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!