Post-construction raptor fatality and nest monitoring is typically conducted at wind energy projects nationwide. However, pre- and post-construction surveys may fail to locate all breeding pairs and most studies at individual wind projects lack the necessary sample size or survey design to assess the effects of turbines on nesting raptors after construction. To address these potential issues, I used an information-theoretic approach to examine the influence of multiple spatial and temporal variables on reproductive success, post-fledging survival, and the distribution of breeding pairs from three sympatric Buteo species in the Columbia Plateau Ecoregion (CPE). Although the probability of detecting breeding pairs was relatively high (71-90%, ± 0.09-0.05), and sampling units were likely to be re-occupied (76-100% ± 0.14-0.10), I was not able to locate all nests in the 2010 or 2011 breeding seasons despite multiple surveys for each species.
The occurrence of breeding pairs was not associated with wind turbines or surrounding habitat types; instead ferruginous hawks (Buteo regalis) and red-tailed hawks (Buteo jamaicensis) selected areas in relation to the density of nesting substrates. Swainson’s hawks (Buteo swainsoni) were more likely to nest in areas with other breeding Buteo pairs, but my results suggest that all three species may have minimized competition through staggered nesting and spatial segregation. According to nest survival models, the daily survival rate (DSR) of ferruginous hawk nests decreased as the number of wind turbines within the home range buffer (32 km2) increased (ß̂ = -0.89, SE = 0.39, 85% CI = -1.47 to -0.30). I found no effect of turbines on the DSR for red-tailed hawk nests or any additional variables affecting the DSR for Swainson’s hawk nests. I radio-marked a combined total of 60 nestlings from all three species. After fledging, none of them died directly as a result of collisions with wind turbines. This was likely due, in part, to the limited size of the natal home range (2.38 km2, SD = 1.48), and the relatively short duration of the post-fledging period (χ ̅ range = 20.75 to 31.60 days ± 1.14 to 3.30). However, the DSR during the post-fledging period was best explained by species, distance to the nearest wind turbine (ß̂ = 1.14, SE = 0.67, 85% CI = 0.19 to 2.10), and a quadratic effect of age. Juveniles of all three species hatched from nests closer to turbines were more likely to die from predation or starvation just after fledging and prior to initiating natal dispersal compared to those from nests further away.
Taken together, these results suggest a greater effect of wind turbines on ferruginous hawk reproduction compared to the other two congeneric species. The causes of this negative association between wind turbines and these reproductive measures are unknown, but could potentially include collision mortality or indirect impacts such as disturbance or displacement of adult hawks. I recommend that methods for raptor nest surveys on wind energy projects be standardized to better facilitate the meta-analysis of long-term data and account for imperfect detection of breeding pairs. Future research should focus on the risk of collision mortality to breeding adult raptors and indirect impacts to reproduction. These data will be vital to understanding the consequences of wind turbine impacts to regional populations.