Low migratory flight altitudes may explain increased collision risk for Scolopax minor (American Woodcock)

Understanding bird migration at low altitudes is critical to evaluating
risk of collision with obstacles. Recent advances in satellite tracking
technologies allow quantifying use of low altitudes by small migrating
birds with a high level of precision, allowing species-level inference
into potential collision risk based on flight altitude. The American
Woodcock (Scolopax minor) is suspected to be a low altitude migrant due to
its frequent collisions with buildings, and subsequent mortality during
migration may be contributing to population declines. We investigated
migratory flight altitudes using satellite transmitters deployed on
woodcock in 2020–2024 and examined how flight altitudes compare to the
heights of common airspace obstacles. Each transmitter recorded a
nocturnal GPS location with an altitude reading every 1–3 days during fall
and spring migrations. We implemented a Bayesian hierarchical mixture
model to identify whether locations were recorded on the ground or during
flight, isolate measurement error, and describe the distribution of flight
altitudes. We found that migrating woodcock fly at mean altitudes of 379 m
above ground level, flying higher during spring (mean: 444 m, 95% credible
interval: 333–578 m) than fall (338 m, 95% CRI: 267–423 m). Woodcock
flight altitudes were frequently lower than could be observed using
weather radar (27% of observations), and 57% of observations fell within
the altitude range of ≥1 airspace obstacle. Our results suggest that
woodcock fly at altitudes lower than reported for most nocturnal migrants,
which likely contributes to their vulnerability to obstacle collisions.
Woodcock provide an example of how vulnerability to obstacle collisions
during nocturnal migratory flights are often species-specific, and
mitigation efforts should incorporate measures aimed at reducing
collisions during both diurnal stopovers and nocturnal migratory flights
to effectively reduce bird collision mortality.