Timing of nest vegetation measurement may obscure adaptive significance of nest‐site characteristics: A simulation study

Abstract

Advances in understanding avian nesting ecology are hindered by a prevalent lack of agreement between nest‐site characteristics and fitness metrics such as nest success. We posit this is a result of inconsistent and improper timing of nest‐site vegetation measurements. Therefore, we evaluated how the timing of nest vegetation measurement influences the estimated effects of vegetation structure on nest survival. We simulated phenological changes in nest‐site vegetation growth over a typical nesting season and modeled how the timing of measuring that vegetation, relative to nest fate, creates bias in conclusions regarding its influence on nest survival. We modeled the bias associated with four methods of measuring nest‐site vegetation: Method 1—measuring at nest initiation, Method 2—measuring at nest termination regardless of fate, Method 3—measuring at nest termination for successful nests and at estimated completion for unsuccessful nests, and Method 4—measuring at nest termination regardless of fate while also accounting for initiation date. We quantified and compared bias for each method for varying simulated effects, ranked models for each method using AIC, and calculated the proportion of simulations in which each model (measurement method) was selected as the best model. Our results indicate that the risk of drawing an erroneous or spurious conclusion was present in all methods but greater with Method 2 which is the most common method reported in the literature. Methods 1 and 3 were similarly less biased. Method 4 provided no additional value as bias was similar to Method 2 for all scenarios. While Method 1 is seldom practical to collect in the field, Method 3 is logistically practical and minimizes inherent bias. Implementation of Method 3 will facilitate estimating the effect of nest‐site vegetation on survival, in the least biased way, and allow reliable conclusions to be drawn.

Phenology largely explains taller grass at successful nests in greater sage‐grouse

“…While our analyses revealed mixed support for relationships between grass height and nest survival in sage‐grouse, they confirmed recent findings that associations between herbaceous vegetation structure and nest success are frequently byproducts of temporally biased sampling rather than indicative of effect of concealing cover on detectability by predators (Gibson, Blomberg, et al., ; McConnell et al., ). Sampling vegetation following nest fate, a pervasive practice in studies of sage‐grouse and other ground‐nesting birds, consistently produces spurious relationships between grass height and nest survival and should, therefore, be avoided.…”

“…Recent evidence has demonstrated that the positive association between grass height, a commonly used metric of herbaceous concealing cover among sage‐grouse nesting studies, and nest survival may be indicative of biased methods rather than a causal relationship (Gibson, Blomberg, et al., ; McConnell et al., ). Using both empirical and simulation approaches, it has been shown that measuring grass height at nests following nest fate (i.e., hatch or failure) produces inflated or even spurious statistical relationships between grass height and nest survival.…”

Nest and brood site selection of eastern wild turkeys

“…We sampled vegetation characteristics within 3 days after of the estimated hatch date or date of brood use to reduce biases associated with changes in vegetation growth (McConnell et al ). To quantify vegetation associated with daytime use sites, we downloaded GPS locations every 3 days and randomly selected 1 location from the day of our most recent brood survey to visit and sample.…”

Fine‐Scale Habitat Selection by Breeding White‐Tailed Ptarmigan in Colorado

“…We used conditional logistic regression implemented via the clogit function in the package survival (Therneau ) to compare ptarmigan nest or brood use locations (case) to dependent random available locations (control). We were cognizant that vegetation can change over time (i.e., added growth or senescence), and when estimating covariate effects on nest survival, inconsistent timing of measurements can result in biased assessment of those effects on survival (Gibson et al , McConnell et al ). We did not measure vegetation at projected hatch dates as is suggested for nest survival analyses (Gibson et al , McConnell et al ), but we measured vegetation at dependent random sites immediately following measurements at the respective use site (typically within a day or 2 of use), directly comparing vegetation available to females within a given area at that given time (Prima et al ).…”