sandy soils near seacoasts.  They are one of the few birds able to
    digest the energy-rich waxy outer coatings of these berries.
  • Waxmyrtle and bayberry fruits grow in small, tight clusters (see below) that Tree
    Swallows can grab, twist off, and gulp down easily and quickly.

  • Jessie Dickson's photo below shows a few of a flock of thousands of Tree
    Swallows she discovered feasting together on waxmyrtle berries.

  • Tree Swallows will stuff themselves with these berries when they're available.
  • The photo below shows 28 bayberries dissected from one road-killed swallow's
    digestive tract.  The waxy outer layer has already been digested from some.

  • Click here to watch an extremely neat YouTube video of a flock of Tree
    Swallows feeding on a waxmyrtle bush.

A third possible reason some birds flock: flocks may contain older birds that
know from experience where to find important resources.
  • Inexperienced birds can follow experienced ones to potential food supplies and
    shelter.  So in a way flocks could be considered information centers.
  • We suspect older Tree Swallows already know where important stopover areas
    are and where patches of bayberry, waxmyrtle, and other foods are located.
  • Perhaps juveniles can learn by following these older individuals to resources,
    and tn this way information vital for survival could pass down generations.
  • We also believe that young swallows could also be following older experienced
    birds south on their migration paths to the wintering grounds, and migration is
    the subject we'll tackle next.   Photo below by Tony Leukering.   

Why do many songbird species migrate away from their nesting grounds?
  • Migration is the regular predictable movement of animals from one location and
    climate to another location and climate.
  • Decrease in availability of food as seasons change is the main force driving
    post-nesting songbird migration in North America.
  • Migration usually demands lots of energy and can be dangerous, but for many
    birds the benefits outweigh the costs, because if they stayed on the breeding
    grounds they would almost certainly die.
  • The nesting season usually coincides with maximum availability of food for
    nestlings.  But when breeding is over and summer passes into fall, and winter
    nears, things change dramatically.  There are fewer daylight hours for foraging,
    at the same time colder temperatures require much more energy for survival.  
  • Many birds that depend on active insect food would starve if they tried to winter
    where they nest, since most northern insect species overwinter as eggs, larvae,
    or dormant adults, unavailable for many songbirds.  Plus seasonal crops of
    plant food are often exhausted long before winter ends.
  • So, many songbirds have no good option but to migrate south to winter in
    warmer areas where food is more plentiful, but they don't wait for winter.  They
    migrate well before winter, while food to fuel their move can still be found along
    their migration routes.
  • These songbird migrants aren't driven by immediate hunger.  They migrate in
    order to avoid future predictable food shortages.
  • Like breeding, migration is not a conscious choice.  Instead, this behavior is
    stimulated by hormonal changes regulated by each bird's "biological clock."

How long does it take Tree Swallows to migrate south and why are stopovers
so important ?
  • Compared to many other songbirds Tree Swallows migrate south at a very
    leisurely pace.
  • It usually takes Tree Swallows 3-4 four months to travel from nesting grounds to
    wintering grounds, but during this long period they may spend less than two
    weeks actually flying south!
  • Recent research using tiny geolocators has revealed that after nesting each
    Tree Swallow usually moves to one of 10 major North American stopover or
    staging regions where it stays for several weeks or even months, roosting,
    resting, molting, and building up energy reserves.
  • Tree Swallow stopovers typically center on large marshes where food can be
    found in surrounding areas and where flocks of swallows can roost at night in
    the relative safety of tall cattails or reeds growing up out of the water.  
  • After their long first stopover each swallow begins moving south again, but it
    may halt for one or more additional brief or even extended stopovers before it
    finally reaches the area or areas where it will winter.
  • Surprisingly, geolocators have also shown that Tree Swallows which nested in
    one area may use different stopover sites and take different migration routes

What is Tree Swallow life like on the southbound migration?
  • As mentioned, much of a Tree Swallow's life during the 3-4 month journey south
    is spent in large flocks at stopover areas where they rest, refuel, and roost.

  • Each morning shortly before sunrise the swallows rise up from the roosting
    marshes and spread out searching for food.  
  • These dispersing flocks are often large enough to create "roost rings" on
    weather radar scans, which detect each bird's body as a large drop of water.
  • The photo below shows a huge ring made by a mixed species flock of swallows
    dispersing from a major roost at Long Point, Ontario, on an August morning, as
    seen by US National Weather Service radar in Buffalo, NY.

  • When not foraging the flocks rest, sunbathe, and preen together on wires,
    beaches, and trees, as in John Gavin's photo below.

  • Unfortunately, Tree Swallows are also attracted to warm road surfaces, and not
    all cars stop.  Photo by John Gavin.

