In the past 23 years, GPS tracking collars have revealed a ton of new information about deer behavior and biology. But a few hunters, skeptical of science or doubtful of the conclusions of specific studies, ask whether the collars themselves change deer behavior and cause bias. If someone attached several pounds of electronics to my neck on a thick, leather belt, it would no doubt alter my outlook on life. It’s a legitimate question with deer or any other tracked wildlife species.
Predictably, scientists are way ahead of us. Identifying potential sources of bias and error, then finding ways to control them in experiments, is part of the scientific method. Just as many deer hunters spend more time practicing with their bow than shooting deer with it, much of scientific inquiry tests equipment and methods before they are deployed for an important real-world experiment.
Above: Andy Olson of the University of Georgia Deer Lab used GPS tracking collars to study deer movements in Pennsylvania’s “Big Woods” in 2012 for his master’s degree. Andy saw no evidence of a “lockdown phase” in the rut, or an “October lull.” (Blindfolds on tranquilized deer help reduce stress during handling.)
You may be surprised at the amount and regularity of self-analysis going on regarding tracking collars, all aimed at improving the performance of the collars and the accuracy of scientific results. It’s also aimed at ensuring humane treatment of research animals, which is required by the federal Animal Welfare Act. Here are a few of the studies I found in my search looking at collar performance, the effects of collars on deer, and even their effects on deer hunters.
Is the Location Accurate?
The first tracking collars deployed on deer in the 1960s were VHF radio telemetry collars. By listening for a radio signal and determining its direction from three different viewpoints, researchers could triangulate a collared deer’s general location. Global positioning satellite (GPS) collars began to appear in the 1990s, and by the early 2000s they were becoming common in whitetail research.
In 1998, Jacob Bowman tested an early GPS collar design by collaring five captive bucks at the Mississippi State University Deer Lab. He climbed into a tower stand to observe their locations, behaviors, movements and head positions at the exact moments that satellites attempted to fix each collar’s location. The satellites had a little trouble acquiring a location when bucks were bedded and their collars nearest to the ground, but otherwise positional accuracy of locations was not affected by behavior, head position or vegetation.
Jacob, who is now a professor of wildlife ecology at the University of Delaware, found that 84% of satellite locations were within 100 meters of the actual location, which even then was considered acceptable error and a huge advancement compared to radio telemetry collars. Satellite and collar technology have, of course, improved significantly in the 25 years since then. These days, under ideal conditions, GPS distance from the true location can be less than 5 meters, and location can be collected much more frequently throughout the day, drawing extremely clear pictures of where and when deer move.
Do They Affect Deer Behavior?
“That is a very difficult question to answer, because you would have to follow deer without collars to see if there are differences in behavior – a very challenging problem to address,” said Dr. Duane Diefenbach of Penn State University’s Deer-Forest Project. “I would say most of the focus has been on whether a collar design causes physical problems, and if it doesn’t then we assume that it therefore does not affect behavior.”
Duane himself led a study, published in 2003, of expandable VHF collars for fawns. In the initial captive phase of the study, he found the collars did not expand fast enough for the growth of the Pennsylvania fawns, so he modified the break-away stitching to open faster. The results then showed the collars caused no discomfort and were used successfully on 86 free-roaming fawns.
In a more recent study from 2018 to 2020, Zach Wesner of the University of Georgia Deer Lab tested five different prototype GPS-collar designs for fawns. He collared 46 different fawns and compared them to 15 fawns that were only ear-tagged as a “control.” He studied how the collars fit, whether they expanded as necessary with fawn growth, whether they affected the fawns physically such as rubbing off neck hair, and how they affected fawn behavior compared to non-collared fawns.
The study report recommended numerous improvements to the new prototypes. “We recommend that GPS collar designs we evaluated undergo additional modifications before deployment in the field, including improved stitching patterns and threads to facilitate more gradual collar expansion, smaller battery housings and improved weight distribution to mitigate behavioral concerns, and smaller band circumferences to improve collar fit across time.”
This UGA study is one of the most comprehensive ever done on the physical and behavioral effects of collaring on deer. It focused on fawns because their small size makes them most susceptible to the effects of carrying around a high-tech collar. Meanwhile, other established designs continue to be used successfully in numerous fawn-survival studies.
