Hi everyone,
Thank you so much for your thoughtful replies. These neighbors were friends of ours, and the dumping was not an accident, just people being stupid. They moved across country last week and this was just one thing on a long list of unbelievable things they did including borrowing our car without telling me and abandoning it in front of the airport with the back hatch open (and then getting on a plane)! I'm still trying to figure out why they didn't just give us the gas to use in our mower instead of trying to dispose of it by dumping it out in their yard! We share a fence line and although the dumping occurred about 20 feet from the fence the water they flooded the yard with ran straight down into our yard and pooled in the chicken run. There doesn't seem to be much loss of grass and plant life in the surrounding area (although the area around the fence and coop is already just dirt), so I'm not sure how bad it could have been and won't know unless/until I test the soil. My father-in-law who has some experience with hazardous spills said that it is unlikely the chickens are continuing to be poisoned at this time because gas evaporates very quickly in high heat, but the two birds dying a week apart could still have been from the initial spill because some animals will take longer to die than others after being exposed. We are not eating their eggs at this time, and I am hoping we are through the worst of it, as the other chickens seem healthy and active and show no signs of poisoning or other illness. My other reaction was that this had something to do with the high temperatures we've been having, but I can't imagine it was just the heat that killed the other birds, it has been hot here since March, and they have been fine. They have plenty of ventilation and access to water 24 hours a day. The only thing it could be is this gasoline incident, or an illness that came on suddenly and showed no signs...
I am watching the rest of our hens carefully and keeping an eye on the garden and fruit trees. I think I will go ahead and have the soil tested just to be sure. There's really no use reporting my neighbors, though I was sorely tempted, as they have already moved and won't be causing problems like this anymore.
Thank you all for your help and I will keep you posted!