California is a strange situation in that generally pheasants are either on one of two types of lands, public and private. Intensely farmed private land like rice fields or upland orchards, pastures and private duck clubs but, generally, there isn't much nesting habitat for upland birds on rice operations. The checks are all that there is with the rest flooded. This makes it easy for the few predators in those areas to just walk the bank and vacuum up a lot of the nests.
The public lands are state and federal wild life areas that tend to be wetlands and uplands with some crops planted for use by all birds. There is good nesting cover and low predator numbers on some of the best places but pheasant numbers remain low. There aren't huge expanses of dry grain fields and natural cover subject to low hunting pressure while still having public access like in Montana. That's a huge generalization about the differences but it's close.
Although winter kill of adults isn't a problem, spring rain and following cold snaps can kill chicks and ruin nesting success. Conversely, not having enough moisture in the spring puts stress on the chicks in their initial days and weeks by not having enough invertebrates to eat and water to drink. Some area managers plant some grain and think that will be all that's needed since pheasants do so well in the grain states while our local mosquito abatement districts are spraying and killing everything that chicks need to grow before they can shift to the grain that's planted.
One 10,000 acre state wildlife area, Gray Lodge, shot a little over 300 pheasants last year with a 5 week season. In the mid 1990s the same area shot 1,700 roosters with a 3 week season. Nothing has changed on the area except a huge increase in mosquito abatement and raiding of the area's budget by the abatement districts to pay for the spraying while exempting the adjoining rice fields. Statewide however, pheasants are in a deep down cycle and nobody seems to have a reason why except maybe the spraying and clean farming practices.