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.