Later measurement largely accounts for taller grass at successful sage-grouse nests

What is it about?

Adequate grass height for hiding cover has been emphasized as an important component of high quality nesting habitat for greater sage-grouse. However, recent research found that the common practice of measuring grass height around nests directly following nest failure or hatch can lead to a false positive signal that indicates grass height is correlated with nest success even when they are unrelated. This is because hatched nests are measured later in the season than failed nests, which gives grasses more time to grow. We evaluated >800 nests from several studies that previously indicated a strong, positive relationship between grass height and nest survival to determine the extent to which this relationship was produced by biased timing of vegetation measurement. After correcting the data to account for grass growth, we found no relationship between grass height and nest fate, confirming a sampling bias, in two of three re-analyzed datasets, and a reduced but still significant association in the third. Following correction, median grass heights at hatched and failed nests were within 0.05 inches of one another across all re-analyzed datasets. These findings suggest that tall grass may not be as crucial to sage grouse nesting success as previously thought.

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Why is it important?

Nest success is a key vital rate driving population growth of sage-grouse. Thus, scientists and managers have sought to identify components of nesting habitat that contribute to greater nest success. Several early studies that found taller grass around successful nests led to broadly applied recommendations that residual herbaceous vegetation should be at least 18 cm (~ 7 in) tall in areas used by nesting female sage-grouse. Our findings, however, suggest the strength and generality of this relationship has been overstated in the literature because of the wide use of biased methods.

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