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Across much of North America, populations of American Kestrels (Falco sparverius) have been in decline for decades (Farmer et al. 2008, Farmer and Smith 2009, Smallwood et al. 2009a, Paprocki et al. 2014, Sauer et al. 2014). Hypothesized causes of kestrel declines include predation by Cooper's Hawks (Accipiter cooperii; Farmer et al. 2008), pathogens (e.g., Nemeth et al. 2006), habitat loss (Sullivan and Wood 2005, Farmer et al. 2008, Bolgiano et al. 2015), pesticides (Smallwood et al. 2009a, Rattner et al. 2015), and climate change (Steenhof and Peterson 2009b), yet no hypothesized factor has been supported empirically (Farmer et al. 2006, Smallwood et al. 2009a). Despite the effort spent evaluating threats, the lack of a “smoking-gun” to explain the decline of this charismatic species has led many professional and citizen scientists to call for action on several unlikely, and unsupported, threats. Here, we evaluate and build on hypothesized causes of declines considered by other authors (e.g., Sullivan and Wood 2005, Farmer et al. 2008, Smallwood et al. 2009a) to synthesize conclusions and articulate research needs.

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This is an author-produced, peer-reviewed version of this article. The final, definitive version of this document can be found online at Journal of Raptor Research, published by the Raptor Research Foundation. Copyright restrictions may apply. doi: 10.3356/JRR-16-73.1

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