Paint By Numbers

Earlier today I sent an email outlining why wild horse advocates such as myself have a difficult time believing the BLM’s population estimates. It didn’t turnout half bad, all things considered, so I figured heck, I may as well post it here too.

As some of you may have noticed, I have grown quite familiar with the White Mountain wild horse herd in Wyoming.  Documenting the White Mountain herd is something of a pet project of mine which makes using White Mountain as a micro example of why the BLM’s data is really messed up is as easy as breathing.  This is kind of a snooze fest of numbers, so no hard feelings if you can’t make it all the way through first.

 Before we get into White Mountain specifically, it helps to start with talking about how the BLM estimates population.  The BLM claims that a typical horse population will increase by 20% every year.  You may be asking yourself, “Gee, where did they get that rate of increase?”  That question haunted me for several sleepless nights until I finally got my research on!


Most of the BLM’s population growth studies occurred prior to 1982.  Those which exist after 1982 primarily revolve around PZP.  This means that when it comes to studying the BLM’s information on population dynamics, it’s 1982 or bust.  There were multiple studies conducted by several professors and one masters thesis.

The first thing to note is that in the various studies conducted, we see dramatically different numbers depending on the herd that was studied and the methodology used.  The growth rates vary from as low as 20% of studied mares giving birth to a foal that survives its first year to as high as 75% of studied mares foaling (mortality not recorded).  Furthermore, in nearly every herd that was studied, the foaling rate of mares differed by at least 20% from one year to another.  

If I were in charge, I would see this as the first item I would want to study: why would the Pine Nut herd report 45% of mares foaling one year and only 27% of mares foaling the next?  Why are herds in Oregon being reported to increase twice as fast as herds in Nevada? Those are answers we may never have, since all of these studies only lasted two years. How could a mere two years possibly give any scientist an accurate picture of the dynamics of a wildlife population in a given area?

What the above information really means is that 40 years after the Wild Horse &Burro Act was passed, we still don’t have an accurate way to measure and predict growth rate.  This is due to a lack of research. To repeat myself a teeny bit: the BLM claims that a horse herd increases about 20% per year.  That 20% estimate does not account for changes to the climate from year to year, herd dynamics, or the effect of roundups.  Whether the horses face snow, drought, or post-roundup chaos, the 20% figure stays the same.

Most wild horse population estimates come from aerial surveys using a double count method wherein two people count horses, they average the number of horses spotted together, and then inflate that number 20-50% to get a population estimate.  The NAS has verified this is an most inaccurate method, and issued recommendations for improving their outdated methods in both the 80’s and 2013.


Now let’s hone in on that 20% yearly increase that the BLM touts all over the place.  BLM documents most often cite the research of L.L. Eberhardt as justification for their 20% yearly increase statistic.  Eberhardt studied two herds in Oregon in the late 70’s.  I have not been able to get my hands on his report, but from what I am able to glean from the documents that mention it, that Eberhardt’s study (via aerial survey) found that for two consecutive years more than 70% of the mares studied gave birth to foals which survived their first year. 

At the time, this study was considered something of an outlier.  Eberhardt’s foaling rate was anwhere from 10-30% higher than what other researchers were reporting as the highest year of increase in other herds.  For example, the studies conducted by L.E. Boyd in the Red Desert and by Siniff, Tester, & McMahon in two Nevada herds found that only 20-30% of mares foaled two consecutive years in a row.  Despite such basic inconsistencies, it is Eberhardt’s figures that provide that 20% per year rate of increase the BLM uses across the board. 

So now that we have brushed over some of the basics, we can talk about White Mountain.  First, let’s briefly discuss the 2011 White Mountain and Little Colorado roundup.  This was a particularly controversial roundup, since the BLM issued a decision that they would have a “non-reproducing herd”, by capturing every horse and only releasing horses that had been castrated or spayed.  A lawsuit was filed and the BLM backed down, deciding instead they would have a standard helicopter roundup, give 2 year birth control to every mare released, and release horses in a ratio of 65:35 stallions to mares.

