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

Abstract

Much interest lies in the identification of manageable habitat variables that affect key vital rates for species of concern. For ground‐nesting birds, vegetation surrounding the nest may play an important role in mediating nest success by providing concealment from predators. Height of grasses surrounding the nest is thought to be a driver of nest survival in greater sage‐grouse (Centrocercus urophasianus; sage‐grouse), a species that has experienced widespread population declines throughout their range. However, a growing body of the literature has found that widely used field methods can produce misleading inference on the relationship between grass height and nest success. Specifically, it has been demonstrated that measuring concealment following nest fate (failure or hatch) introduces a temporal bias whereby successful nests are measured later in the season, on average, than failed nests. This sampling bias can produce inference suggesting a positive effect of grass height on nest survival, though the relationship arises due to the confounding effect of plant phenology, not an effect on predation risk. To test the generality of this finding for sage‐grouse, we reanalyzed existing datasets comprising >800 sage‐grouse nests from three independent studies across the range where there was a positive relationship found between grass height and nest survival, including two using methods now known to be biased. Correcting for phenology produced equivocal relationships between grass height and sage‐grouse nest survival. Viewed in total, evidence for a ubiquitous biological effect of grass height on sage‐grouse nest success across time and space is lacking. In light of these findings, a reevaluation of land management guidelines emphasizing specific grass height targets to promote nest success may be merited.

Timing of vegetation sampling does not influence associations between visual obstruction and turkey nest survival in a montane forest

“…In contrast to other recent studies (Gibson et al, ; McConnell et al, ; Ringelman & Skaggs, ; Smith et al, ), we did not find evidence that timing of sampling (projected hatch date vs. failure date) influenced our estimates or inferences regarding the positive association between VOR and nest survival. Most nests were constructed in evergreen shrubs (primarily common juniper, Juniperus communis ); retention of leaves and slow growth of these plants likely prevent appreciable changes in visual obstruction during the incubation period.…”

“…Specifically, measuring vegetation following the termination of incubation (either nest failure or hatch, a method commonly used in nesting studies, Gibson, Blomberg, & Sedinger, ) may result in increased levels of concealment at successful nests simply because plants at successful nests will have, on average, more time to develop and produce cover than plants at failed nests. This sampling issue may overestimate the influence of vegetation on nest survival because of a relationship which is correlative, but not causative: Successful nests may be more concealed because they were sampled later, not because greater concealment lead them to be successful (Borgmann & Conway, ; Burhans & Thompson, ; Gibson et al, ; McConnell, Monroe, Burger, & Martin, ; Ringelman & Skaggs, ; Smith et al, ; Vega Rivera, Montaño, Rappole, & Cerda, ).…”

“…Suggestions to avoid this confounding issue include date‐corrected estimates of vegetation measurements (Gibson et al, ) and measuring vegetation at the expected hatch date for failed nests (McConnell et al, ). Smith et al () used date‐corrected vegetation measurements to reanalyze data from multiple studies across the range of greater sage‐grouse ( Centrocercus urophasianus ) and, after correcting for plant phenology, found little evidence for a meaningful effect of grass height on the survival of sage‐grouse nests. A true‐positive effect of concealment was found for duck nests, but the effect size was over‐estimated when data were collected at nest fate (Ringelman & Skaggs, ).…”

Factors influencing nesting ecology of lesser prairie‐chickens

“…Although no other lesser prairie‐chicken study has reported this threshold relationship, it is consistent with other grouse studies (Wiebe and Martin , McNew et al ). Similarly, nest survival was maximized between 2.0 and 4.0 dm in a quadratic relationship with 75% visual obstruction where visual obstruction was a better predictor of nest survival than vegetation composition; however, this relationship may be biased high because we sampled nest vegetation within 3 days of assessing nest fate, allowing vegetation to grow taller surrounding successful nests (Gibson et al , Smith et al ). Our findings are consistent with those from greater prairie‐chicken ( T. cupido ) research; however, greater prairie‐chicken peak nest survival occurred at a greater visual obstruction, likely because of greater vegetation growth resulting from increased precipitation relative to lesser prairie‐chicken range (McNew et al ).…”

Vegetation phenology and nest survival: Diagnosing heterogeneous effects through time

“…However, measuring vegetation at each nest on its fate date (depredation or hatching) may also be misleading, because vegetation at hatched nests developed over the full period of the nesting cycle, whereas vegetation at depredated nests was measured at an earlier point in the cycle. Critically, as several researchers have recently pointed out, measuring vegetation on the fate date can automatically lead to the erroneous conclusion that taller/denser vegetation is associated with higher nest survival when no effect is actually present (Borgmann & Conway, 2015;Gibson, Blomberg, & Sedinger, 2016;McConnell, Monroe, Burger, & Martin, 2017;Smith et al, 2018). Gibson et al (2016) and McConnell et al (2017) published concurrent and independent studies demonstrating that measuring vegetation at nests on their fate date leads to inflated estimates of the importance of vegetation on nest survival (Figure 2a).…”