Vegetation phenology and nest survival: Diagnosing heterogeneous effects through time

Abstract

Birds should select nest sites that minimize predation risk, but understanding the influence of vegetation on nest survival has proven problematic. Specifically, the common practice of measuring vegetation on nest fate date can overestimate its effect on nest survival, simply because vegetation at hatched nests grows for a longer period of time than vegetation at nests that were depredated. Here, we sampled the literature to determine the prevalence of this bias in studies of duck breeding ecology. We then used survival data collected from ~2,800 duck nests to empirically evaluate evidence of bias in four different vegetation metrics: vegetation density measured when the nest was found, density when the nest was fated, and date‐corrected regression residuals of these two. We also diagnosed the magnitude of vegetation effects on nest survival by restricting our analysis to only nests which were fated contemporaneously (thereby removing potential bias in the timing of measurement). Finally, we examined whether systematic phenological differences exist between vegetation at hatched and depredated nests that have the potential to further obfuscate the relationship between vegetation and nest survival. We found evidence for a true‐positive effect of vegetation density on nest survival that appeared to be inflated when using raw vegetation measurements collected at fate date. However, taken in combination with the literature review, our results suggest that the majority of duck nesting studies have evaluated the role of vegetation on nest survival using a relatively less biased metric—vegetation density when the nest was found. Finally, we found that over the course of a nesting attempt, vegetation increased in density at successful nests, but decreased in density at depredated nests. As a consequence, duck researchers using vegetation data collected when the nest was found may actually be underestimating the magnitude of the effect. This seasonal change potentially points to an important new metric for understanding predation risk, but further experimental research is required to fully eliminate potential biases in the timing of vegetation measurements.

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.…”

“…Additionally, researchers' ability to detect the relationship varies among species. As noted by Ringelman and Skaggs (), many studies of duck nest survival measure vegetation when the nest is found because nests are located by flushing the female. This method is appropriate for some taxa (e.g., some waterfowl and songbirds), but not for species where investigator disturbance may induce nest abandonment.…”

“…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, ).…”

“…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, ). However, Ringelman and Skaggs () also note that most studies of nest survival for ducks collect data on vegetation density at the time the nest is found, which is likely less biased.…”

“…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, ). However, Ringelman and Skaggs () also note that most studies of nest survival for ducks collect data on vegetation density at the time the nest is found, which is likely less biased. Additionally, vegetation density increased during the nesting period at successful nests, but decreased at failed nests, suggesting that in this system, measuring vegetation at the expected hatch date may introduce another source of bias (Ringelman & Skaggs, ).…”