Detecting capture-related mortality in radio-marked birds following release

Abstract

ABSTRACT. A fundamental assumption of avian survival analysis is that the act of capture, handling, and marking birds does not affect subsequent survival. This assumption is violated when animals experience injury, physiological stress, or disorientation during capture and handling that increases their mortality risk following release. Such capture-related effects must be accounted for during analysis, typically by censoring individuals from the survival history, to avoid biasing the resulting survival estimates. We reviewed studies of radio-marked upland game birds to characterize researcher approaches for addressing short-term effects of capture on survival, and used data from a study of Ruffed Grouse (Bonasa umbellus) to illustrate an empirical approach for evaluating such effects and identifying time thresholds to censor individuals that die shortly following release. A majority of studies (65%) reported using some form of censoring for mortality that occurred within one to three weeks after release, although only 8% of studies reported an empirical approach to identify a threshold for censorship. We found that Ruffed Grouse mortality was greater from one to six days following release when compared with 7 to 30 days. This threshold, and the proportion of birds censored as a result of it, is consistent with a number of previous studies of radio-marked Ruffed Grouse. We also found that short-term mortality of Ruffed Grouse following release was reduced by checking traps twice each day and by adequately concealing traps. We recommend that future studies of radio-marked birds employ empirical methods for detecting postrelease mortality thresholds, which will allow for reduced bias while minimizing unnecessary censorship of birds that die for reasons unrelated to capture.Détection de la mortalité attribuable à la capture chez des oiseaux marqués par radiotélémétrie RÉSUMÉ. L'analyse de la survie des oiseaux repose sur l'hypothèse de base voulant que l'acte de capturer, manipuler et marquer des oiseaux n'affecte pas leur survie subséquente. Cette hypothèse est violée lorsque les animaux subissent des blessures, un stress physiologique ou une désorientation pendant la capture et la manipulation, puisque leur risque de mortalité une fois relâchés augmente du coup. Ces effets imputables à la capture doivent être considérés au moment des analyses, en suivant habituellement l'histoire de survie des individus, afin que les estimations de la survie ne soient pas biaisées. Nous avons passé en revue des études réalisées sur des oiseaux terrestres considérés comme gibier afin de caractériser les approches utilisées par les chercheurs pour tenir compte des effets à court terme de la capture sur la survie. Nous avons également utilisé les données d'une étude sur la Gélinotte huppé (Bonasa umbellus) afin de présenter une approche empirique destinée à évaluer ces effets et à déterminer les seuils temporels où on devrait interrompre le suivi des individus qui meurent rapidement une fois relâchés. La majorité des études (65 %) ra...

Effect of Net‐Gun Capture on Survival of Mule Deer

“…Capture‐related mortality has most commonly been defined as any mortality that occurs within a predetermined number of days after release (Bartsch et al 1977, Harthoorn 1977, Chalmers and Barrett 1982, DelGiudice et al 2005). A common approach to avoid bias caused by capture‐related mortality has been to exclude the first period after capture from the data set completely, but there is limited scientific support for the use of any particular timeframe (Norton et al 2016, Blomberg et al 2018). Underestimating the required timeframe keeps biased information in the data set, whereas overestimating the timeframe reduces precision and eliminates valuable and expensive data (Quinn et al 2012).…”

“…Additionally, if captures always take place around the same time of year, information on survival and mortality in the period following captures will be completely missing (Norton et al 2016). Therefore, recent recommendations have been to empirically identify a threshold for censorship (Quinn et al 2012, Norton et al 2016, Blomberg et al 2018). In our study, mortality rates of captured mule deer were twice as high as those of non‐captured deer during the first week following capture.…”

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