The Firefly Forest

House Wrens (Troglodytes aedon) are small, plain wrens found throughout most of the Americas in wooded thickets and in areas of dense, shrubby vegetation from southern Canada to the southern tip of South America. House Wrens are not only widespread, they are also quite common, and I have observed them in many places here in the United States and in Costa Rica. In June of 2008, we observed this House Wren below near the 9,157 foot (2791 m) summit of Mt. Lemmon in Arizona's Santa Catalina Mountains. Here in southeastern Arizona, House Wrens are common summer residents in mountain pine forests and fairly common winter residents in lowland brushy areas.

House Wrens favor low, dense cover, and with their small size and drab coloration, they would be difficult to spot if it were not for their constant activity and frequent dry chattering and scolding calls. We had apparently strayed too close to this one's nest, and the angry wren hopped from branch to branch in a nervous, agitated fashion and furiously scolded us. The House Wren's scolding diversion proved successful and we never did spot its well-hidden nest. Once we moved away, the relieved House Wren quieted down and disappeared into the bushes.

House Wrens are among the plainest of wrens due to their drab brown coloration and lack of distinct markings other than faint brown barring, a buff eye-ring, and an often indistinct light eye-line. This plainness distinguishes them from all other wrens here in the U.S. except for western Winter Wrens (Troglodytes troglodytes). However, Winter Wrens have only a short stub of a tail and are thus easy to distinguish from House Wrens.

As their common name indicates, House Wrens will nest in birdhouses, especially those designed for wrens. These eager little cavity nesters have even been known to puncture the eggs of other cavity nesting birds and usurp their nests. Despite this sometimes naughty behavior, House Wrens are quite beneficial to have around because they eat insect pests and will nest in disturbed, fragmented habitat like suburban backyards avoided by some other insectivorous birds.

Robber Flies (Family Asilidae) are medium to large flies that prey on other flies and insects. These predatory flies are usually seen either waiting on low perches in open, sunny areas or buzzing around as they look for prey. I observed this female Robber Fly below embracing a captured Eristalis Fly (Eristalis sp.) in Arizona's Santa Catalina Mountains. Eristalis Flies are bee-sized, so one of these plump, flower-loving flies would make a good-sized meal for a hungry Robber Fly.

As this photo shows, Robber Flies are quite capable of handling large prey, and they will even take on large, dangerous prey like bees and wasps. However, unlike bees and wasps, Robber Flies do not sting, and the "stinger" on this Robber Fly is actually a harmless ovipositor that she uses for laying eggs. Although they do not sting, Robber Flies do bite and their saliva contains neurotoxins to quickly immobilize prey and proteolytic enzymes to dissolve its internal organs so that they can later be easily sucked out. The proboscis of a Robber Fly is specialized for stabbing and sucking, and after one of these flies captures something, it will return to a perch in its territory to suck out its liquefied meal.

Robber Flies are highly beneficial insects because they prey on a wide variety of pest flies, so I rescue any that get trapped in my house or swimming pool. Although they don't attack or bother humans, Robber Flies can bite quite painfully, so do not capture or pick up these flies with your bare hands.

Although some species of Robber Fly appear bee-like, most species of Robber Fly have a characteristic elongated shape, almost like that of a short, fat dragonfly. Robber Flies have a single pair of powerful wings, spiny legs that aid in grasping prey, a heavily bearded face, and large compound eyes that give them excellent vision for spotting prey.