Hi Amos, I am not familiar with the Israeli fauna but do wonder if you have any species of burrowing crayfish that may be creating these burrows?

Diane, the area is an extremely dry desert (although it may not look like it in the photo due to the Vachellia (Acacia) trees) receiving only 25 mm of rain per year. The only land crustacean that we have is the burrowing woodlouse Hemilepistus reaumuri, but even they do not occur in that extreme part of the desert, but in areas of 80-200 mm. They do make mounds, but they look very different and are made of their feces.

Thanks for the clarification Amos. It was just a thought based on the limited information. Hope you get an answer soon.

Ants? Ants will build mounds of debris collected from the area about the nest, and they can easily dig a tunnel of that size. The are all hiding further down to escape the heat of the day.

A small lizard?

A ground nesting wasp?

Solifugid?

Scorpion?

What if you come back at night?

In Spain I have seen those piles closing nest entrances of the harvester ant Messor barbarus. No hypothesis as why they are doing this.

Thanks to all responders.

 Timothy - Ants certainly could do it, but since it is clearly done from outside - there is no tunnel in the stone pile - just in the soil underneath- why would they block from outside an entrance? The same question refers to all other organisms you mentioned - we do see burrows of some of those organisms, but if they are blocked, they are blocked from inside, and it looks very different. The only exception from your list is the wasp, that may cover a nest from outside, and that is a valid option that need to be proven. We did visit the structures at night, and there was no activity during the two nights. 

Daphna - same question - why would ants after a nuptial flight cover a hole from outside? wouldn't they want to be inside the burrow?

Xavier - this is very interesting. Do you have a photograph of similar structures in Spain?

The way a pile is built is determined by where the materials originate. If the pile is built by moving dirt from inside a hole it will look differently from a pile that is built by gathering material from the surface and moving it to one location.

Build a pile for protection, or to control the environment (manage temperature or water loss).

I could build a similar pile by moving stones from the outside to cover the entrance. I would leave a special stone to the end where I would pick it up, walk into my tunnel, and pull the stone over the entrance. As far as a person could see I would then have a stone pile as you describe. So I don't think that you can tell whether the builder is inside or outside based on the construction of the stone pile.

If the wasp was on the outside and finished with the nest then I would expect that the tunnel would be filled with sand. I would also not build a rock pile because it alerts predators/parasites that there is a nest. Usually, the solitary wasps try to disguise the entrance to the nest, not advertise it. So my guess is that the builder is home.

You might have to look at many rock piles. In arid environments a rock pile might not be destroyed for many years after the builder has died. It might be instructive to look at the rock piles after a heavy rain. It might also work to look at them after a big sand storm to see which piles look fresh.

Hello Amos;

     In southern California structures similar to those in the images are made by various crickets and wasps. If the pebbles are really as large as 5 mm. it is unlikely to be an ant.

     Dig carefully! 

Best regards, Jim des Lauriers

@ James, crickets?

@Amos, The piles in the picture look like they are in the wash. Are there any piles under a bush or tree? Are the piles associated with a particular plant?

 A few species of ants drop stones at the nest entrances of other species, making it difficult for these last species to forage outside. I could observe this nest plugging behaviour in Tapinoma nigerrimum (in Madrid, Spain):

Data Nest plugging by Tapinoma nigerrimum / Taponamiento de nido ...

Amos and Jose;

     If some of the pebbles on the piles are as large as 5 mm diameter, Tapinoma couldn't possibly carry such a large stone. Hmmm.

        Jim Des Lauriers

 @ Timothy, Indeed, most of the piles were in the open. Not always in a wash, but sometimes in small dry washes. but I do recall two mounds that were next to a large shrub, Nitraria retusa, but that's not typical. Haven't seen any under a tree yet (even though the Acacia trees are fairly abundant in that area). 

 @ Jose, That's very interesting, thanks. We do have an endemic Tapinoma in the country (T. israele) but they only reach 3 mm in size, and as Jim noted, they wouldn't be able to move the stones on the mounds we have. In addition, they are not found in such extreme parts of the desert. But thanks to your note, we now have a possible explanation for the behavior, even if it is a different ant species. Incidentally, one of my students caught a lCataglyphis ant carrying a small twig on top of one of those mounds, at around midnight. Quite unusual for a diurnal ant. The ant has been sent now to an expert to identify the species and the cast.

Jose's note connects to Xavier's observation in the first few responses to this question, also from Spain.