Weak effects of geolocators on small birds

Biologging is a powerful tool and often utilised to study animal movement patterns. But how can researchers be sure that the tag itself does not negatively impact the study animal? A recent meta-analysis published in the Journal of Animal Ecology investigated the effects of geolocators on small birds. Lead author Vojtěch Brlík explains the #StoryBehindThePaper and the study results.

When I was discussing suitable topics for my bachelor’s thesis with my supervisor Petr Procházka and co-supervisor Jaroslav Koleček after a memorable journal club, we were trying to find a balance between the mandatory thesis format as a literature review and my deep desire for data analysis. It was more than two years prior to the expected date of defence so we were not in a rush but I was looking forward to already pursuing a clear aim. Suddenly, Petr suggested half in jest to conduct a meta-analysis and, to his surprise, I shortly agreed. As Petr’s lab studies bird migration and I already analysed light-level data from geolocators retrieved from Eurasian Reed Warblers, the thesis topic was soon arranged. Besides, several people from the lab had already been wondering about the potential geolocator impacts on tagged birds many times as they used them in several species. A few years later, the meta-analysis of geolocator effects on small birds got published with extensive contribution from a large number of authors.

Study timeline

Our team intended to take a novel view on the impact of tracking devices as we expected that published geolocator studies are inevitably based on archived light-level data so one needs to recapture at least some tagged birds after one or more years of recording and storing the information. This could mean that studies with only few returning birds as a result of a potentially detrimental effect of geolocator-tagging would have only a little chance to be published. To avoid obscuring results of the meta-analysis by such publication bias, we thus contacted all geolocator producers and asked for distributing a data request to their customers and we were very lucky as all of them agreed to send the request around. During the next year, I was exchanging numerous e-mails with leaders of many projects to get detailed information on unpublished data. Moreover, I contacted authors of published studies to send the information missing in their papers. This effort led to a compilation of tagging information about over 25,000 birds of 69 species.


We found slightly lower recapture probabilities of geolocator-tagged birds in following years compared to control birds that were marked with metal or colour rings only. However, body mass changes, timing of events of the annual cycle or breeding performance did not significantly differ between tagged and control birds. Moreover, higher relative load of geolocators led to decreased recapture probability in the following seasons and these negative effects were also found in smaller species. Interestingly and despite the enormous effort spent on the collation of unpublished information, we did not find statistically significant differences in the tagging effects between published and unpublished studies. In conclusion, the results encourage further use of geolocators on small bird species but the ethical and scientific benefits should always be carefully considered.

More Info:

Brlík et al. (2019) Weak effects of geolocators on small birds: a meta-analysis controlled for phylogeny and publication bias. Journal of Animal Ecology: https://doi.org/10.1111/1365-2656.12962

One thought on “Weak effects of geolocators on small birds

  1. Pingback: BES Journal Blogs Round Up: March 2019 | Animal Ecology In Focus

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