“As researchers, we aim to keep collar weight less than 5% of the body weight of the individual,” said Dr. Gino D’Angelo, who leads the UGA Deer Lab. “Generally, I think the differences in behavior due to collaring are minimal and short-term. Collared deer are still breeding and appear to be operating as normal members of populations.”
Do Collars Cause Stress?
Another way of looking at the effect of collars on deer is to study stress levels. Scientists can measure stress in deer through levels of stress hormones in their blood, feces and other places.
In 2007, Dr. Remington Moll led a University of Missouri study of captive deer fitted with dummy collars designed to mimic the size, weight and fit of various types of GPS tracking and video collars. They ranged in weight from 2% to 4% of the body weights of the deer wearing them. Remington found no significant difference in stress hormone levels between collared deer and non-collared deer in the study. In fact, air temperature at the captive facility had a greater effect on the stress levels of all deer than the collars did.
As GPS-collar technology has improved over time, they’ve also gotten smaller, lighter and thus less likely to affect deer behavior. New designs also better accommodate expansion of a buck’s neck circumference during the rut and across years of body growth. From 2014 to 2016, Jacob Haus of the University of Delaware tested expandable elastic and stitched collar designs on free-roaming adult bucks (photo below). His report provided information that would help improve collars to remain on bucks longer, even across years as they grew, without causing harm.
The latest designs now incorporate magnets to allow collars to expand and contract with the size of a buck’s neck. GPS collars that weigh under a pound are now available for use on adult deer.
While it’s difficult to study collar effects on wild, adult deer, it’s clear from study results that GPS collars don’t stop normal deer behaviors like breeding, feeding and long-term survival. And GPS collars have documented deer doing some exceptional things as well, many of them observed in multiple studies repeatedly. Just consider these few examples:
- GPS collars showed deer swimming across incredible spans of open water.
- A GPS collar documented a Missouri buck traveling 186 miles over 22 days on a journey that took him from one side of the state to another.
- Multiple studies using GPS collars revealed bucks and does taking long-range rut “excursions” outside of their home ranges.
- They’ve shown bucks effectively avoiding the hunters who were after them.
- They’ve revealed strategies you can use to encounter bucks in daylight.
Do Tracking Collars Change Hunters?
Aside from whether collars provide an unbiased look at deer locations and behaviors, scientists have also wondered whether collars affect hunters. If you’re studying deer survival and hunter-harvest rates, but hunters hesitate to shoot collared deer, that’s bias. In 2009, researchers with Wisconsin DNR and UW-Madison set up a test to learn whether hunters even noticed collars, and, if they did, how that affected their decision to shoot. I wrote a separate article about that study.
In the reverse, you may not be studying hunter-harvest mortality and need every collared deer to stay alive as long as possible. It’s costly and time-consuming to capture a wild deer, and a deer hunter can erase that investment with a trigger-pull. If you ask area hunters to pass collared deer, will they do it?
In a study at Auburn University published in 2017, Kevyn Wiskirchen examined this question. While tracking a number of GPS-collared deer on public and private land in Alabama, researchers contacted landowners, spoke to public-land hunters, and posted signs and other advertisements asking hunters not to shoot deer wearing orange tracking collars. Compliance with the request was voluntary, since it was not a violation of any law or regulation to kill collared deer.
Out of 122 GPS-collared deer Kevyn studied, 12 or 10% were killed by hunters. Collared bucks (nine out of 58, or 16%) were more likely to be shot than collared does. There was also bias toward public-land deer, as 17% of the GPS-collared deer on public land were harvested, compared to only 8% of private-land deer. The hardest-hit group of GPS-collared deer in the whole study were public-land bucks: 25% of those eight deer were harvested. Kevyn also noted a buck age relationship. He calculated that a visibly-marked, 5½ -year-old buck was 11 times as likely to be harvested by hunters as a marked yearling buck.
The education campaign was mostly successful, since 90% of the collared deer were not killed by hunters, but big bucks and public-land bucks faced the greatest risk. I’m not indicting a few hunters for failing to assist with science. They did nothing illegal. The lesson was for scientists. This study helps scientists take hunter harvest into consideration as they design future experiments using GPS collars.
That’s the overall take-home from my digging into this topic: Deer scientists are aware of the potential for bias from collars, and a lot of other kinds of bias most hunters don’t think about. They’ve been working since the dawn of tracking technology to steadily improve it through self-analysis. It’s clear from the many amazing things deer do while wearing GPS tracking collars that we’re gaining accurate insights into deer behavior never possible before.