At the Project:
The Tree Swallow nesting season is over.  Your swallows are gone now, but as you go
about your days you may find yourself wondering at times where they are 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 staging places juveniles can become adept at finding and capturing
    food, and both juveniles and adults can build up their energy reserves.  
  • Now, instead of isolated territorial breeding pairs you are more apt to find Tree
    Swallows in large flocks of hundreds, thousands, or even millions!  
  • 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 members of a group take turns looking and listening for danger, each
    individual 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 grabbed a Tree Swallow and
    appears to be in the act of severing its spine behind its skull.  Photo by
    Catherine Hamilton of Birdspot.

Another benefit of flocking: more eyes can find food more efficiently.  
  • Bird species tend 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 flocks in western NY eating fruits of bayberry, shrub
    dogwood, red cedar, and viburnun 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 fruits of waxmyrtle and bayberry bushes that
    grow in sandy soils near seacoasts.  They are one of the very few birds able to
    digest the energy-rich waxy outer coatings of these berries.  
  • 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 can be considered information centers.
  • We suspect older Tree Swallows know from past experience where stopover
    marshes are and where to find local patches of bayberry and waxmyrtle, and
    that juveniles can learn by following older individuals to resources.  In this way
    information vital for survival can pass from generation to generation.
  • We also believe that older swallows in flocks could play an important role in
    leading younger ones south on their first migration 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.  
  • Most 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, all unavailable for many songbirds.  And seasonal crops of
    plant food are often exhausted 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 must
    migrate in order to avoid future predictable food shortages.
  • And like breeding, migration is not a conscious choice.  Instead, this behavior is
    stimulated by hormonal changes regulated by each bird's "biological clock."

What is Tree Swallow life like on their southbound migration?
  • If you combine post-breeding migration south and pre-breeding migration back
    north you'll see Tree Swallows spend up to 4-6 months of each year migrating.
  • You could consider the flocking of Tree Swallows at large marshes after nesting
    the first step in their migration, because it begins a pattern of rather short step-
    wise moves down a series of traditional foraging and roosting sites, a pattern
    that continues for months until they finally reach their wintering grounds.
  • Many other migrant bird species use their own traditional stopover points, where
    in most years they can count on food; stopovers where the migrants can rest,
    and refuel before taking the next step on their journey.
  • Stopping over can be crucial for successful migration and can last from a day or
    two to several weeks, depending on the species and its needs.
  • Traditional Tree Swallow stopover points typically center on large marshes
    where many thousands of swallows can roost at night in the relative safety of tall
    wetland cattails or reeds growing up out of the water.   

  • Shortly before sunrise the swallows rise up from the marshes and spread out in
    all directions searching for food.  These dispersing foraging flocks are often
    large enough to create "roost rings" on weather radar scans.
  • The photo below shows a huge ring formed by a mixed species flock of swallows
    dispersing from an overnight roost at Long Point, Ontario, in August, 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 evenings the swallow flocks return to marshes to roost, either the one they
    left that morning or the next one along in the migration chain.  
  • Going to roost is a dangerous time because predators are attracted to such
    large concentrations of potential prey.  Photo below by Jo-Anna Ghadban.

  • As if not wanting to be the first to come in, thousands of swallows swirl about
    high overhead as dusk falls.  Then, often forming a funnel cloud, wave after
    wave of swallows dashes down into the marsh with a roar of wings.  
  • 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 one of nature's amazing sights, and it's one you
    can see in person!  If you're in southern New England in September and
    October consider taking one of Connecticut River Expedition's "Swallow
    Spectacle" cruises, where you can witness hundreds of thousands of Tree
    Swallows converging at sunset on their roosts in Connecticut coastal marshes.
  • Roger Tory Peterson, who knew a bit about birds, and who lived not far from
    these Connecticut marshes, 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 that nest in eastern Canada and northeastern United States
    usually gravitate to the Atlantic seacoast and follow it south to Florida.  Some
    may continue on as far as Cuba.
  • Populations from central Canada and the Great Lakes states appear to follow
    Mississippi River basin waterways south to Louisiana, where they may either
    winter or continue on to Florida, Mexico or Central America.
  • Migration routes of Tree Swallows that nest in western North America are less
    well known, but it's thought they follow north-south mountain ranges, valleys,
    and the Pacific seacoast, south to southern California, Arizona, and Mexico.
  • Although most Tree Swallow migration is overland, they are quite able to cross
    large bodies of water, such as the Gulf of Mexico from Louisiana to the Yucatan
    Peninsula of Mexico, when necessary.

How long does it take Tree Swallows to reach their wintering grounds?  
  • While a few songbirds migrate quickly in long distance flights, most move south
    in a series of short flights of 200 miles or less, with rest and refuel stopovers of
    one to several days between each flight.  
  • Tree Swallow post-breeding migration progresses in a sequence of quite short
    steps, averaging only 50 to 100 miles each, from one traditional roosting marsh
    to the next.  
  • And compared to most other songbirds Tree Swallows migrate at a very
    leisurely pace.  It may take some Tree Swallows a full four months to travel from
    nesting to wintering grounds!

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
    during the day.
  • Although day migrants are more exposed to predators there are some
    advantages.  Day migrants can orient themselves and navigate using land
    formations they can see, 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 late July and
    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 life, for instance, not during
    migration.  However, Tree Swallows are one of the few exceptions to this rule.  
    They molt on migration.
  • Adult Tree Swallows have one complete molt that starts when nestling is almost
    over and extends over the several months of migration into autumn.  
  • Juvenile Tree Swallows also have a complete molt that starts soon after they
    fledge.  By October or November young males will have attained the 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).
  • Note the mix of old faded wing and body feathers and bright new ones in both
    the photo above and the one below from  

Why are some flocks of wintering Tree Swallows so huge?
  • By late autumn most of the south-bound flocks of migrating Tree Swallows have
    reached their wintering grounds.
  • The vast majority of Tree Swallows winter in or near wetlands in Florida, coastal
    Louisiana, and parts of Mexico, Central America and Cuba.  Obviously, the
    continued availability and integrity of this limited winter habitat is important for
    the future of the species.
  • When people living where Tree Swallows winter see this species' large flocks
    they may not realize its entire population is now packed into a very small region
    compared with the vast spread of their North American nesting grounds.
  • And people viewing huge 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 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 overwater marsh roosts at dawn,
    spreading out in groups to feed with periods spent preening and sunning, and
    returning and reconcentrating at the marsh roosts around dusk.
  • And, as on the southward migration, waxmyrtle berries and bayberries are
    crucial 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.  Each year a few hardy
    ones try to tough it out in coastal wetlands as far north as New England and
    Atlantic Canada 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 shorter stopovers, as they home in on the locations where they nested in
    the past or where they were raised the previous year.
  • 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.
  • 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, some others, both adults who nested and young that fledged from
    your boxes, should make the journey south and winter successfully.
  • And with luck many of these survivors and others that travel with them will return
    north to spend another nesting season with you at your project.

Question for the next topic:  Project Wrap-Up.
  • What's left to do after your swallows have gone?

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