A recently published article in Colorado Birds magazine has assessed the amounts of bird mortality from the 2020 September snowstorm that blanketed the San Luis Valley. In the article titled “An Assessment of the Bird Mortality from the 2020 Summer Snowstorm in the San Luis Valley, Colorado” it is estimated that more than 100,000 birds likely died in the freakish summer snowstorm.
Local field ornithologist and author John J. Rawinski of Monte Vista conducted the research based on information obtained from volunteer birders within a five-county area including Alamosa, Conejos, Costilla, Rio Grande and Saguache counties.
On Sept. 8-9, 2020, an abnormally cold snowstorm descended on the San Luis Valley covering most of the Valley with deep snow and freezing temperatures. Alamosa reported 15 inches of snow in this strange summer snowstorm. Temperatures plummeted from 87 F. to 27 F. the next day. The cold temperatures persisted for additional days, keeping the snow present and deep upon the ground.
The calendar season said summer, the bird season said fall, and the storm looked much like winter. The drastic changes in weather conditions had direct impacts on both migratory and resident songbirds that resulted in scores of bird deaths.
Rawinski surveyed numerous birders in the San Luis Valley, and they provided data on the numbers and kinds of birds they saw deceased on their property. Bird data were then extrapolated to the number of rural residences in the San Luis Valley. Bird mortality was estimated to be in the hundreds of thousands. Those rural residences acted like beacons in a storm and drew birds desperately looking for shelter. The mortality is a conservative estimate since no urban areas were surveyed, and it is likely birds died within towns as well.
Insect-eating birds suffered the greatest losses, comprising 93 percent of all the documented mortality — warblers, bluebirds, thrushes, flycatchers and swallows. The deep snow and cold effectively shut down insect activity, leaving the birds in a desperate situation. Seed-eating birds also had less available food and suffered to a lesser extent.
Many of the respondents reported seeing birds in torpor. Torpor is defined as a state of inactivity in birds that is brought about by physiological changes such as lowering heart rate, breathing rate, metabolism and a reduced response to external stimulation. Cold temperatures or lack of food seem to be contributory factors. In their weakened condition, birds were unafraid and could be easily approached.
It is believed that the deep snow and cold made food sources unavailable, and birds went into torpor to maintain dwindling energy. The snow on the ground lasted for a few days, and this was a circumstance from which they could not recover.
Another source of bird mortality from the snowstorm was that large numbers of birds perished as a direct result of highway collisions. The deep snow effectively cut off food sources for birds, so they were forced to gather along plowed roadsides where plants and seeds were exposed along the highway edges.
When vehicles passed by, the birds would flush, and collisions resulted. It is estimated that more than 11,000 birds perished along the roadsides alone in the five-county area. Collision mortality was not limited to insect-eating birds and now included a wide variety of species such as Green-tailed Towhee, Vesper Sparrow, Mountain Bluebird, Wilson’s Warbler, American Robin, and Hermit Thrush.
Respondents observed no significant dead birds in their locations in weeks prior to the summer snowstorm. It was concluded that any pre-existing conditions from wildfire smoke, migration fatigue or drought was not directly resulting in massive bird die-offs in the San Luis Valley.
However, based on this research, it is believed that the abnormal summer snowstorm had direct, primary, and acute impacts causing massive bird mortality in the San Luis Valley region, Colorado. Birds likely starved to death from lack of food and in combination with extremely cold temperatures, were unable to sustain their metabolism.
Did the storm affect local bird populations the following year? In general, the spring migration 2021 in the San Luis Valley had fewer overall bird numbers and this trend was apparent in many parts of Colorado based on broader chatroom dialogues. Local surveys near Mosca showed bluebird numbers to be much lower in 2021 than in prior years. The full paper is available through Colorado Birds magazine or by contacting the author.
Photos by John J. Rawinski
A Wilson’s Warbler was one species heavily affected by the snowstorm. Here a bird is shown in torpor, and is easily approached.
Mountain Bluebirds had a difficult time in the storm. Many succumbed to the cold and snowy conditions.
Wilson’s Warbler suffered tremendous mortality from the snowstorm. This bird was lucky enough to find a spider at the water’s edge.