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 very
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 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
- 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 primary stopover or staging area where it
stays for several weeks or even months, roosting, resting, and building up
- 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 step-wise south down
a series of other traditional foraging and roosting sites, but it may halt again for
one or more additional extended stopovers before it finally reaches the area
where it will winter.
- Surprisingly, geolocators have shown that Tree Swallows which nested in one
area may use different stopover sites and take different migration routes south.
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 that nest in eastern Canada 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 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.
- 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
- 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 Birdfreak.com.
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 (54%) and Louisiana (12%),
with fewer numbers wintering in other parts of the US southeast, California,
Mexico, Central America and Cuba.
- Obviously, the continued availability and integrity of this limited winter habitat is
vitally important for the future of the species.
- People witnessing Tree Swallows in large winter flocks may overestimate the
species' abundance, not realizing the entire population is now concentrated into
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
- 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 that have nested
successfully are very apt to return to their breeding location, and even young
swallows entering their first nesting season usually return to the general area
where they were raised
- 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: Box Care and Project Assessment
- What's left to do after your swallows have gone?
Molt, and Wintering
Learn About Birds at Tree Swallow Nest Box Projects
|Life History and Nesting Guide, Spring Return, Songbird Behavior, Song and Calls,
Nest Site Competition, Pair Formation, Nest Building, Bird Flight, Mating,
Eggs and Egg Laying, Incubation, Nest Takeovers, Feather Care, Hatching and Begging,
Parental Care, Sexing and Aging, Nestling Growth, Mortality Causes, Older Nestlings,
Fledging, Ectoparasites, Juveniles, Flocks, Migration, Molt, and Winter,
Box Care and Project Assessment