Ecological responses by Mexican spotted owls to environmental variation in the Sacramento Mountains, New Mexico

I evaluated the hypothesis that Mexican spotted owls ( Strix occidentalis lucida) could be conserved by manipulating microhabitat conditions that increased abundance of one or more of its common prey species. I evaluated this hypothesis by (1) determining which common prey were preferred by this owl, (2) which prey species were most likely to influence the owl's reproduction, and by assessing (3) which prey species were most likely to increase in abundance following microhabitat manipulation while accounting for climate and habitat effects. I focused on one population of Mexican spotted owls over a 6-year period (1991--1996) in the Sacramento Mountains, New Mexico, where vegetation communities have changed considerably during the past 100 years.; I found that Mexican spotted owls preferred Mexican woodrats ( Neotoma mexicana) but consumed greater numbers of white-footed mice (Peromyscus spp.) and voles (Microtus spp.) when abundant. Average number of young produced per pair was comparable to other spotted owl populations but reproduction was more variable over time. The owl's reproductive output covaried with total biomass of mice and voles. Biomass of Mexican woodrats contributed little to the correlation.; Abundance of Mexican woodrats per unit area was consistently low in all habitats during the study but slightly more abundant in late-seral mixed-conifer forest. I estimated that vast reduction of this seral stage could have decreased woodrat biomass by an average of 22 kg, for an overall net loss of 13 kg of common prey biomass per owl foraging range. I also found that shrub diversity and volume of large (≥30-cm diameter) logs, both which were correlated with the abundance of Mexican woodrats, were less abundant in mid-seral stages of mixed-conifer forest. Thus, succession following past timber harvest may have contributed to greater variation in the owl's reproduction by diminishing habitat quality for its preferred prey while fostering more temporally dynamic populations of alternative prey.; Overall, the hypothesis was supported. Forest-thinning experiments designed to enhance microhabitat conditions for Mexican woodrats, and in turn reduce temporal variation in reproduction of Mexican spotted owls, should be conducted to evaluate the hypothesis further.