The idea is to use a "normal" GLS in combination with a pre-programmed, possibly weight-reduced GPS logger that switches on/off at defined dates (let's say 4 weeks before/after the equinoxes) and also under constant light (or dark) conditions; the latter is basically in function like a saltwater switch, but working with information on light. When active, the GPS would log 2 positions a day (no finer resolution needed than with the GLS working for the rest of the year), therefore saving battery life and reducing size. No trouble in using a GPS logger, since the bird has to be recaptured to get at the GLS anyway.
This would be an alternative to a) having to skip several weeks to months of logged data for analysis because latitude could not be determined reliably with GLS and b) calculations/modelling environmental variables recorded during equinoxes/midnight sun to get an idea of likely geographical locations at the time.
The idea crossed my mind in Sept 2015 and I asked around a bit in Dec 2015 at uni, etc. Neither then nor now, have I found literature on anybody having tried/working on this. There is some literature on "combined-" or "double-tagging", but with the purpose of estimating the GLS' s accuracy, not to get data during the equinoxes.
Has anybody tried this?
What would be obvious pitfalls? (I know, weight of the GPS logger is an issue for smaller species.)
How small can a GPS logger be, when it's necessary to log only about 500 locations? (4 weeks before/after each equinox = 112 d = 224 loggings PLUS midnight sun on Svalbard max. ~ 126 d = 252 loggings. That gives a total of 476 loggings.)
How difficult is the programming?
- Getting it to switch on and off at a defined calendar date cannot be too complicated. How about programming it to switch on when there's 24 hrs constant-light conditions?
Which companies/working groups are specialized on non-standard things like that?
(Literature: Afanasyev 2004; Bogdanova et al. 2011; Evans et al. 2013; Frederiksen et al. 2012; Gaston et al. 2015; Gilg et al. 2013; González-Solís et al. 2011; Guilford et al. 2009; Leat et al. 2013; López-López 2016; Millspaugh et al. (date?; Wildlife radio-tracking and remote monitoring); Phillips et al. 2004; Phillips et al. 2007; Ponchon et al. 2013; Reiertsen et al. 2014; Rutz & Hays 2009; Seavy et al. 2012; Wilson et al. 2002).
Dear Judith,
these two companies may help you.
Kind regards,
Michał Filipiak
http://www.e-obs.de/index.html
http://www.biotrack.co.uk/index.php
Hi Judith,
If you find a suitable (for your species) light weight GPS that can store 500 positions, than I think you can forget about the low accuracy GLS and just use the GPS with one fix a day (nearly 17 months of monitoring).
Best,
Olivier
Hi
I concur with Olivier's answer. If your goal is to have better data on the whereabouts of your focal species, then if already integrate a GPS unit in your tracking tag, you are more likely to get better data by using the weight to maximize battery size and allowing additional GPS fixes than having the overhead of another, inaccurate sensor and a separate battery.
Having said that, I think that the idea of combining tags (GPS+geolocator) can be very useful if your goal is to improve tracking methodology. If eventually your goal is NOT to get the best possible data on your focal species, but to develop methods for better interpretation of GLS tracks, having this dual dataset will be very useful. This combination (where GPS fix can be regarded as the 'true location' for a given time point since its measurement error is negligible in large scales) will allow you to compare different filters and post-processing approaches of the GLS data, and see which one provide best fit to 'real data'. This comparison can help us to get better inference from GLS data in other system, where dual tracking is impossible. Among other advantages it will also allow you proposed ways one might do to compensate for GLS inaccuracy at different contexts, as you initially planned.
We have taken a similar approach when contrasting ACC-identified behaviors with GPS-tracking of vultures, to validate our behavioral classification (e.g., how many of the behaviors identified as 'feeding' actually happen at a feeding station or a foraging habitat? how many of the 'active flight' behaviors are actually when a vultures is moving fast?).
Eobs, mentioned above were very helpful in implementing custom made set-ups, although integrating the GPS sensor with the light sensors sound nontrivial, and presumably time consuming. Also, i know Yossi Yovel from Tel Aviv university (http://www.yossiyovel.com/) have a custom made miniature GPS tag for bats that can be integrated with external sensors like sunlight sensor.
Best
Orr
Thanks, Michal, Olivier, and Orr, for your thoughts and recommendations.
Much appreciated.
Best,
Judith
Dear Judith,
sorry, I couldn't say something important in response to your question, being not as much familiar with this technology. But I always track the information about these devices, hoping to get the possibility to use ones in future.
Best,
Igor
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