Avian surveys

Anderson (1970, 1972) noted the location of his plots, which were quarter sections (65 ha) within the Township and Range divisions of the Public Land Survey System. The exact locations of Anderson's bird surveys within quarter sections were unclear but most quarter sections were within more extensively forested areas, as determined from aerial photos. Therefore, we conducted our surveys within the same quarter sections and assumed, because we were surveying the same habitats, that our survey results would be reliably comparable to Anderson’s. In two cases (plots 6 and 7), access was reduced because of COVID-19-related regulations so some counts we conducted were outside of the quarter-sections, but within 250 m of plot boundaries and in the same habitats. We categorised our survey locations as “In” plots or “Out” of plots so we could assess potential differences in bird estimates.

Anderson counted birds using four different methods (strip census, strip map, point quarter and sample count methods), but stated that the sample count method produced the most accurate density estimates, which were consistent with his estimates from information combining all his techniques (Anderson 1970, Anderson 1972). The sample count method involved conducting ten 10-minute stationary point counts spaced approximately 95 m apart along an “irregular” transect. We assumed that “irregular” referred to a transect with a non-straight vector, but are unsure if Anderson's transects followed roads and trails or simply transected forest. The forest understorey in his plots is characterised by dense vegetation and steep terrain, conditions that would disturb birds and potentially reduce the number of birds counted, so we hypothesise that his irregular routes followed logging roads and trails to minimise disturbance. At each point, Anderson recorded all birds seen within 18 m and, if a bird was heard, he attempted to visually observe it. He counted birds starting one hour after sunrise at least once per week between 2 November and 1 March 1968 to 1970, on days with good weather.

To maximise our chances of reliably comparing our results with Anderson’s, we also used stationary (point) counts that were spaced approximately 100 m apart along roads and trails and were 10 minutes in duration. Plots were visited four times total, during which we conducted 10 to 15 counts per visit. All counts were conducted from dawn to noon on days with minimal rain or wind. Each plot was surveyed twice each year in January through to March, usually once by each skilled observer (except for plots 3 and 4 which were surveyed twice by the same observer in 2021). The total effort amounted to 35 to 55 total counts in each plot. Numbers of points varied amongst years and plots and are preserved in the data archived here. We made no attempt to repeat surveys at the same locations across years. All survey locatons are identified by their latitude and longitude (see below). During each count we recorded each bird that was detected, the distance to it (checked with laser rangefinder), the cardinal direction of its initial detection and detection type (singing, calling, visual, flyover, drumming). If a bird was heard within approximately 20 m, but was not seen, an effort was made to visually locate that individual. Thus, we repeated Anderson’s methods as closely as possible. At the same time, we used modern counting methods with unlimited radius count areas and time-interval tracking of individual birds where each count was divided into ten 1-minute intervals (or in a few cases two 5-minute intervals). We also measured the latitude and longitude of each point location with hand-held GPS units, typically Garmin etrex 10 (accurate to within 10 m).

Counts were conducted as back-to-back 5-min surveys totalling 10 min each. Within each 5-min count, each bird was given a detection-no detection (dnd) history, based on whether or not it was detected in each of the five one-minute intervals. A 1 indicates the bird was detected (irrespective of detection cue type) within that interval. A 0 indicates it was not detected. Thus, dnd histories are strings of five 0 s and 1 s with one entry of a 1 or 0 per interval. An exception was for 1455 records where the dnd histories included just two total intervals for a 10-min count. In the latter case, if a bird was detected during the first 5 min of the 10 min count, it was noted as 1 in the min1 interval and, if it was not detected, it was noted as 0 in the min1 interval. Likewise, for the second 5-min of a 10-min count, if a bird was detected, it was noted as 1 in the min2 interval and 0 if it was not.

Habitat surveys

Anderson recorded trees per acre, average canopy cover, height class distribution, shrub density and qualitative descriptions of vegetation in each plot. Anderson used a variety of methods to produce his results. We used methods described by James and Shugart (1970) to produce the same metrics. We spaced vegetation plots at intervals of approximately 100 m along the same roads and trails where we surveyed birds. The centre of each circular plot of 12-m radius was located 15 to 20 m perpendicular to the trail or road. We identified the species and diameter at breast height category of each tree, measured maximum canopy height with laser rangefinders and counted the number of shrubs over 1 m tall in a 2-metre-wide swath along north-south and east-west transects through the diameter of the plot.

We surveyed avian communities and vegetation structure at seven of Anderson's original 10 plots in the eastern foothills of the Coast Mountain Range outside of Corvallis, Oregon, USA. These plots are described below using their quarter-section and the degrees-decimal latitude and longitude coordinates of each corner (Tables 1, 2).