Fatal attraction for an imperiled songbird: is cropland in the northern Great Plains an ecological trap for breeding thick-billed longspur?
Swicegood, Amber Elizabeth
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Thick-billed longspur (Rhynchophanes mccownii) populations have declined 4% annually during the past 50 years. This species nests in recently disturbed or sparsely vegetated patches within native mixed-grass prairie and is also known to occur in crop fields in northeastern Montana during the breeding season. Maladaptive habitat selection may result in crop fields operating as ecological traps, but information on thick-billed longspur use of and demography in crop fields are lacking. We hypothesized that crop fields provide cues for territory selection, but frequent human disturbance and increased exposure to weather and predators should result in reduced reproductive success relative to native grassland habitats. To address this hypothesis, we 1) used dynamic occupancy models to compare arrival times of territorial male longspurs using data collected with autonomous acoustic recorders, 2) used open population distance sampling models to compare trends in longspur abundance over the breeding season, 3) compared indices of nest density and number of young fledged, and 4) used nest survival models to compare survival rates of nests between crop and native sites. Arrival times were similar in both site types and occupancy ranged from 0.52 + or - 0.17SE on April 7 to 0.99 + or - 0.01 on April 30. Bird abundances appeared to be mediated by vegetation biomass and drought conditions. Standardized nest densities were 0.15 + or - 0.22SD and 0.23 + or - 0.32 nests/hour/plot in crop and native sites, respectively; the number of young fledged per successful nest was similar in crop and native sites and averaged 2.9 + or - 0.18SE. Nest survival was similar in crop and native sites and averaged 0.24 + or - 0.03 SE (n=222 nests). The data did not support our ecological trap hypothesis: longspurs did not exhibit a clear preference for crop sites and reproductive output was not significantly reduced. Our results suggest that crop fields may expand nesting opportunities for thick-billed longspur in a region where native habitat may be limited.