In November of 2007, the BLM had a helicopter roundup in this area. At the end, the BLM  estimated the post-roundup population was 274 horses total in both herds.  In April of 2010 an aerial survey was conducted. The BLM reported that the results of that aerial survey showed there were a total of 564 horses in the two herds. That implies that in two breeding seasons, the herd increased 105%.  For those who are less acquainted with horse physiology,  an adult mare can only produce one foal per year, making a 105% increase in two years impossible.  So if the 2010 survey numbers were accurate (and I would agree they were fairly close), then the only conclusion that can be reached is that there were more horses in White Mountain and Little Colorado than were reported in 2007.

In 2011,  I questioned this in my comments to the BLM Advisory Board.  Because I’m nothing if not a smartass, I told them their numbers implied that every horse, including the stallions, were clearly giving birth to twins . In order for the herd to double in two years, every single mare and multiple stallions would have to foal.  So clearly, the BLM cannot stand on the concept that the wild horse population numbers are high due to “out of control” foaling rates.  Rather, the BLM’s lack of accuracy and refusal to conduct a real census on any given herd, e.g. not just a flyover count, has led to inflated estimates of reproduction rates.

Rather than questioning the number of horses found in the 2010 aerial survey, the BLM assumed there had to be 564 horses in White Mountain and Little Colorado.  In fact, not only did the BLM assume that those numbers were correct, but they increased that figure by 14% to account for horses not counted.  Apparently they were all hiding in the imaginary juniper.  On top of that 14% increase, the BLM tacked on an assumed foaling rate of 21%.   

If we expand that even further, the BLM predicted that from April 2010 to July of 2011, the population would increase by another 169 horses (21%), for total increase of 71.2% from the aerial survey performed in 2010.  A practical person might take a step back and question these numbers.  Yet the BLM used them to justify the roundup, claiming that by August of 2011 (when the roundup occurred) there would be 970 horses on the range. 

This leads us to the roundup itself.  The BLM planned to capture a total of 873 horses, or 90%, of the horses in the herd.  In reality they could only locate and capture a total of 699 horses, including foals.  That implies there were 770 horses total, which means the BLM overestimated the population by 200 horses.  So while I would agree that there were more horses out there than the BLM reported in 2007, the difference in numbers shows that the BLM is overestimating the number of births by a considerable amount.

White Mountain Roundup, 2011

White Mountain Roundup, 2011

Now all of the above looks like it should still come out in the BLM’s favor.  There were more horses than they thought there were in 2007, so why would that not also be the case post-2011 roundup?  Part of this can be attributed to the increasing number of roundups post-2000.  Prior to 2000, roundups in any given horse herd would occur maybe once every 8-10 years, if that.  This in combination with poor survey techniques led to vastly different populations than those that were estimated.  Following changes in management under the Bush and Obama administrations, wild horse herds began to be rounded up every 3-4 years like clockwork.  The increasing level of roundups has led to an increased accuracy in the population numbers reported after a roundup.  Meaning, the more times they round up the horses, the closer the population estimates get to the actual population. 

The 2011 White Mountain/Little Colorado roundup lasted only 10 days.  The contractor, the Cattoors, were given three weeks to complete the roundup.  In 2011, the numbers in holding had not reached the “crisis level” that they are at today, and there was additional space in long term and short term holding for horses.  Few facilities were full.  It was common for the BLM to remove more horses than they planned to in the Environmental Assessments, under the claim that they located more horses than they predicted.  But instead of finding more horses and removing more horses, they captured fewer horses than expected in White Mountain and Little Colorado.  Roundup contractors are paid by the head, to encourage them to capture as many wild horses as possible.  It would have been in both the contractor and the BLM’s interest to continue the roundup if there were more than 699 horses to be found.  

Some may argue that there could have been horses who were “hiding” and unable to be seen by the helicopter. The White Mountain and Little Colorado herds are hilly, but they easily viewable from the air and contain no juniper or trees to provide cover for the horses. Approximately 65% of the horses in  White Mountain are sorrel or chestnut, with the second most common color being bay.  Those dark horses would have stood out well against the sage and yellow rocks in the area. 

Additionally, if the BLM’s numbers were inaccurate in 2011, I would expect to see more uncaptured mares in the herd.  Every mare who was released in 2011 was given a hip brand.  This makes it extremely easy tell captured mares from uncaptured mares, and to analyze the data accordingly. 

