Bird Strikes and the Discovery of an Unusual Vireo: The Perils and Insights of Fall Migration
By now, fall migration has largely wrapped up in the Western/Northern region. Migration presents a host of dangers as birds run the gauntlet from their Canadian breeding grounds to their wintering grounds in the south. Dangers include predation, exhaustion and starvation, adverse weather events, and building strikes.
Recently, CWHC Western-Northern received multiple birds from a building strike at a site in the Saskatoon region. Multiple species of warblers and vireos were likely forced down by poor weather conditions and collided with the building. All the birds necropsied had blunt force trauma consistent with colliding with a solid object.
Most land bird species (warblers and vireos among them) migrate at night to avoid predation, heat stress and adverse weather. Birds have excellent eyesight and usually have no problem migrating at night, however artificial light from buildings can disorient birds1,. Usually, birds migrate at higher altitudes where human-made obstacles are not much of an issue, but poor weather can force birds to migrate at lower altitudes, and potentially collide with objects. On the night prior to discovering these birds, rain, fog, cloud cover, and moderate winds were recorded in the area.
While unfortunate, bird strikes like these sometimes offer a ‘silver-lining’ of sorts: the opportunity to document species that are less frequently encountered. In this case, one individual stuck out as particularly interesting and was identified as a Yellow-throated Vireo (Vireo flavifrons). This vireo is an eastern species and ranges only into the extreme southeastern corner of Saskatchewan, where it prefers mature deciduous forests with tall trees.
Overall, Yellow-throated Vireos are considered rare in Saskatchewan, and are generally considered uncommon even in their core breeding range4. Finding one outside of its normal range is of interest, especially for an understudied species which may not have been detected otherwise.
This case has since been finalized and shared with conservation partners including the SK Conservation Data Centre. Additionally, the remains of the vireo are to be sent to the Royal Saskatchewan Museum to enhance the provincial collection and documentation for this species.
 FLAP Canada. Bird Migration. https://flap.org/bird-migration/
 Audubon. What Makes Bird Vision So Cool. May-June 2013. https://www.audubon.org/magazine/may-june-2013/what-makes-bird-vision-so-cool
 U.S. Fish and Wildlife Service. 1998. Migration of Birds. https://www.csu.edu/cerc/researchreports/documents/MigrationofBirdsCircular.pdf
 Birds of the World. Yellow-throated Vireo. 2020. https://birdsoftheworld.org/bow/species/yetvir/cur/