Sampling protocol has sage grouse implications

For grassland and shrubland birds, such as sage grouse, grass height is important for the success of nests. However, the nature of the relationship has potentially been confounded in some of the most recent studies due to the timing of sampling, as revealed in a new study published by Daniel Gibson from the University of Nevada-Reno.

In this study, the authors assess how researchers have been studying the relationship between when grass height was measured relative to nest success. The authors concluded that, in a majority of studies, researchers measured vegetation at failed nests at the time of nest failure, usually early in the season, and then later measured vegetation for successful nests at some predicted hatch date.

The problem with this approach is that plant growth stage, otherwise known as “phenology” in the scientific world, is at an earlier phase when sampled for a nest that did not survive to the expected hatch date than it was if sampled later at the expected hatch date. By sampling failed and successful nests at different times, this sometimes leads to a grass height and nest success relationship that is “positively biased relative to true effects,” according to the researchers.

This problem can lead to a “bias sufficient to the change the overall direction of the effect, as well as its magnitude.”

To quote the authors, “The fate of successful nests occurs inherently later in the season. Therefore, vegetation biomass will increase prior to sampling for successful nests when compared with unsuccessful nests, which fail and are sampled earlier.”

When you read the paper, make sure to pay attention to Figure 3, which shows the difference in the relationship between sampling vegetation at the failure date versus sampling vegetation at the predicted hatch date for sage grouse in Nevada. You will note that the slope of the line and variability around the line is quite different for the two approaches. The authors conclude that, when this confounding effect is considered, the positive effect of grass height on sage grouse nest survival is a very weak effect.

I am not sure at this point how these findings will affect our current situation with sage grouse, grass height and grazing, but I know that this paper is very insightful with very strong implications for this major issue. I also suspect this is not the last we will hear on this issue.

Please contact me any time with thoughts or concerns at jscasta@uwyo.edu. Check out my blog “Rangelands4u” at wyoextension.org/rangelands4u.