Let’s review my personal data for the White Mountain herd, shall we? At this time, I have located and can actively track roughly 70% of the horses in White Mountain (I anticipate this number will rise in 2014 as I spend more time in the North).  I define “actively tracking” as horses I am able to locate once per year at minimum, and can track their movements and foaling rates.  Note that my sample set is considerably higher than sample sets used in the BLM’s late 70’s and early 80’s population studies.  At this time, my records indicate the following:

–          The BLM captured at least 85% of the horses in White Mountain.  One band consisting of one stallion, two mares, two 2010 offspring and one 2011 offspring eluded the helicopter

–          In 2012, the herd saw a 10.8% increase overall.  Note that the 2012 foal crop would not have been effected by the contraceptive drug (PZP-22) given to the released mares in August 2011

–          In 2013 the herd saw an increase of 5.7-10%.  There were two bands I did not locate in late summer of 2013, but who I am confident I will locate in the spring of this year. If every mare in those two bands foaled (which is unlikely) then the increase would be the 10% figure.  If none of those mares reproduced, it would be the 5.7%

–          The above increases do not account for mortality rates.  Wild horse mortality rates are typically averaged as 15% mortality in foals and 5% mortality in adults per year

My statistics indicate that if every horse (including foals) survive the winter, that is, there was no mortality among foals or adults observed in 2012 or 2013, then there will be 206 horses in White Mountain this Spring.  Compare this to the BLM’s estimate of 246 horses (17% increase) in White Mountain in 2013.

Fermat's Band contains three adult mares, all of whom were given PZP in 2011.  Two foaled in 2012, none foaled in 2013, and while it is a little early yet to tell, none of them look pregnant in 2014

Fermat’s Band contains three adult mares. Two foaled in 2012, none foaled in 2013, and while it is a little early yet to tell, none of them look pregnant in 2014

So why are my numbers in the field so radically different from what the BLM is reporting?  That is something that wild horse advocates have never received a complete answer on.  Additionally, the White Mountain herd is a 15 minute drive from the Rock Springs Field Office, so why can’t the wild horse specialist, Jay D’Ewart, find the time to do something so simple as put his boots on the ground and get a more accurate rate of increase? I’m a twenty-something who lives 6 hours away from this herd, and yet I have a better bead on White Mountain than the individuals we pay to manage them.

If the BLM wishes to open dialogue; if they want to start earning the trust of suspicious, cantankerous wild horse lovers like me then a true population census is a good place to start. I love crunching numbers, but trying keep up with the BLM’s mountain goat leaps in logic as they look for excuses to inflate the wild horse populations as much as possible is freaking exhausting.

(If they’d quit having closed door sessions and memos talking about euthanizing removed horses en masse or shooting horses out in the field wouldn’t hurt either)


5 thoughts on “Paint By Numbers

  1. Prairie girl says:

    I’ve read ‘wild horse Annie and the wild mustangs’ .
    You are now my new hero, Rachel.
    I know you won’t give up on them.
    After reading this, you’re just getting started.
    And I’m so happy about that. I’m behind you all the way.
    “Heads up BLM”.

  2. Puller Lanigan says:

    You go gurl!!! We don’t get answers from the BLM because in the gov’t it’s ‘shoot, shovel and shut up’. Nobody admits they don’t know or don’t have an answer.

  3. Anonymous says:

    Thank you – the first analysis that stands next to the numbers of the BLM. CENSUS is the FIRST STEP to take for better management. “WE CANNOT CHANGE WHAT WE CANNOT MEASURE”. That is where the budget should go. And so easy here in Colorado with very well documented herds in Springcreek, Sand Wash Basin, Little Bookcliffs and Douglas. Unfortunately the reading and study takes BOOTS ON THE GROUND (surprise) and ONE FLIGHT PER YEAR by the USGS is NOT sufficient to claim a functioning scientific CENSUS PROGRAM (USGS referred to limited funding)

  4. CSZ says:

    This is awesome. Thank you so much for devoting your time and efforts to collecting this information and writing this clear, concise brief. I want to know how to get this very information to people who will read it (!) and be willing and able to do something about it. I am getting nowhere on Facebook (friends have their own issues I guess), and have gotten no responses to letters to media or state (OK) or federal government. I have started appealing to people in their “pocketbooks” by talking about the costs of the roundups and holding, versus letting the horses stay on the range, but even that doesn’t seem to make a difference.

  5. Anonymous says:

    Don’t give up the fight! This is not a priority for any Government (any lvl). Only those who care about the horses will continue to make a difference.

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