wildlife.state.co.us
Colorado welcomes returning swallows, known for two things: eating millions of insects each summer and building mud nests on the sides of buildings, bridges and man-made overhangs.
Coloradans worried about mosquitoes carrying the West Nile Virus may be looking for new ways to make their communities bird-friendly, including educating residents about the benefits of having swallows visit each summer.
Colorado is fortunate to be the summer home for many species of migratory birds, including several species of swallows, notably the barn swallow and cliff swallow. These birds are known for two things: eating millions of insects each summer and building mud nests on the sides of buildings, bridges and man-made overhangs.
Swallows eat insects almost constantly while in flight. For this reason, attracting swallows to a property may be one positive step to managing insects of concern. Swallows capture hundreds of insects, including flying ants, termites, aphids, mosquitoes, crane flies and moths. On average, insects make up 99.8 percent of the swallow’s diet.
“We want to educate the community about the benefits of the swallows, especially in the wake of West Nile Virus, which can be carried by mosquitoes. Any species that feeds voraciously on mosquitoes helps reduce the risk,” said Aimee Ryel, wildlife officer for the Colorado Division of Wildlife (DOW.)
Swallows typically winter in southern climates, as far south as Central and South America. They return to Colorado and other northern states and provinces in spring – although the legend of San Juan Capistrano in California states they return punctually in March, in Colorado it can be as early as February or as late as June, depending on the weather.
Particularly in Colorado’s resort areas, there are many buildings, condominiums, and vacation properties that often go unused between winter ski season and summer outdoor recreation season. During the muddy season between April and June, the birds are often busy building and repairing nests. When people return for Colorado’s glorious summer weather, many management companies and homeowners spruce up their properties.
Their distinct tails can differentiate barn swallows and cliff swallows: the barn swallow has a sharply forked tail, while the cliff swallow has a square-shaped tail. Both barn swallows and cliff swallows build nests out of mud pellets that they carry in their mouths to a nest site protected by an overhang.
Barn swallows build open, cup-shaped nests. Cliff swallows build enclosed, gourd-shaped nests with a downward facing entrance. It is easy to distinguish the species based on the nests alone: Barn swallows do prefer to have their nests several feet apart. However, Cliff Swallows tend to form large colonies and nests are often packed very close together. The typical nest, which contains as many as 1,000 mud pellets when finished, is lined with grasses and feathers.
Some people find mud-based swallow nests an unattractive decoration, but destroying an active nest and/or the young in the nest is illegal. Swallows, their active nests and eggs are all protected by the federal Migratory Bird Treaty Act of 1918 and may not be destroyed. The U.S. Fish and Wildlife Service allows vacant nests to be destroyed, but nests with active birds, their young or the presence of eggs must be left alone. Otherwise, it is a violation of federal law.
“We have a lot of swallows nesting on local bridges, sometimes near bike paths. I have seen well more than 100 nests at one time on a bridge. That means quite a few swallows helping us with the mosquito population,” Ryel said.
To prevent swallows from returning to places where they may be a nuisance, inactive nests should be removed only above doorways, walkways, and places where visitors spend time. Do not take down nests that are on the back side or between buildings, or in public areas such as bridges and overhangs. This allows the birds to nest in a community, while encouraging them to find places that are less visible to human visitors.
“People who find a swallow nest near their front door or otherwise causing a problem may find that placing an artificial nest off to one side of the door or even on a different wall may be the surest way to coax the swallows to move,” Ryel said.
Studies on swallow relocation indicated the birds often prefer building on a vertical wall of rough wood, sometimes on a small projection such as a protruding knot or nail. Placing a piece of wood, particularly rough wood such as weathered barn wood, approximately 1 ½” from the building’s eave or overhang, appeals to swallows. The swallows seem to prefer to build right up near the roof edge. This likely helps them avoid predators and protects nests from wind and rain. If mud nests plastered on a wall get wet, they will fail.
Swallows do not like continuous ledges or shelves, probably because predators, such as rats and mice, can reach the nest by crawling along such horizontal surfaces. Although barn swallows seem to prefer a distance of several feet between nests, they tend to build nests near other swallows. Since swallows tend to raise two to three broods each year and generally change to a new nearby nest with each brood, empty nests can be removed carefully once they are vacant. Creating a nearby shelf acceptable for nesting may encourage the birds to move towards a more agreeable location.
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For more information on the International Migratory Bird Treaty Act (MTBA) of 1918, see, http://laws.fws.gov/lawsdigest/migtrea.html including range of fines and penalties for violations of the Act.
For digital photos of swallows and nests found in Colorado, go to
http://wildlife.state.co.us/imagedbdow/index.asp
and enter the keyword “swallow.”
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Division of Wildlife
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