Short-eared Owls (Asio flammeus) are of conservation concern in North America due to their decline of >70% over 40–50 yr on Christmas Bird Counts and North American Breeding Bird Surveys (Environment Canada 2010, Sauer et al. 2011, National Audubon Society 2012). This owl is also of conservation concern in Europe (Calladine et al. 2010). Monitoring is essential for successful conservation (Buckland et al. 2008). To be effective, an understanding of variation in annual abundance is necessary (e.g., Poulin et al. 2001). The Short-eared Owl's continental-scale movements, large interannual variability in local abundance, the difficulty in finding its nests, and its largely crepuscular and nocturnal activity (Beske and Champion 1971, Clark 1975, Holt 1992, Holt and Leasure 1993, Houston 1997, Priestley et al. 2008, Swengel and Swengel 2009) create challenges for effective monitoring (Calladine et al. 2010, Keyes 2011). Here we report long-term observations of Short-eared Owls at a Wisconsin site from two research groups: Hamerstroms during 1955–94 (Hamerstrom 1969, 1986, Hamerstrom et al. 1985) and Swengels during 1997–2011 (Swengel and Swengel 2002, 2009). We obtained additional published and unpubl. data on Short-eared Owl observations at Buena Vista and in the state of Wisconsin to construct and corroborate this long-term dataset. We documented patterns of variation in Short-eared Owl abundance on annual and decadal scales.

Methods

Our study site was the Buena Vista Grassland (or Marsh) Wildlife Area in Portage County, Wisconsin, with 5000 ha of public land (Toepfer 2003). Frances and Fred Hamerstrom began recording raptors here by 1939 (Berger et al. 1963). In 1959, they and numerous associates began an intensive study of Northern Harrier (Circus cyaneus) breeding that pioneered the use of observation and knowledge of raptor behavior to minimize researcher disturbance (Hamerstrom 1969, Follen 1975). This team recorded all raptors seen, including crepuscular and nocturnal observations of Short-eared Owls (Beske and Champion 1971) and raptors noted by observers in blinds for observing Greater Prairie-Chickens (Tympanuchus cupido). They also used snap traps (minimum 1200 trap-nights/yr) to estimate vole (Microtus) abundance during 1964–83, because voles strongly affect harrier reproduction (Hamerstrom 1986). Using these data, we ranked vole abundance (one for the lowest year and 20 for the highest) in Hamerstrom (1986) and grouped these into four categories (one for lowest and four for highest) for comparison to annual owl abundance indices.

DHJ made photocopies of all pages mentioning Short-eared Owls in the Hamerstroms' “red books” (data books) archived at the Julian W. Hill Library Collection, Acopian Center for Conservation Learning at Hawk Mountain Sanctuary. From these and from Hamerstrom et al. (1961) and Beske and Champion (1971), DHJ tabulated all Short-eared Owl observations made by the Hamerstrom team at Buena Vista from 1955–91, as well as the 1994 observations by Stan Moore (pers. comm.).

SRS and ABS chose to emphasize observational study of owls and did not attempt to find nests. We conducted daytime walking surveys at Buena Vista during 1997–2011 in the context of extensive surveys for butterflies and grassland birds in the growing season at 150 grasslands in seven states during 1988–2011 (Swengel and Swengel 1999, 2000). We also conducted driving surveys year-round (usually 1–2 times/mo) during the day, evening, and after evening civil twilight from November 1999–November 2011 at Buena Vista specifically to detect Short-eared Owls and other raptors (Swengel and Swengel 2002, 2009). We noted whether each owl contact (record, or episode of observing an individual owl at a particular time and place) was distinguishable as a juvenile, but likely classified some fledged young as adults in an effort to count young conservatively. Compared to adults, young were darker overall (Beske and Champion 1971), especially on the facial disk, underwing, and back, with upper chests darker or more darkly barred (Mikkola 1983, Johnsgard 1988, Wiggins et al. 2006, König and Weick 2008), and many still had shorter tails. If not clearly identifiable as a juvenile, the owl was considered an adult. We coded each contact as a new or repeat observation of an individual previously recorded during a driving session that day, counting birds in later sessions in the same general area as repeats unless they had different plumages or more individuals were seen at once. This was not an attempt to identify individual birds, but instead to calculate a minimum number of owls present that day based on location and, to a lesser extent, obvious differences in plumage. These tallies were likely to be conservative because individual Short-eared Owls in daylight hours have a very low detection probability (e.g., <5% in Calladine et al. 2010).

