Evidence of Continuing Downward Trends in American Kestrel Populations and Recommendations for Research into Causal Factors

Abstract

We analyzed long-term datasets from nest box programs, Breeding Bird Surveys run by the US Geological Survey, Christmas Bird Counts run by the National Audubon Society, and counts from two key fall migration watch sites, Hawk Mountain Sanctuary and Cape May Hawkwatch. We found that populations of American Kestrels (Falco sparverius) in North America are generally still declining, albeit with some caveats. Some populations are actually increasing, while others are remaining stable. Overall, the magnitude of annual change appears to be decreasing slightly during recent years. To understand potential causes of the decline, we recommend the following courses of action in no particular order: (1) determine whether the increase in Cooper's Hawk (Accipiter cooperii) populations is restricting kestrel distributions by the mere presence of the larger raptor; (2) study the effects of habitat loss and/or degradation on the falcon's wintering range; (3) further investigate on a broader spatial scale whether within-season habitat alterations are creating ecological traps for breeding kestrels; (4) determine the importance of arthropods in the diet of kestrels, especially the long-term population trends and timing of emergence of grasshoppers in relation to kestrel breeding chronology; (5) discover whether rodenticides pose a serious risk to American Kestrels across North America; (6) learn more about possible effects, both direct and indirect, that the use of neonicotinoids may have on kestrels; and (7) continue ongoing studies of the effect of climate change on these birds.

The Effect of Local Weather Conditions on American Kestrel (Falco sparverius) Reproduction

“…The American Kestrel ( Falco sparverius ) is North America's smallest and most widespread falcon (Smallwood and Bird 2020). Kestrel populations in the United States and Canada have been declining in recent decades (Farmer et al 2008, Smallwood et al 2009, Bird and Smallwood 2023). Data from the North American Breeding Bird Survey (BBS) suggest that the decline was underway by the mid-1960s and continued to 2019 (Sauer et al 2020).…”

“…Data from the North American Breeding Bird Survey (BBS) suggest that the decline was underway by the mid-1960s and continued to 2019 (Sauer et al 2020). Further evidence of the decline since the 1990s comes from migration counts (Oleyar et al 2023) and occupancy rates for kestrel nest box programs across North America (Smallwood et al 2009, Bird and Smallwood 2023). Although a number of factors that may have either increased mortality or decreased reproduction in kestrels have been examined, the cause or causes of the decline remain elusive, especially in our study region, as McClure et al (2021) found that American Kestrel population declines in New Jersey were not due to reproductive failure.…”

The Full Annual Cycle of the American Kestrel: State of the Knowledge, Information Gaps, and Conservation Needs

“…The American Kestrel ( Falco sparverius ) is a relatively common and popular raptor that has often been used as the model species for a variety of scientific studies (e.g., Bird and Bowman 1987). Despite the kestrel's ubiquity, long-term data from the Breeding Bird Survey (BBS), raptor migration counts, and occupancy of nest boxes all suggest that American Kestrel populations are undergoing widespread declines across North America (e.g., McClure et al 2017, Bird and Smallwood 2023, Oleyar et al 2023, and references therein). The most severe declines seem to be occurring in populations along the Atlantic coast and the Appalachian Mountains (Farmer and Smith 2009, Bird and Smallwood 2023), although some level of decline seems to have occurred throughout much of North America (Bird and Smallwood 2023, Oleyar et al 2023).…”

“…Despite the kestrel's ubiquity, long-term data from the Breeding Bird Survey (BBS), raptor migration counts, and occupancy of nest boxes all suggest that American Kestrel populations are undergoing widespread declines across North America (e.g., McClure et al 2017, Bird and Smallwood 2023, Oleyar et al 2023, and references therein). The most severe declines seem to be occurring in populations along the Atlantic coast and the Appalachian Mountains (Farmer and Smith 2009, Bird and Smallwood 2023), although some level of decline seems to have occurred throughout much of North America (Bird and Smallwood 2023, Oleyar et al 2023). Regional declines in kestrel populations have been reported beginning as early as 1951 (McClure and Schulwitz 2022), and although many potential drivers of decline have been investigated, no clear conclusions have been reached (Bird and Smallwood 2023).…”

“…The most severe declines seem to be occurring in populations along the Atlantic coast and the Appalachian Mountains (Farmer and Smith 2009, Bird and Smallwood 2023), although some level of decline seems to have occurred throughout much of North America (Bird and Smallwood 2023, Oleyar et al 2023). Regional declines in kestrel populations have been reported beginning as early as 1951 (McClure and Schulwitz 2022), and although many potential drivers of decline have been investigated, no clear conclusions have been reached (Bird and Smallwood 2023).…”