The Institute for Bird Populations has just published a study of all known Great Gray Owl nest sites in Yosemite area of the Sierra Nevada. Here is the official citing for the article: Wu, J.X., R.B. Siegel, H.L. Loffland, M.W. Tingley, S.L. Stock, K.N. Roberts, J.J. Keane, J.R. Medley, R. Bridgman, and C. Stermer. 2015. Diversity of nest sites and nesting habitats used by Great Gray Owls in California. Journal of Wildlife Management 79:937-947.
It cost me $38 to see the whole thing so I will give you a summary.
As I found when writing my Great Gray Owl book this species is NOT a strictly montane bird, nor is it confined to conifer forests. It may now be most likely there because we humans have driven them out of valleys and more oak-dominated terrain like the Sacramento and Willamette River Valleys where they may have nested three centuries ago.
One-fifth of the nests in this study were found below 1000′ in elevation.
30% of nest trees were oaks, large oaks, within conifer stands.
The GGO prefers well-rotted trees beneath a fairly dense canopy. Message to land managers: don’t rip down all your old, rotting oaks or Doug-fir.
Many of the preferred nest snags lasted about five years after the first nesting use. Good naturally occuring nest trees are ephemeral
Slowly deteriorating trees like incense-cedar and sugar pines are not used for nesting.
This study included no man-made platforms which are frequently used by Oregon populations of GGOs, both in the Wallowa Mountains and southern Cascades.
This study covered 56 nest sites in the California Sierra confirmed between 1973 and 2014. The southernmost one is on the Tulare-Fresno County border. That’s the southernmost GGO nest in the known world.This picture shows female with owlet in nest that was probably originally built by Ravens, not in a rotted out, broken tree trunk as most often found in Yosemite area. The pictured nest was used only once in the southern Cascades of Oregon. One of the two owlets survived fledging from this nest in a ponderosa.