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TREE SWALLOW
By Lisa Bate
It usually happens sometime in March. I am outside
working on the farm when I hear what sounds like
bubbling water flying overhead. Then I just smile
knowing that the tree swallows (Tachycineta bicolor)
have returned from their wintering grounds and with
them, have brought the real beginning of spring to
northwest Montana.
These aerodynamic masters of the sky are the first of
the swallow species to return every year. Their unusual
diet allows this. Unlike other swallows in our area,
which rely solely on insects for their food, tree
swallows can subsist on fruits, seeds and berries for
extended periods of time. In fact, up to 20 percent of
their diet can be vegetative. In the east, they have
wintered as far north as Long Island, New York, where
they feed on the waxy fruits of bayberry or wax-myrtle (Myrica
sp.). Here in Montana, their strategy is a little
different. Remember the freak snowstorm we had in June
2008? That storm left about seven inches of snow on our
farm. It appeared that the swallows had just left. That
same day while out on a walk behind our place I found a
surprise. There I witnessed a flock of about 40 tree
swallows hovering above the snowy fields where grass
seed heads protruded. I stopped to watch. The swallows
were knocking grass seeds from the stems to forage on
them along with a few insects and spiders!
The tree swallow is a medium-sized swallow with a
slightly forked tail. As their species name (bicolor)
suggests, tree swallows are bi-colored with white
coloring below and iridescent blue/blue-green above
(males). The upper sides of the females are a mixture of
brown and greenish blue. Juveniles are brown above, with
a light brown wash across the chest. They can be
distinguished from violet-green swallows by the absence
of a white flank and eye patch. Their genus name,
Tachycineta, comes from the Greek words “tachýs”
which means ‘swift’ and “kīnéō” which means 'move.'
Swallows are one of the most aerodynamic birds in the
world.
Tree swallows breed throughout central and North
America. Their northern distribution overlaps with the
northern limit of the boreal forests. Male adults are
first to arrive, followed by the females. Yearling
swallows are the last to arrive. During very cold
periods they will roost together in cavities on their
breeding grounds. They spend winters in areas along the
coasts of Florida, the Caribbean, Mexico, and parts of
Venezuela. They are highly social birds, especially
outside the breeding season. During migration, these
birds can roost in flocks numbering in the hundreds of
thousands. They exhibit interesting behavior before
settling into their nighttime communal roosts. About an
hour before dusk, tree swallows form large moving clouds
above the marsh or forest canopy. Then they fly upward
in a spiral like a bee swarm. Just before dark they
begin flying back and forth over the trees or cattails.
With each passing, large numbers will drop down from the
cloud to roost.
Tree swallows are secondary cavity-nesters, meaning they
rely on woodpeckers and other primary cavity-nesting
birds to create the cavities where they nest. They also
nest in natural cavities. Their populations are severely
limited by suitable nest trees, and the competition for
these sites is intense, as shown by their early spring
arrival and intense territorial defense. The males pick
the nest site, the female picks the male and builds the
nest and incubates. Both adults help to feed the young.
Because tree swallows take readily to nest boxes, they
are one of best studied birds in North America. On our
farm we have had as many as 20 out of 25 nest boxes
occupied in a single year by tree swallows. The great
thing about this is that tree swallows feed voraciously
on mosquitoes. Some estimates suggest they can take up
to 1000 mosquitoes per day!
They prefer to nest in the open areas away from forest
edges. In this way they avoid predation from house wrens
which are known to destroy the eggs of many
cavity-nesting birds.
Although they will nest in solitary situations, more
often they nest in small groups. They prefer to forage
in open meadows with tall grass or over open water.
During the early part of the breeding season in
Colorado, they have been documented flying over 100 km
from their nest sites in the mountains to forage in open
fields on the plains.
Although primarily monogamous, about five percent of
birds will also mate with other individuals. This varies
widely among populations. Up to 50% of nests in some
brood studies contained chicks from different fathers.
Infanticidal behavior is well documented in this species
where independent males have killed chicks of widowed
females to take over the nest site and subsequently
breed with the widowed female.
Also intriguing is the female version of infanticidal
behavior. In these cases the females will kill nestlings
at another female’s nest. This is likely an attempt to
gain access to a new nest site or one with better
habitat. Contrary to what a lot of people think, tree
swallows are not cooperative breeders, meaning that a
pair may have “helpers” to feed and care for the young.
Single males and young birds investigating cavities may
have led to this erroneous conclusion.
Swallows are like the canary in the mine for some of our
grassland areas, because they can indicate where high
levels of certain pollutants such as PCBs and DDE
persist. In areas where these substances are common,
researchers have seen mortality rates of swallows
increase with age. Introduced species such as house
sparrows and European Starlings, which compete for the
same nest sites, are also a threat to this species. That
is why I evict these two species whenever I see them
taking over my swallow (and bluebird boxes). But the
most significant threat to this species is the loss of
suitable nest trees. So remember, retain your snags, cut
a tree or snag to a safe height if it is a hazard, and
put up nest boxes. In return, you will be rewarded with
the sound of water flying overhead, astounding
aerodynamic flight displays, and FEWER mosquitoes!
Thanks, swallows! |
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