Detecting ecological traps in human-altered landscapes: A case study of the thick-billed longspur nesting in croplands

Conversion of the North American prairies to cropland remains a prominent
threat to grassland bird populations. Yet, a few species nest in these
vastly modified systems. The thick-billed longspur (Rhynchophanes
mccownii) is an obligate grassland bird whose populations have declined 4%
annually during the past 50 years. Thick-billed longspurs historically
nested in recently disturbed or sparsely vegetated patches within native
mixed-grass prairie, but observations of longspurs in spring cereal and
pulse crop fields during the breeding season in northeastern Montana, USA
suggest such fields also provide cues for habitat selection. Maladaptive
selection for poor-quality habitat may contribute to ongoing declines in
longspur populations, but information on thick-billed longspur breeding
ecology in crop fields is lacking. We hypothesized that these crop fields
may function as ecological traps; specifically, we expected that crop
fields may provide cues for territory selection, but frequent human
disturbance and increased exposure to weather and predators would have
negative consequences for reproduction. To address this hypothesis, we
compared measures of habitat selection (settlement patterns and trends in
abundance) and productivity (nest density, nest survival, and number of
young fledged) between crop fields and native grassland sites during
2020–21. Across both years, settlement patterns were similar between site
types and occupancy ranged from 0.52 ± 0.17SE to 0.99 ± 0.01 on April 7
and 30, respectively. Early season abundance differed by year, and changes
in abundance during the breeding season appeared to be associated with
precipitation-driven vegetation conditions rather than habitat type. While
an index of nest density was lower in crop than native sites, the number
of young fledged per successful nest (2.9 ± 0.18SE) and nest survival
(0.24 ± 0.03 SE; n=222 nests) were similar for crop and native sites.
Collectively, 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 indicate
that croplands may provide alternative breeding habitat within a
human-dominated landscape.