AUTHOR BLOG: Geolocator Effects May Have More to Do with Marking Method Than Mass

Henry Streby

Linked paper: Comment on ‘‘Mixed effects of geolocators on reproduction and survival of Cerulean Warblers, a canopy-dwelling, long-distance migrant’’ by H.M. Streby and G.R. Kramer, The Condor: Ornithological Applications 119:4, November 2017.

Author Blog picUnprecedented numbers of species and individual birds have been marked with all sorts of tracking devices in recent years, and those numbers will continue to rise in the future. The data ornithologists are gathering by marking birds with tracking devices are providing a wealth of previously elusive knowledge about all stages of birds’ life cycles. The rapidity with which new tracking studies are being initiated places an ever-growing burden on the USGS Bird Banding Lab (BBL) and other agencies charged with assessing auxiliary marker requests and determining permission to mark birds on a case-by-case basis. In many cases, those agencies have little to no species-specific information on which to base their decisions, either because a species has never been marked before or because those who marked them did not study or did not report marker effects during the course of their research. We as research biologists have an ethical responsibility to objectively assess the potential effects of our research activities on the animals we study for the sake of minimizing harm to the animals and potential bias in our data caused by markers impacting animal behavior or survival.

We applaud the authors of Raybuck et al. (2017) for honoring a request from the BBL to band additional birds as control groups during their geolocator study of Cerulean Warblers. We believe Raybuck et al. conducted their study and published their paper with ethical and admirable intentions and it was not our intention to criticize the integrity of their research. However, their conclusion of an overall geolocator effect on annual survival of Cerulean Warblers was overreaching in light of the modest sample-sizes and the confounding factors of year, site, and marking method. We believe our analysis and interpretation of their apparent annual survival data is more appropriate and clarifies some small but critical shortcomings in their assessment. Of primary concern, the negative geolocator effect on annual survival of Cerulean Warblers was driven by a strong negative effect associated with one marking method in one year, and there was no support for a negative effect of geolocators when they used a different marking method in the second year of their study. This difference was not driven by geolocator mass, because the reduced survival occurred in the year when a lighter geolocator was used.

In our opinion, there are two broad take-home messages from our paper. First, when marking a species for the first time, it is critical to use available knowledge and replicate methods known to work well in closely related species. When the smallest details are considered, there are almost as many methods for deploying a geolocator on a songbird as there are species that have been marked with geolocators. It should not be necessary for each research group to reinvent methods and relearn unfortunate lessons when simple, safe, and effective methods have been developed and made widely available. Second, there is only one control group that is relevant for comparison to a marked group of birds during a geolocator study or any other study of marked birds: a control group identified at the same site(s) during the same period of the same year. Parameters of interest, especially survival, can vary widely among years and populations. It is therefore not appropriate to compare parameters like survival of geolocator-marked birds with those of control birds from any other population or year or even long-term averages from the same population. Just as we have a responsibility to assess the potential effects of our research on the animals we mark, so too do we have a responsibility to design and present those assessments in a statistically rigorous and scientifically appropriate manner. It is then that we achieve our goal of providing accurate information that may be useful to other researchers and those in oversight positions who use this type of information to decide whether continued or new research is permissible.

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