Bighorn water visits peaked in hot months; lions came year-round

STEP 1 · Hot months

Sheep stacked most visits into four or five months

In the Chihuahuan Desert, 70 percent of desert bighorn visits fell from April through August across 5 site-years. In the Sonoran Desert, 85 percent fell from May through August across 9 site-years. In the Mojave Desert, 83 percent fell from May through September across 21 site-years. These shares use full years only. The crew divided each month's visits by the camera-days running that month. An even pace through the year would put 33 percent of visits in any four months and 42 percent in any five months.

Mountain lions placed 42 percent of Chihuahuan visits, 54 percent of Sonoran visits and 32 percent of Mojave visits inside those sheep windows. Our read is that Chihuahuan lions put an even year's share into the sheep window and Mojave lions put less. Sonoran lions leaned into the hot months and still stayed under the sheep share. Those lion shares varied widely from site-year to site-year.

The months outside each window held 30 percent of Chihuahuan sheep visits, 15 percent of Sonoran sheep visits and 17 percent of Mojave sheep visits. That arithmetic is only a share of visits in this camera set. It does not count sheep living nearby and it does not forecast a new water. Our read is that a quiet cool-month camera at one of these waters is not a test of local sheep abundance.

STEP 2 · Dry weeks

A drier six weeks moved lions more than sheep

Higher maximum temperature and less rain over the prior six weeks lined up with more visits by desert bighorn sheep and mule deer. Less rain over those six weeks lined up with more mountain lion visits. Long-term precipitation influenced lion visits more than prey visits at every site except Armendaris Ranch. Weather covariates were standardized to a mean of zero and a standard deviation of one, so the comparison is a relative model response. It is not an individual's chance of visiting and it is not a kill rate.

At Armendaris Ranch the sheep and mule deer response to a drier six weeks ran stronger than the lion response. The authors report that on this standardized scale sheep and mule deer there were 2 times more likely than lions to add visits as that rain total fell. They suspect the exception occurred because the ranch abuts Elephant Butte on the Rio Grande, and sheep catchments sit about 2.2 miles (3.5 km) from the reservoir. They suggest lions may drink at the reservoir and show a weaker rain effect at the sheep catchments. Our read treats that as one ranch and an untested guess. If a large permanent water sits near the catchment you watch, treat the dry-spell pattern from the other sites as less certain there.

STEP 3 · Same catchment

Sheep and lions rose together at two Sonoran sites

A positive link required the predator and the prey at the same catchment in the same weeks. A negative link can come from different weeks or from different catchments inside the site. Sheep visits lined up with more lion visits at Kofa National Wildlife Refuge and Cabeza Prieta National Wildlife Refuge and with fewer lion visits at Barry M. Goldwater Range East. Mojave and Chihuahuan sites showed essentially no relationship between sheep visits and lion visits on the managed waters in the model. That description covers weekly co-visits at watched catchments. It does not show that lions ignore sheep.

Mule deer visits lined up with more lion visits in the Mojave and Chihuahuan and with fewer lion visits in the Sonoran. In those two deserts our read is that a lion photo at a sheep water tracks deer use of water more than sheep use of water. On Barry M. Goldwater Range East, 1 percent of desert bighorn visits were recorded where 82 percent of lion visits occurred. Our read is to keep lion and sheep photos on separate tallies. One shared week does not show the weekly link the paper found at Kofa and Cabeza Prieta.

Every site also held managed waters outside desert bighorn habitat, and no camera watched those waters. Lion timing there is outside this study.

STEP 4 · Coyote visits

Deer visits tracked coyotes at every site

More mule deer visits lined up with more coyote visits at every site, and the strongest link was at Kofa. Sheep visits lined up with more coyote visits only in the Mojave and at Cabeza Prieta and Kofa. Our read is that a coyote at these waters follows the deer tally more consistently than the sheep tally.

STEP 5 · Visit log

Effort-corrected counts test the hot-season pattern on your water

The authors say year-round water for desert bighorn sheep appears unnecessary because visits center on four or five months. They also say year-round water matches visits by mountain lions, mule deer and coyotes. That conclusion rests on visit timing. Every site provided water through the whole study. The discussion leaves restricting the water season and any change in predator density or predation risk as a later question. Our read is to leave any drop in sheep kills out of this result.

If you can legally run a camera on one water, our test is a full year with the camera-days written down. Record the date and species for each detection you are allowed to review. Count a new visit for that species only after one hour. Use a camera that covers the whole water and runs continuously, and divide each monthly total by the days it ran. The log can show whether sheep visits at that water stack in a hot window and whether sheep and lions shared that catchment in the same weeks. It cannot count unique sheep, recover the model's rain slopes, or show a kill.

Use April through August for a Chihuahuan water in this set, May through August for a Sonoran water and May through September for a Mojave water. Away from these nine sites treat the windows as a starting guess. Our reading calls the hot-season stack a fit when your effort-corrected sheep share sits well above the even-year mark for that window and lions still appear in other months. A share near 33 percent across four months or 42 percent across five months would make that stack a poor description of your water. A few weeks with both sheep and lions on that catchment can occur even where the paper found no link, so they do not by themselves make your water look like Kofa or Cabeza Prieta.

A handful of sheep visits cannot separate these patterns, because each desert share here rests on 5 to 21 site-years. A small cool-month sheep count does not measure how many sheep live nearby. The results stop at these nine sites from July 2009 through December 2016. Season rules vary, so check the state agency for the herd in front of you.