We adapted breeding bird atlas methods (Cutright et al. 2006) to group owl contacts by increasing confirmation of breeding: (1) breeding territory (adult seen repeatedly in one area during summer; presence of a pair; territorial defense/vigilance; courtship such as singing, high altitude wing-clapping, flight display, talon-touching [as described by Clark (1975), Mikkola (1983), and Holt and Leasure (1993)]); (2) nests; and (3) family groups (e.g., recently fledged young, adult feeding or supervising a fledgling). We counted the number of areas that had evidence of territoriality, nesting, and family groups separately, then the number of different breeding areas combining these. When we repeatedly observed a pair or family group in one area, this was considered as one breeding area, even though these might not be the same birds. If we observed territoriality, then a nest (rarely), and then a family group in the same area, it was counted as one breeding area. Only when we saw two pairs or family groups in the same area at the same time did we record two breeding areas there.

Data Analysis

DHJ devised categories (Table 1) to consistently classify years by owl abundance and extent of breeding behavior observed, even though the Hamerstrom and Swengel datasets were gathered using different methods. In the combined dataset, there were 13 years for which there was no Hamerstrom red book. We treated these years as missing values, and used the corroborative dataset to infer owl abundance at Buena Vista during these years.

Table 1. 

Categories of breeding abundance for Short-eared Owls. A breeding area is a location where any breeding evidence (territoriality, nest, and/or family group) was observed in spring or summer (see Methods).

We analyzed the raw number of counties reporting Short-eared Owls in summer, as well as the number of counties corrected for observer effort, as there was a marked increase in the number of observers after 2005. It can be beneficial to adjust opportunistic data like season summary reports for observer effort because of the bias resulting from years with considerably more observer effort (Smith and McKay 1984, Bielefeldt and Rosenfield 2003). Because about 80% of the ca. 300 observers contributing records electronically reported just a few birds, this increase in observers did not represent 3–4 times as much effort as the 75–100 “hard core” people who reported before eBird. As a result, we corrected for effort by dividing the number of Wisconsin counties reporting Short-eared Owls by the square root of the number of observers instead of by the raw number of observers. This corrected variable was then multiplied by 10 to make it within the same order of magnitude as the raw county total: the mean number of Wisconsin counties statewide reporting Short-eared Owls each summer (uncorrected)  =  1.0 (range 0–6); the mean number of Wisconsin counties corrected by dividing by the square root of the number of observers and then multiplying by 10  =  1.436 (range 0–7.385).

We computed all statistical tests using ABstat 7.20 software (Anderson-Bell 1994). Significance was set as a two-tailed P < 0.05. We used the Spearman rank correlation to test for significant correlations between different owl datasets and between owl and vole abundance, and the Mann-Whitney U-test to test for significant differences between samples.

Results

Summer detections and evidence of breeding for Short-eared Owls at Buena Vista varied greatly among years in the Hamerstrom and Swengel datasets (Fig. 1, Appendix). Three major peak years of owl abundance (1970, 1974, 2000) occurred at Buena Vista during 1955–2011. At least three other years (2001, 2006, 2009) had relatively high numbers. We also categorized 11 years as having mid-range abundance, 13 as low years, 14 as no-owl years, and 13 years with missing data—subsequently inferred by corroborative analysis to be low or no-owl years.

Figure 1. 

Short-eared Owl abundance categories at Buena Vista compared to the number of Wisconsin counties reporting Short-eared Owls the same summer in the Passenger Pigeon. Abundance category bars are gray and set to 0.5 for the 13 years of missing data likely to be category 0 or 1 based on patterns in Table 3.

Records from the Passenger Pigeon season summary correlated well with Buena Vista data. Abundance categories (Fig. 1) and number of breeding areas at Buena Vista correlated positively, usually significantly, with the number of Wisconsin counties (raw or corrected for the number of observers) reporting Short-eared Owls in summer (Table 2). Owl indices from Portage County and other counties were 3–4 times as high when Hamerstroms or Swengels observed a higher owl breeding category (2–5) as when they detected the two lowest categories (Table 3). Whether Short-eared Owls were reported in Portage County in summer in the Passenger Pigeon correlated with the species' incidence at Buena Vista: presence-absence, abundance categories, and especially the number of breeding areas recorded at Buena Vista (Table 4).

Table 2. 

Spearman rank correlations of Short-eared Owl abundance (categories and number of breeding areas in Appendix) by Hamerstroms (1955–96) and Swengels (1997–2010) at Buena Vista with the number of Wisconsin counties reporting the species in summer in the Passenger Pigeon (both untransformed and corrected for the number of observers–see Methods for details).

Table 3. 

