4.4 | Data Limitations
A broader demographic analysis including seasonal adult, annual adult, juvenile, and post-fledging survival rates would increase our ability to identify population sources and sinks and more fully test the ecological trap hypothesis. Our estimates for nest survival and the number of young fledged per successful nest are similar to estimates reported in other studies of thick-billed longspur and similar species in native prairie habitats, including horned lark (Eremophila alpestris ) and chestnut-collared longspur (Calcarius ornatus ) (Sedgwick, 2004; Lloyd and Martin, 2005; Mahoney and Chalfoun, 2016; Gaudet et al., 2020; Pulliam et al., 2021; Reintsma et al., 2022). Because double-brooding in longspurs can be affected by seasonal habitat conditions that vary differentially in crop fields and native prairie, future research should evaluate the relative contribution of each habitat type to overall fecundity (i.e., fledglings per female per season). To better understand mechanisms driving longspur population declines, additional information is needed on vital rates across life stages (e.g., renesting rates, juvenile survival, adult survival). Grassland birds are highly sensitive to variation in annual adult survival (Sedgwick, 2004; Perlut et al., 2008), however, low site fidelity in breeding areas for longspurs may make estimation of these vital rates difficult (Sedgwick, 2004; With, 2021). Recent advancements in VHF technology (e.g., Motus Wildlife Tracking System) may allow for expanded assessments of seasonal fecundity and annual survival of both adults and juveniles, in addition to movement rates between habitat types.
Evaluating stress hormone levels sensu Des Brisay (2018) of fledglings, juveniles, and adult longspurs in crop sites could provide additional insight on habitat quality. Body condition at the start of migration often influences survival of adults and juveniles during migration and winter (Merilä and Svensson, 1997; Angelier et al., 2011; Labocha and Hayes, 2012). Lower post-fledging survival in crop sites, reduced condition of adults or young, or lower seasonal fecundity in crop sites would provide evidence for reduced habitat quality of crop sites.
4.5 | Conclusion
The loss of native grasslands through conversion to cropland is a primary driver of grassland bird population declines, including thick-billed longspurs (Samson et al., 2004; Blann, 2006; Ellis et al., 2010; Wright and Wimberly, 2013). Nevertheless, we show that longspurs breeding in a region of relatively intact grasslands in northeastern Montana can successfully nest in nearby crop fields. While hazards for nests (e.g., farming activities, predation) differed between crop and native sites, overall seasonal reproductive effort and nesting success were similar. Because suitable native prairie habitat may be reduced under modern rangeland management, crop fields may effectively expand nesting habitat for this grassland obligate bird, supporting higher overall carrying capacity than existing native sites in this region alone. However, our study area contained large tracts of both crop and native habitats and we have not assessed the use of crop landscapes by longspurs in areas with less-extensive native prairie. Potential population-level benefits of expanding nesting sites to crop fields may be limited to areas close to historical native prairie habitats, depending on habitat selection behavior at higher spatial scales (e.g., 1st order habitat selection; Johnson, 1980).
Though overall nest survival rates were similar in crop fields and native prairie, reproductive output might be improved with modified farming practices that minimize the destruction of nests. Early seeding (< 10 May) and reducing summer fallowing should result in greater use and higher nest survival by longspurs in crop fields. Pesticides, including fungicides, can harm adults and nestlings (McEwen and Ells, 1975; Martin et al., 1998; Mineau and Whiteside, 2013). We recommend reducing or eliminating widespread application of herbicides, pesticides, and fungicides when possible, until their potential impacts on the survival of nesting longspurs can be evaluated. Given the great weight of evidence that conversion to cropland is detrimental to grassland bird populations, we strongly recommend against any conversion of native prairie to benefit longspurs. Future research should explore management practices that promote dynamic patterns of disturbance, bare ground, and short grass in native prairies, especially in early spring when longspurs select territories. Further investigation into population demographics, body condition, and resource availability may provide additional insight into the relationship between longspurs and agricultural landscapes.