During the 2000 field season (March - September) we located the nests of over a dozen female desert tortoises and monitored the hatching success of their eggs at a study site located in the foothills of the San Bernardino Mountains near Palm Springs, CA. Locating nests required that we mount a thread trailing device (Figure 1) on the back of each tortoise once they were determined to be carrying shelled eggs with x-radiography.
	Figure 1. This photo shows a female desert tortoise wearing a temporary thread trailer.
The loose end of the thread was tied to a shrub so that as the female went about her daily activities, she would leave a trail of thread over her exact route of travel. Each female with a thread trailer was located on a daily basis and weighed to determine if oviposition (egg laying) had occurred (Figure 2).
	Figure 2. Most tortoises nested in the mouth of their burrow. This female is in the process of nesting. The nest chamber is excavated and covered with the hind legs.

Since each egg weighs about 30 grams, we were able to estimate the expected weight loss that would occur at oviposition by multiplying the number of eggs the tortoise was carrying by the estimated weight of each egg. When we suspected that the female had nested (because she lost the estimated weight), we retraced her trail from the last 24 hours and searched for the nest until it was located. After 75 days of incubation, we fenced in each nest using a cage created with wire mesh hardware cloth to capture hatchlings when they emerged from their underground nest chambers (Figure 3).
	Figure 3. This photo shows two newly hatched desert tortoises inside their screen enclosure. The screen not only allowed us to measure, mark and attach radios to the hatchlings before they left the nest, but also provided temporary protection from predators. The pipe in the background was installed to provide an immediate burrow and source of shade for the hatchlings.
We checked each nest every day until all the eggs hatched. Radio transmitters were affixed to several hatchlings to monitor their movement and survival (Figure 4). Our efforts in the 2000 field season allowed us to locate 24 nests that produced 74 hatchlings. Only three (12.5%) of the nests were destroyed by predators.
	Figure 4. Small radio transmitters were attached to the shells of several hatchlings. The antenna can be seen in the right side of the photo.

Ultimately, these data, and data on survival of hatchlings, can be used to build more accurate demographic models to understand the recovery potential of desert tortoises.

Sequence of photos showing a hatchling desert tortoise emerging from its nest chamber