Comparison of Short-eared Owl abundance categories in the Hamerstrom and Swengel studies (see Table 2) to whether the species was recorded in Portage County in summer in the Passenger Pigeon (0  =  no, 1  =  yes) and to the number of Wisconsin counties reporting Short-eared Owl in summer in the Passenger Pigeon, and the latter corrected for the number of observers. Swengels did crepuscular and nocturnal surveys in summer 2000–10 but not 1997–1999.

Table 4. 

Comparison of Short-eared Owl detection in summer (0  =  no, 1  =  yes), their abundance categories, and number of breeding areas (see Appendix) observed by the Hamerstroms and Swengels at Buena Vista by whether the species was recorded in Portage County, Wisconsin in summer in the Passenger Pigeon 1955–2010. P-value from Mann-Whitney U-test presented for each paired comparison within a column.

Four peak years of vole abundance (1966, 1970, 1974, 1979) occurred at Buena Vista during 1964–83 (Fig. 2, Hamerstrom 1986). Owl abundance categories and number of breeding areas both covaried strongly with vole abundance (ranking, abundance categories; Fig. 2, Table 5).

Figure 2. 

Short-eared Owl abundance categories and number of different breeding areas at Buena Vista (see Tables 1–2) compared to vole abundance (from Hamerstrom 1986) expressed as ranks and abundance categories. See Table 5 for statistical tests.

Table 5. 

Spearman rank correlations of vole abundance rankings and abundance categories (based on data in Hamerstrom 1986) to Short-eared Owl abundance categories and breeding areas (see Appendix).

Reports in the Passenger Pigeon were useful for inferring owl incidence at Buena Vista in years of no data. Whether Short-eared Owls were reported from Portage County strongly related to the number of Wisconsin counties reporting owls (Table 6). In years of no data or no owls at Buena Vista (Appendix), Portage County also had no summer owl records in the Passenger Pigeon (Table 7). Passenger Pigeon data suggest that 1997–99 (low owl detections in Portage and other Wisconsin counties; Fig. 1, Table 3), when Swengels had less effort and no systematic evening or nocturnal surveys, were correctly categorized as 0–1 in owl abundance at Buena Vista (Appendix). Based on Passenger Pigeon data (Table 7), the years without Hamerstrom data would also be categorized as having low owl abundance (categories 0 or 1), so that most years (n  =  40) during 1955–2011 were probably low owl years at Buena Vista. There was a trend suggesting that owl abundance from 1955 to 2011 was increasing for both owl abundance categories (r  =  0.186, n  =  44, P < 0.10) and number of breeding areas (r  =  0.223, n  =  44, P < 0.10), but neither of these was statistically significant.

Table 6. 

Comparison of number of Wisconsin counties reporting Short-eared Owl in summer (raw or corrected for the number of observers) by whether the species was recorded in Portage County, based on records in the Passenger Pigeon 1955–2010. P-value for Mann-Whitney U-test presented for each paired comparison within a column.

Table 7. 

Comparison of Short-eared Owl detection in summer reported by the Hamerstrom team (1955–96; see Appendix) to whether the species was recorded in Portage County that summer in the Passenger Pigeon (0  =  no, 1  =  yes) and to the number of counties reporting Short-eared Owl that summer in the Passenger Pigeon (raw or corrected for the number of observers). Within a column of mean values, no pairwise Mann-Whitney U-test was significant (P < 0.05); for the number of counties recorded in WI, no data and present were P < 0.10.

Within a summer, the number of breeding areas and number of adult owls seen (Appendix) correlated highly within Hamerstroms' (r  =  0.762, n  =  24) and Swengels' datasets (r  =  0.972, n  =  15: 1997–2011) and for both studies combined (r  =  0.889, n  =  39; P < 0.001 in all tests).

Discussion

Short-eared Owls occurred in summer and bred regularly at Buena Vista during 1955–2011, but in low numbers most years, with rare and unpredictable peaks in abundance (Fig. 1). Summer abundance at Buena Vista was a rough barometer of county and statewide owl numbers (Tables 2–Table 3. 4, 6–7). Most or all of the 13 years without data (Appendix) were probably low years for Short-eared Owls at Buena Vista, based on correlation of these data to Passenger Pigeon reports. As a result, we believe we identified all years of high owl abundance at Buena Vista from 1955 to 2011. We did not find a way to account for observer effort by the Hamerstrom and Swengel teams. Both teams may have had enough effort to characterize owl abundance, so that correcting for effort would not change the results. Observer effort correction did not seem to improve accuracy of relating Passenger Pigeon data to the 1955–2010 Buena Vista data (Table 2). But for periods more influenced by the recent boom in online reporting, corrected owl indices may have greater concordance with other datasets (Swengel and Swengel 2013).