The Outside Story

“What (f)lies beneath”—avian blood-suckers

By Meghan Oliver
When you find a bird feather in the woods and stoop to pick it up, does your mom’s voice echo in your brain? Can you hear her say, “Birds have lice, don’t pick that up!” Mom was mostly right. Birds can have lice (although you won’t catch lice from a bird). But what Mom probably didn’t know is that birds have something far creepier lurking in their feathers. It’s six legged, leathery, and flat. And it sucks blood.
The good news? It does not want to suck your blood.
Avian hippoboscid flies—also called flat flies and louse flies—survive on bird blood. Estimates vary for the number of different species in our region, but there are likely more than ten and fewer than twenty.
These flies are ectoparasites, living on the bird rather than in it. Alan Graham, Vermont’s state entomologist, has an extensive collection of avian and other louse fly species, and he has identified a diversity of bird hosts, ranging from northern flickers to song sparrows, barred owls to blackpoll warblers. While Graham said larger birds often host larger species of flies, this is not always the case. Birds may be infested with more than one hippoboscid species at a time.
To the casual observer, hippoboscids look a lot like the common house fly, except flatter, as if someone had stepped on them. This low profile helps them scuttle sideways, crab-like, over and under feathers. They have claws that help them cling to feathers and a special needle-like proboscis protected by mouth parts on either side that unfold at mealtime. When a fly is ready to dine, it pierces the bird’s skin with its proboscis and slurps up a meal.
Catherine Greenleaf, director of Saint Francis Wild Bird Hospital in Lyme, N. H., finds them most commonly on birds of prey (eagles, hawks, owls), particularly those on death’s door. She described the insects as grim reapers and vampires. In her experience, emaciated birds covered in hippoboscid flies almost always test positive for anemia.
Greenleaf also described another unpleasant feature of these flies—their penchant for flying off bird patients and directly into the hair of human rehabilitators, where they don’t bite but are “really gross” and “very hard to catch.”
While Graham couldn’t give an estimate as to the lifespan of these flies, he said he’s collected specimens in winter, suggesting they can survive the cold. Living against the skin of a warm-blooded bird with the extra protection of insulating feathers helps keep them alive.
As for offspring, the flies have one of the lowest reproduction rates in the insect world. According to the “Manual of the Neartic Diptera,” hippoboscid females may produce just seven to eight offspring in a lifetime. Whereas the common housefly may lay clusters of thousands of eggs at a time, with hippoboscids, only one egg is produced, and it’s not laid at all. A single larva hatches within the mother louse fly’s uterus. In this protected environment, the young insect completes its larval development. The female lays the larva, which pupates. When the adult fly emerges from the pupa it immediately begins to hunt.
One theory for these flies’ low reproductive rate is that a proliferation of them might eliminate host populations. John Burger, a professor of zoology at the University of New Hampshire, also suggested a limiting factor on adult hippoboscid numbers: sometimes, the birds bite back. “My guess,” he said, “is that the life of a hippoboscid fly is quite chancy,” with some being eaten by their host if the host can nip them.
So if you happen upon an injured bird, know that the avian species you’re looking at may not be the only winged creature down. Louse flies may lie beneath. But take comfort in the fact that they have no interest in you. Mom’s got no need to worry.
Meghan Oliver is a freelance writer and the assistant manager at The Norwich Bookstore. She lives in Vermont. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: wellborn@nhcf.org.