A new study published in the journal Ecology and Evolution raises questions about whether sampling bias might be affecting the estimates of herbaceous cover needed for ground nesting birds. Evaluating vegetation effects on animal demographics: the role of plant phenology and sampling bias, authored by Daniel Gibson, Erik Blomberg and James Sedinger from the Program in Ecology, Evolution and Conservation Biology at the University of Nevada, Reno, analyzed the timing of nest survival surveys to determine required vegetative cover.

The authors used simulated and observational data to model the relationship between vegetation structure and nest survival of greater sage grouse. They evaluated height measured at the time of nest fate (nest failure versus hatch) versus height measured at a standardized date (predicted hatch date). Based on their studies, the measurements taken at predicted hatch date more accurately predicted the influence of grass height on nest survival; the findings were replicated in both the simulated and observed data. The researchers outline how surveys timed for nest fate can come at different times within the life cycle of the vegetation resulting in significantly taller grass cover the later the surveys are done. This can cause a sampling bias for relationships between plant height and nest success.

"We found that studies of grassland and shrubland birds predominantly used nest site vegetation metrics sampled at the time of nest fate, which was more likely to yield a positive effect of grass height or cover when compared to data collected on a standardized date, such as predicted hatch," the authors write. "Our simulations, and evidence from real data, also show that grass height measurements recorded at nest fate produced effect sizes that were biased high relative to the true effect of grass height, and in some cases, this bias was sufficient to change the overall direction of the effect as well as its magnitude. These results are undoubtedly related to the inherent confounding between grass growth and timing of fate for failed versus successful nests; 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."

The study provides an interesting new context to grazing management decisions, particularly as land management agencies consider policies regarding minimum stubble height required to maintain sage grouse populations.