aAbandonment often occurred after weather or partial predation events in both site types.
bWeather events included flooding, hail, or storm damage which resulted in nest destruction or destruction of nest contents.
FIGURE CAPTIONS
Figure 1. Estimated decline of thick-billed longspur populations based on Breeding Bird Survey data from 1966 – 2019 (Sauer et al. 2020). Estimated population size is shown on the y-axis and years are shown on the x-axis. Outer lines indicate estimated 95% confidence intervals.
Figure 2. Map of study area and study plots on crop and native habitat sites in Valley County, Montana, 2020–21. Clustering of native plots is due to patchy distribution of thick-billed longspurs in native habitats.
Figure 3. Conceptualization of layout of initial breeding bird survey transects in Valley County, Montana, 2020–21. The dashed lines indicate the transect walked and the outer edge represents the 64-ha survey plot.
Figure 4. Conceptualization of layout of line transect surveys in Valley County, Montana, 2020–21. The dashed lines represent the transect walked and the outer edge represents the 16-ha survey plot.
Figure 5. Estimated probability of detecting a thick-billed longspur on a song meter recording relative to Julian day (top left), daily minimum temperature (top right), and minutes past sunrise (bottom) in Valley County, Montana, during the month of April 2020–21. Shaded regions depict 95% confidence intervals.
Figure 6: Estimates of latent occupancy (proportion of sites occupied by thick-billed longspurs) in both crop and native sites in Valley County, Montana, from 7 April – 30 April in 2020 and 2021. Whiskers indicate 95% confidence intervals.
Figure 7. Effect of observer group (1 = high detection, 2 = low detection) on the distance detection function for thick-billed longspur surveys conducted in 2020 (top) and 2021 (bottom).
Figure 8. Bayesian estimates of mean site-level abundance of thick-billed longspurs in both crop and native sites in Valley County, Montana, 2020 (top) and 2021 (bottom). Whiskers depict 95% confidence intervals. Survey rounds were evenly spaced between 10 May – 15 July each year.
Figure 9. Estimated nest initiation dates in both crop and native sites for 222 thick-billed longspur nests found in Valley County, Montana, 2020–21. Results are based on 139 nests in 2020 (68 crop, 71 native) and 83 nests in 2021 (28 crop, 55 native). Overall nest initiation patterns were similar between crop and native sites given the year; 2020 was relatively cool and wet and 2021 was a drought year.
Figure 10. Differences in habitat conditions between crop and native sites and changes in conditions over the growing season (May, June, and July, survey rounds 1–3) for four habitat measures collected in northern Valley County, Montana, 2020–21. Measured variables include visual obstruction reading (VOR; top), percent bare ground cover (second), percent litter cover (third), and litter depth (bottom). VOR and litter depth were log-transformed to meet the assumptions of linear regression.