Covered in Flies

In a season that is usually highlighted by migrant birds and rare vagrant species, this incipient fall field season has been anything but, and to our chagrin, our attention has been focused on something entirely different. Since the arrival of the fall bird crew, Southeast Farallon Island (SEFI) has been inundated and plagued by a surfeit of flies. If any bird were interested, SEFI would be the ultimate buffet, but unfortunately there just aren’t enough migrant songbirds (thanks to the current weather pattern) to put a dent in the daunting number of flies. As they have spread over the island in one of the worst excesses seen in recent years, our incessant swatting, slapping, and twitching is driving us all mad. Being field ornithologists, 75% of our work requires us to be outside. To deal with the horrendous nuisance, we take preemptive measures to cover every inch of our bodies, even on calm, hot days, by tucking pant legs into socks, cinching sleeves tightly around gloves, and wrapping bandanas around our faces. Alas, the flies still manage to crawl under our protective layers to tickle us until we smash or release them. Imagine the perseverance it takes to stand motionless in a swath of flies for an hour conducting a bird survey. Quite frankly, the flies are wearing on us.

For healthy birds, the flies provide a much needed snack.  For a sick or weakened bird, the flies turn the tables and swarm the moribund individual. Take for instance this recently fledged Western Gull that has clearly lost the battle to live. 

A few of us have found a comical, to some vile, way to handle the flies. Ryan Potter, the carryover seabird biologist, has decided that letting the flies run amuck is the best way to overcome the problem. He theorizes that by exposing more skin, there is an overstimulation that is easier to deal with than just one or two flies crawling under heavier wraps. Thankfully, there are days when the flies don’t seem as bad and being outside becomes bearable. This mostly occurs when a steady force of wind keeps the flies clinging to the rocks, or when a cold fog keeps them at bay. Unfortunately, these are not the conditions that bring songbirds to the island. The few birds that are on the island, though, are making the most of this fly cornucopia, such as this Townsend’s Warbler making a tasty afternoon snack out of this hapless bugger, or this juvenile Western Gull picking away at a fly-covered sea lion corpse.  

Prior to writing this blog, it was assumed that the pest we were dealing with was the widely dispersed kelp fly, Coelopa frigida. This species of fly is common along beaches where it utilizes deposited kelp or seaweed for its reproductive cycle. However, thanks to a bit of recent sleuthing, we now know we are dealing with a completely different species. And it makes sense. SEFI is a rocky outcrop with little area or space for kelp to deposit, and kelp flies don’t tend to display the relentless desire to land on humans in massive numbers anywhere else. In fact, the fly that has proliferated on SEFI is better known as a Cormorant Fly. The species we appear to have here, Fucillia thinobia, primarily lays its eggs on dead cormorants and is only found near cormorant colonies. Dr. Robert Kimsey, an entomologist from UC Davis, recently studied this fly on Alcatraz Island in the San Francisco Bay, where it annoys tourists, and discovered the link between this species and cormorants. As it turns out, Alcatraz is also experiencing a profusion of flies and recently posted the following advisory: 

Visitor Advisory – Why are there so many flies on Alcatraz Island and on Alcatraz Cruises vessels?

During this time of year (September & October), one of the 17 identified species of flies on Alcatraz Island is found in large numbers around the dock area and on Alcatraz Cruises boats. These flies are commonly referred to as Cormorant flies, and they do not bite or pose a health risk to island visitors. The presence of these flies is an indicator of a healthy population of cormorants on the island. In the last two years, the cormorant population on the island was decimated by several natural events. This year, the population is rebounding, so the fly population has temporarily increased. These flies are a very important part of the island ecosystem and are not caused by any adverse conditions on Alcatraz Island or on Alcatraz Cruises vessels. We apologize for the annoyance these flies may cause and hope you will enjoy your visit to Alcatraz Island.

Because the type-specimen (the first of a species to be collected and described to science) of Fucillia thinobia was collected on SEFI, we currently believe that Corm Flies are also pestering us. In terms of its general biology, the adult flies deposit their eggs on dead cormorant bodies, and the larvae feed and mature in the guano-stained soil. One thing that makes this species of fly unique from other types of carrion flies is that it specializes on a specific type of carrion. By restricting itself to a particular group of birds in a seabird colony, the Corm Fly has evolved to take advantage of a predictable time and place of high mortality (many seabird young die before fledging) where it may reproduce. Here on SEFI we have three breeding species of cormorants with the most populous being the Brandt’s Cormorant. With 2013 being an early breeding season for seabirds in general and a rather productive year for the Brandt’s Cormorants, this year produced a bumper crop of corm flies. And with the seabirds having recently fledged, it appears that the flies may be exploring the rest of the island for a secondary host in which to lay their eggs. 

Now that we know the species we are dealing with, we may begin to search for answers to our many burning questions. For example, why are adult corm flies attracted to humans? It has been proposed that perhaps they land on humans as they would on any other substrate, but it is obvious to us that they swarm on us in much higher concentrations than on the surrounding substrates. So, are they attracted to us because we smell like carrion? Perhaps they wish to lay their eggs on us and this is just a case of mistaken identity. Originally we wondered if they could be feeding on us, but we do not observe them “licking” us with their proboscis (fly mouth-parts). Either way, we have found, as you may have wondered, showering does not deter them from swarming on us.

It is also apparent that some years have more flies than others, and it would be interesting to know exactly what factors affect their population. Is it just numbers of cormorants breeding, or do Corm Flies reproduce in greater numbers following a couple years of poor cormorant productivity? The last really big year for Corm Flies was in 2004, and an intern related his experience this way:

"A plague of flies during [my internship] provided few places for rest when outside, and howling NW winds became my friend. While on shark-watch, I would stand in the brunt of a gale, and watch flies launch out towards me from the protected south wall of the lighthouse, only to be immediately caught in the high winds and vanish. Standing there with a fly swatter I could kill 20 at a time, only to have 20 more instantly begin to feed on the smashed bodies of their cousins, which I would then swat again. But there was no end to this game... Scoping from behind the carpenter shop was perhaps worse still. Clothing had to cover EVERY bit of skin to avoid insanity. Only a small hole in your hood to view the eyepiece was possible. Afterward, your clothing was peppered in tiny fly turds, innocent enough until you contemplated the sheer number of them"

With a list of numerous unanswered questions, we look forward to delving deeper into researching these little buggers. Until then, though, we are simply trying to avoid being carted off the island in straitjackets. And we are extremely hopeful that our next blog will be about the swarm of birds that descended upon the island and devoured every last Corm Fly.

                         Here's a look at what we really mean by Covered in Flies.