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 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 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 vary for each species, and the advantages of flocking for
Tree Swallows may be different from those of other songbirds.
One reason birds flock is that many eyes and ears can detect predators
- 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 spinal cord behind its skull. Photo by
Catherine Hamilton of Birdspot.
Another benefit of flocking is that 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 food whenever they find it, regardless of the
- We've often watched flocks 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
- 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 is that 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 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 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 most 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 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 birds use traditional stopover points, where in most years
they can count on food; stopovers where the migrants can stop, 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 grasses or reeds growing 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 "ring echos" on weather radar scans. Photo below by
- 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.
- For Tree Swallows those 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
- 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 Birdfreak.com.
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.
- This species' numbers may seem huge to people living where the swallows
winter but this is because the entire swallow population is now packed into a
very small region compared with the vast spread of their North American nesting
- People viewing huge wintering flocks should 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
- 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 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 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?
Molt, and Winter
Learn About Birds at Tree Swallow Nest Box Projects
|Nesting Guide, Spring Return, Songbird Behavior, Song and Calls, Nest Site Competition,
Pairing Up, Nest Building, Bird Flight, Mating, Eggs and Egg Laying, Incubation,
Takeovers, Feather Care, Hatching, Nestling Care, Sexing and Aging,
Nestling Growth, Mortality, Older Nestlings, Fledging, Ectoparasites, Juveniles,
Flocks, Migration, Molt, and Winter, Box Care and Project Assessment