  • In the evening after sunset the swallows return again to marshes to roost, either
    in the one they left that morning or the next one along the migration chain.  
  • Going to roost is a dangerous time because predators are attracted to such
    large concentrations of potential prey.  As if not wanting to be the first to come
    in, thousands of Tree Swallows swirl overhead as dusk falls.

  • Then, wave after wave of swallows dashes down into the marsh vegetation with
    a roar of wings.  Photo below by Jo-Anna Ghadban.

  • Click this link to watch Brett Slattery's YouTube video of Tree Swallows
    funnelling as they come to roost.  It's truly awesome!
  • And here's another beautifully filmed YouTube video, this one by Mark Vance,
    of Tree Swallows going to roost, which includes two minutes of unusual low
    light action toward the end.  
  • Tree Swallows going to roost is an amazing sight, and it's one you can see in
    person.  If you plan to be in New England in late August, September or early
    October consider taking one of Connecticut River Expedition's "Tree
    Swallow Sunset Cruises" where you can witness hundreds of thousands of
    Tree Swallows converging at sunset and diving into their island roost.
  • Roger Tory Peterson, who knew a bit about birds, and who lived not far from
    this Connecticut stopover marsh, once said "- - for sheer drama, the tornadoes
    of Tree Swallows eclipsed any other avian spectacle I have ever seen."  

What routes do Tree Swallows take on their migration south?
  • Each bird species that migrates uses one or more general routes of travel.
  • Tree Swallows migrating south typically follow one of three major flyways.
  • Tree Swallows that from eastern Canada, the Great Lakes and northeastern
    United States usually gravitate to the Atlantic seacoast and follow it south to
    Florida.  Some continue on as far as Cuba.
  • Populations from Alaska, central Canada and the U.S. Midwest follow Mississippi
    River basin waterways south to Louisiana, where they may either winter or
    continue on to eastern Mexico, Central America, or Florida.
  • Tree Swallows from western North America migrate west of the Rocky Mountains
    to wintering grounds in western Mexico and Central America.
  • EBird now offers an excellent "Abundance Animation" video showing Tree
    Swallow annual movement patterns.  
  • Although most Tree Swallow migration is overland, they are quite able to cross
    large bodies of water, such as from Florida to Cuba, or from Louisiana to the
    Yucatan Peninsula of Mexico.
  • Data collected by light-weight geolocators like the ones below is helping
    ornithologists determine songbird migration routes with greater precision.  
    Photos courtesy of the Alaska Songbird Institute.

How do migrating birds know where to go?  How do they find their way?
  • It may surprise you to learn that most songbirds migrate at night.
  • The fascinating navigational techniques used by night migrants are beyond the
    scope of this web site, but you should know these birds must possess both a
    "map" to pinpoint where they are at any moment and a "compass" to set their
    flight directions.
  • By contrast Tree Swallows are among the minority of songbirds that migrate
    primarily during the daylight hours.
  • Although day migrants are more exposed to predators there are some
    advantages.  Day migrants can and navigate using land formations they can
    see easily, such river valleys, coastlines, and mountain ranges.
  • And, unlike night migrants, those that migrate by day can feed as they go, in the
    Tree Swallow's case on both flying insects and their favorite berries.  

What is molt and when does it take place in Tree Swallows?
  • If you are able to get a close-up look at Tree Swallows between mid-July and
    mid-November you may be surprised at their odd appearance, for this is molting
    season for this species.  Photo below by Jeremiah Trimble.

  • Molting is the gradual and systematic replacement of all or portions of a bird's
    feathers by new ones.  In songbirds molt takes place gradually so they can
    continue to fly.
  • Different songbird species have different timing and patterns of molts, and some
    species molt more than once a year.
  • Molting is "expensive" for songbirds.  Replacing old feathers with new takes lots
    of raw materials and energy, plus it's harder to fly and keep warm when some
    feathers are missing or only partly regrown.
  • Because it does demand lots of energy to grow new feathers most birds molt
    when nothing else major is going on in their lives, for instance, not during
    nesting or during migration.  However, Tree Swallows do things a bit differently.  
  • Adult Tree Swallows have one complete molt that starts when nestling is almost
    over and continues through the several months of migration.  
  • Juvenile Tree Swallows also have a complete molt that starts soon after they
    fledge.  By October or November young males will have acquired the bluish AHY
    plumage they will retain all their lives, and young females show their distinctive
    brownish SY plumage.  (See Sexing and Aging for more on this subject).
  • It has been suggested that one reason Tree Swallows typically make one or
    more long stopovers on southward migration is to allow time for molting.
  • Note the mix of old faded wing and body feathers and bright new ones in both
    the photo above and the one below from  

