Owl Team response to questions from Steve Brink, 10/4/2012

Owl Team response to questions from Steve Brink, 10/4/2012 by Kim_Ingram, at 3:33 p.m. on 4 October 2012,

The following are questions and responses regarding the CA spotted owl: Q1:Since 1993, the owl team stated that they have surveyed roughly 90% of the DSA each year. This was determined by placing a 1/2 mile buffer around each of the recorded survey points. Would it be possible to determine the % "covered" for each year 1986 through 1992 using the same methodology? If so, can the percentage of coverage be provided?

A1: We previously estimated the survey coverage on the Eldorado Density Study Area (EDSA) for the years 1989-1992:

1989: 78% 1990: 82% 1991: 86% 1992: 91%

Although the coverage was reasonably high in these years, many portions of the EDSA were only surveyed once per year from 1989-1992 because we did not have sufficient funding to hire an adequate number of technicians. This lack of survey effort made it more likely that we failed to detect owls that were actually present. 1993 was the first year that we conducted more than 1 survey at >90% of our survey stations.

Q2:Could the occupancy analysis be run on the owl sites that were first found in the study area from ~1986 through 1992? Has the occupancy rate for those owls sites dropped as well? Premise of question: The range -wide high reproductive year of 1992, (most likely due to abundance of prey and very mild spring weather) also lead to a high survival rate (young and adult) and potentially a population that was above the carrying capacity of the landscape. The population in 1993 with nearly 100% occupancy has been slowly adjusting back "down" to the “normal or potential” carrying capacity of this landscape.

A2: We appreciate Mr. Brink’s suggestion, but we currently lack the time and resources to pursue an occupancy analysis for the years 1986-1992. As we stated at the Owl IT meeting in August, we chose 1993 as the starting point of our analysis because it was scientifically defensible. In other words, a peer reviewer could assert that any observed occupancy trends were confounded by changes in survey effort if we used data prior to 1993.

We also question his premise that a single year (1992) of high reproductive output is still affecting our study population 20 years later. Spotted owls are long-lived, but with an annual survival rate of ≈ 0.83, it is unlikely that a birth pulse would continue to influence population trends 20 years after the pulse.

This post is a part of the following discussions: