What is a Flat Fly? Curiosity and Research at the Center for Wild Bird Rehabilitation

By Anna Morris
Lead Environmental Educator
A flat fly (Christian Hugues)
The birds that arrive for care at VINS’s Center for Wild Bird Rehabilitation have had a rough time out in the wild. Not only may they be injured from car accidents, window strikes, or cat bites, but they often have acquired parasites. Some of the parasites are internal, in the blood or intestines, but some live on the skin, under feathers, sucking blood from their host and staying hidden from predators.
One such curious hitchhiker on wild birds is known as the “flat fly”. Insects of the family Hippoboscidae parasitize birds and mammals, and shuffle sideways across their hosts, ducking under feathers to hide, but not much is known about them. Our rehabilitators find them an annoyance, but will always grab a plastic tube after one is spotted, and dunk the fly in alcohol, labeling the tube and storing it for later. We save them for Alan Graham, the former Vermont state entomologist, who aims to find out more about these little-known invertebrates.
Alan Graham (Seven Days VT)
In addition to his role as state entomologist, Graham has worked for the Stroud Water Research Center and for the Vermont Agency of Agriculture studying tick- and mosquito-borne diseases. He has many years of experience looking closely at parasitic insects and their populations, and VINS provides specimens for his continuing study.
But why the flat fly? I asked Graham what drew him to these little insects. “Curiosity,” was his answer. “Few people know what they are,” or even where they are. Vermont is an under-collected region for flat flies, and Graham argues that even with the good job our state does in protecting land, we should also know about what we’re protecting. Ever since he first collected a flat fly from a pigeon in graduate school, Graham has sought to answer fundamental questions about their life history.
Feather lice parasitizing a flat fly. (Alan Graham)
So what is known? Though some flat flies live on deer and other mammals, a great diversity live on birds. Flat flies that secure a good host will then snip off their own wings, which prevents them from flying again. They can travel long distances under the feathers of a bird’s wing, and survive the freezing temperatures of a New England winter next to their warm skin. A flat fly female lays only one egg, and it hatches mature, skipping through the larval stage common to many other flies. (Several veterinarians in the area also collect botfly larvae for Graham to document).    
But what is their phenology like–when do they mate? What time of year are they most abundant? Do they cause significant harm to their hosts, or like many parasites, are generally benign? Can they transmit diseases between hosts (Fascinatingly, there are none that Graham knows of—flat flies seem incapable of transmitting West Nile Virus, for example). Fueled by pure curiosity Graham has already collected many observations that will help him pursue the answers to these questions. VINS is glad to provide this help!


  1. Lupe Asplund on January 4, 2024 at 6:09 am

    Really appreciated reading this article! The insights you shared about the subject were enlightening and inspiring. I particularly found the part about the article to be very intriguing. It’s great to see such well-researched and educational content. Looking forward to your next post!

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