A few days ago I had the opportunity to take images of California Gulls feeding on brine flies along shoreline of the Great Salt Lake next to the causeway to Antelope Island State Park, which is something I have wanted to do since I moved to Utah a few years ago. The first time I saw this behavior I thought it was very comical as well as interesting and wanted to document the behavior with photographs.
When visiting Antelope Island State Park during the summer I can often see piles of the pupal casings of brine flies on the shoreline that have been blown there by the wind. Perhaps “piles” is a bit of an understatement since there can be over a billion pupal casings in a one mile stretch of shoreline. They remind me of the mounds of Turtle Grass I’d often see on the wrack line on the beaches of Florida but in this case it isn’t grasses but the shed casings of the pupae of the brine flies.
The shoreline and the surface of water nearest to the shore is also dark with the newly emerged brine flies themselves, millions of them! They seem to move in choreographed mass when anything approaches them.
Though other bird species feed on the brine flies of the Great Salt Lake I find the way that California Gulls do it in the most humorous and entertaining way. Franklin’s Gulls seem to stay in one place then as the flies move past them they snap their bills to capture the flies. Many of the shorebirds; like the out of focus Black-necked Stilt above pick the flies off of the water.
The California Gulls seem to stake out a stretch of the shoreline as their territory and chase off other gulls that enter it by spreading their wings and running after the “intruder”. They do not seem to mind resting gulls though and they don’t chase them off. In the photo above the gull in the foreground is actively chasing after the brine flies while the other resting gull appears to be taking advantage of the first gull’s chase by passively feeding on the flies stirred up by the other one’s movement.
It is quite a challenge to photograph the gulls during their chases of the brine flies as there are times the camera wants to focus on the mass of flies rather than the birds. I had begun to think the only way for me to get the gulls sharp was to manually focus on them but gave up on that idea because the gulls move through so fast. I used auto focus instead and it seemed to do well though I have a bunch of out of focus gull photos I will simply delete.
Most of the time the California Gulls lower their heads, scrunch their bodies down and then begin to chase the flies where they are floating on the water while snapping their bills closed around the flies. At other times they lift and spread their wings during the chase which appears to act something like a “scoop” or a “net”. As the gulls move in the direction of the flies , the water which had appeared black because there are so many thousands of flies concentrated there briefly turns blue again as the flies take flight. Rocks or other debris along the shoreline also turns black with the masses of flies that land on them.
There are two main species of brine flies that inhabit the Great Salt Lake, Ephydra hians and Ephydra cinerea, of the two the Ephydra hians is the largest in size but only accounts for about 1% of the total population of brine flies that live in the Great Salt Lake area.
The individual California Gull’s “hunting” territory appears to be about 25 to 30 feet in length, they chase the length of the shoreline, stop for a few seconds then turn and chase in the opposite direction. I have wondered how many of the brine flies the gulls consume during their fast moving forays through the cloud of flies as it appears the gulls expend a lot of energy in the chase.
Brine Flies live most of their lives underwater as larvae who feed on cyanobacteria, other bacteria, diatoms, detritus and bottom-dwelling algae. After the larval stage of a brine fly’s life it grows a pupal case while it changes into an adult by a process called metamorphosis. Adults only live for a few days and the females deposit their eggs. Larvae over winter in the lake and as soon as it warms up again in the spring, the cycle continues.
In this last image I wanted to show just how thick in the air that the brine flies can be while the gulls are chasing after them.
I thought I might be able to add some more images of California Gulls chasing brine flies when I visited there today, but the weather conditions weren’t right. The strong south wind must have been keeping the brine flies from that section of the shoreline. I’m just glad I had the chance to photograph the California Gull’s feeding on the brine flies and capturing the behavior.
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