Don’t Let the Bedbugs Bite

Jeff Mitton

Natural Selections (Appeared November 24, 2005 in the Daily Camera)

Following a dry irrigation ditch on Marshall Mesa, I ducked through a tunnel beneath Route 36. I was delighted to find, in such an unnatural place, dozens of cliff swallow nests plastered into the junctures of the walls and the ceiling. While the ranges of many species are shrinking, cliff swallows have used tunnels and bridges to increase their range in North America.

Historically, cliff swallows, Hirundo pyrrhonota, spent spring and summer in western North America, from the mountains of Mexico to Alaska. As their common name implies, they nested on cliffs, but they have learned to take advantage of the tunnels and bridges that have sprung up across the country. The abundance of new nest sites has allowed them to expand their range across the Great Plains and through the eastern deciduous forest to the Atlantic.

Cliff swallows collect mud, work it into a malleable pellet, and plaster it to a protected vertical surface. Several hundred pellets are needed to build a gourd-shaped nest, usually with a downward projecting entrance tunnel. Nests are often lined with dry grass. The nests must be placed where they will be protected from rain; if the nests are soaked in a summer shower, they dissolve and fall. Well-made nests placed in dry locations can last for years.

Cliff swallows nest colonially, and colony size ranges from about a dozen to the record of 4,000 nests attached to a single barn in Wisconsin.  Advantages accrue to larger colonies. Although larger colonies attract more predators, including loggerhead shrikes, American kestrels, roadrunners, red-tailed hawks, Cooper’s hawks, and red-shouldered hawks, predators take proportionately fewer swallows from large colonies. In addition, swallows watch returning foragers to gain information on the direction of a food source. This inadvertently-shared information reduces time and energy spent foraging for food.

The advantages of large colony size are balanced by the increasing load of parasites, including hematophagous swallow bugs, ticks, and fleas. The swallow bugs, close relatives of the blood-sucking bedbugs that parasitize humans, inflict the most damage. When swallow bugs are common, they increase the mortality of nestlings by 50%. The swallow bugs overwinter in the nests, waiting for the swallows to return in spring. Persistent parasite loads cause swallows to avoid some nests, and have even caused swallows to abandon a colony and construct an entire new colony. Swallow bugs and ticks are rare in small colonies.

Large colonies also provide optimal conditions for cliff swallows to be nest parasites. A female typically lays four to six eggs in her own nest, but she might also drop an egg in a neighboring nest when the adults are foraging. Females need only about one minute to enter a nest, lay an egg, and leave; the record time is 15 seconds. Some swallows lay an egg in their own nest, but pick it up in their beak and fly to a neighbor’s nest to deposit it. In large colonies, about 25% of the nests contain eggs that do not belong to the attending parents.

Swallows are aerial insectivores; they catch bees, wasps, ants, beetles, bug, flies and mosquitoes in the air. During summer evenings, the acrobatic maneuvers of swallows dining on the wing provide natural entertainment. Because insects do not fly in the rain, cliff swallows fast when it rains. When insects disappear for the winter, cliff swallows migrate to South America to find an abundance of flying food. The departures and arrivals of cliff swallows are so reliable we use these events to mark the seasons. Cliff swallows, as harbingers of spring, return to California’s Mission San Juan Capistrano on March 19 of each year.

Cliff swallow nests

Photo by Jeff Mitton

Swallow nests used year after year become infested with swallow bugs and ticks, which take blood meals from the chicks. Infestations of swallow bugs (relatives of our bed bugs) increase chick mortality by 50%; not all of the chicks survive to fly south for the winter

Photo by Jeff Mitton

A casual inspection revealed three nests (out of about 80) with dead chicks in the entrances; perhaps other nests had cadavers hidden deep in the nests. The swallows will probably abandon this nest site

Photo by Jeff Mitton

Photo by Sarah Rogers