Where do Tree Swallows winter and why are some flocks of wintering Tree
Swallows so large?
  • By late autumn most of the south-bound flocks of migrating Tree Swallows have
    reached their wintering grounds.
  • The majority winter in or near wetlands in Florida, Louisiana, and eastern
    Mexico, with fewer numbers wintering in other parts of the US southeast,
    California, western Mexico, Central America and Cuba.  
  • Geolocators have determined that as many as half these swallows will travel
    from one wintering region to another during the winter season, perhaps driven
    by competition for food and/or roosting opportunities.
  • Obviously, preserving the availability and integrity of these crucial but limited
    winter habitat sites is vitally important for the future of the species.
  • People witnessing Tree Swallows in large winter flocks may overestimate the
    species' abundance, not realizing their entire population is now concentrated
    and vulnerable at this time in these relatively very small regions.
  • Contrast the spread of Tree Swallows at the height of breeding (below left) with
    how concentrated they are by mid-winter (below right).  Photos from eBird.

  • People viewing large wintering flocks should also be aware that these are
    migration survivors.  Many of the swallows that started south are already dead.  
  • It's believed that up to 85% of some songbird species' annual mortality occurs
    on migration, not while nesting or wintering.  Bad weather, predators, disease,
    and accidents all take their toll.  Chris Wood's photo below shows a Merlin
    carrying a captured migrating Tree Swallow in its talons.

What is life like for Tree Swallows on their wintering grounds?
  • For Tree Swallows daily life during winter appears to be much like that on
    migration stopovers: rising from large roosts at dawn, spreading out in groups to
    feed, with periods spent preening and sunning, and then returning again to the
    roosts around dusk.
  • And, as on the southward migration, waxmyrtle berries and bayberries are
    important food resources during winter.  Click this link to view Jill Kusba's
    YouTube video of a flock of thousands of Tree Swallows feeding on
    waxmyrtle berries in Florida.
  • However, not all Tree Swallows winter in the deep south.  Some winters a few
    hardy ones try to tough it out along the Atlantic coast as far north as New
    England and Nova Scotia in the east, and up the Pacific coast to Washington
    State in the west, and occasionally some survive to spring.  As you might expect
    their ability to endure this far north depends largely on availability of bayberries
    and waxmyrtle berries.
  • To get a sense of how some Tree Swallows managed to endure a northern
    winter click here.  John Elliot's report is anecdotal but interesting to read.  

Is Tree Swallow migration north similar to their migration south?
  • Eventually, as the daylight hours lengthen in late winter and temperatures warm
    once again, internal hormonal changes stimulate Tree Swallows to begin the
    return north to their species' nesting grounds.
  • However, migration north is different.  For one thing many of the swallows that
    began the move south last year have perished.  And instead of autumn's great
    flocks, surviving Tree Swallows head north early in spring as individuals and
    small groups, older swallows migrating first, followed a few weeks later by
    second-year birds.
  • Wind directions that prevail in spring may cause the swallows to follow migration
    routes that differ from the ones they used last autumn.
  • And rather than a slow step-wise progression from roost to roost, Tree Swallows
    heading north appear to fan out across the continent rapidly, with longer flights
    and much shorter stopovers, as they home in on the locations where they
    nested in the past or where they were raised the previous year.
  • Northward migration usually takes Tree Swallows 1-2 months instead of the 3-4
    month the birds spend moving south.
  • Tree Swallows migrate north extremely early compared to other birds that rely
    primarily on insect food.  It's thought this early return is driven by the need to
    obtain a nesting cavity before they are all taken.
  • However, migrating north so soon is dangerous.  Flying insect food and even
    berries may be scarce or absent, and death from starvation and hypothermia is
    a very real risk.  Marty Burke's photo below shows a flock of Tree Swallows in
    Ontario halted by cold, snowy weather in early April.  Note how the swallows
    have huddled together trying to conserve body heat, which is quite different
    from keeping their usual "individual distance."
  • To learn more about the northern migration and spring return click here.

Will your swallows come back?
  • As we've seen migration and wintering have their own sets of dangers, and
    unfortunately it's inevitable that some, perhaps many, of your birds, especially
    the younger ones, will die.
  • However, banding data has shown that adult Tree Swallows, especially males,
    which have nested successfully in a location are apt to return.  Females have a
    lower rate of attachment to previous nesting sites.
  • Banding has also demonstrated that even young swallows entering their first
    nesting season usually return to the general area where they were raised, and
    that they have a statistically better chance of returning if they were larger and
    stronger when they fledged.
  • So, if they survive the challenges of migration and wintering, those adults who
    nested with you the previous year and the young that they fledged should
    attempt to return north to spend another nesting season at your boxes.  And
    other Tree Swallows, newcomers prospecting for places to nest, will probably
    arrive with them.

Question for the next topic:  Nest Box Care and Project Assessment
  • What's left to do after your swallows have gone?

Flocks, Migration,
Molt, and Wintering
Learn About Birds at Tree Swallow Nest Box Projects