Interesting question! The situation for Grasshopper Sparrows in western Oregon is somewhat different than in north-central/north-eastern Oregon -- still affected by succession but grazing regimes and/or active management play a big role.
There are a few places with fire-managed native prairie (such as the North Prairie and adjacent restorations at Finley NWR) where occasional burning and other treatments seem to maintain the habitat well enough that we can find a few, year after year. On NRCS-type private-land prairie restorations in the Willamette Valley, there tends to be an early flush of Grasshopper Sparrows in the initial phase of a project, then they peter out as the native vegetation grows too thick.
In the Umpqua region, most Grasshopper Sparrows are found on grazed lands which are a mix of native and non-native pasture grasses, though usually with a native component. Stocking density seems to play a role. In western Oregon, often the problem is "not enough cows" per acre rather than "too many cows" as can be the problem east of the Cascades. In some places ranchers seem to have reduced their herds, or the land has changed ownership to people who aren't running cattle anymore. In some places, grassland landscapes are being fragmented by vineyards, and/or rural residential development.
I wouldn't draw too strong of a conclusion regarding the Umpqua population, based on my brief visits in the last couple of years. When Bob Altman and I surveyed down there a few years ago, we found the bulk of the population on private land where we had access to go well away from the road corridors. On one ranch where we estimated 15-20 pairs, probably only 3 or 4 birds could have been detected by roadside birding. In lands that are cross-fenced, you can find big differences when you go from one pasture to the next, likely depending on how the cows have been moved around that year.
Another takeaway from that experience was that, for grassland birds, there are limitations to the idea of estimating bird populations based on land cover data! A lot of places that look very similar from aerial photos or satellite images end up being very different in terms of the numbers of grassland sparrows. When you get out and walk around you can see differences in the finer-scale structure of the habitat, depending on how it's been grazed, in combination with soil types. Those more subtle differences seem to be key for persistence of Grasshopper Sparrows and Vesper Sparrows.
On Mon, 2019-06-10 at 21:54 -0700, David Bailey wrote:
K & R hits the spot. It's a birding tradition for many of us birding down that way to stop there for shakes and burgers! Grasshopper Sparrow habitat is successional in Oregon for the most part, isn't it? They like native mature grasslands with a few native shrubs in my very limited experience with them in Sherman and Morrow Counties. Do you have a hypothesis as to why you are not detecting the species in areas where you are used to finding them? > > > David in Seaside >