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Summer Snow

Aaron Holmes

The High Desert that characterizes much of the northern Intermountain West is full of extremes. Field biologists in PRBO's shrub-steppe research program quickly become familiar with daily temperature swings—from morning lows well below freezing to warm afternoons that require shedding layers of clothing. Such fluctuations are common in deserts, because the atmosphere contains little humidity to block solar energy during the day or trap it at night.

In the high elevation (5,000–7,000 feet) sagebrush landscapes where we work in Wyoming and Nevada, most songbirds initiate nesting between late April and early June. Weather at that time of year is predictably unpredictable and can switch from sunny and warm to violently hailing in short order. Our work focuses on "sagebrush-obligate" birds (see box below) that are well adapted to extreme weather. Even so, episodic events can have major impacts that ripple through their short nesting season and beyond.
Snow in sagebrush country. Photo by Corey Lampert/Flickr.

In 2003, a June snow event that persisted for several days at our Wyoming study sites resulted in massive reproductive failure for breeding birds: 35% of active songbird nests failed over several days. Our field biologists recounted stories of Sage Thrasher nests buried under snow, with attending adults entering and exiting through a tunnel-like entrance to an igloo! By day two, the frozen eggs, dead nestlings, and snow-filled nests made it clear this was a defining event for the season, especially for the diminutive Brewer's Sparrow.
Sage Thrasher. Photo by Peter LaTourrette.

We documented similar events in other years in Wyoming and Nevada. We saw the latent effects of storms in the form of nestlings with stunted development that ultimately take 10–30% longer in the nest to fledge. This exposes them to additional risks from predators and decreases the likelihood their parents will attempt a second brood. In Nevada, video footage from one of our nest cameras shows a Sage Thrasher removing two of four live nestlings on the day after a snowstorm. We speculate that this, as well as the delayed maturation of nestlings, was in response to post-storm crashes in food availability.

A number of nests in the egg stage appeared to survive the storms each year but were later abandoned when they failed to hatch. Inspection revealed embryonic death at a stage in development coinciding with the harsh weather. In several cases, Brewer's Sparrows incubated for about three weeks (or up to 30 days) after the expected hatching date—before finally giving up. Weather events can have effects that persist long after the snow is gone and the desert sun returns.