AMERICAN KESTREL (FALCO SPARVERIUS)

The first falcon identified by most beginning birders is an American Kestrel. And no wonder. The species breeds over a wider contiguous range than any other North American falcon, and it is by far the most numerous falcon in the New World, with an estimated global population of about 1 million to as many as 5.8 million individuals, about 75% of them occurring in North America (Farmer et al. 2008, Marks et al. 2016). Too, kestrels are highly conspicuous when they perch on a roadside power line or hover over an open field in search of prey, and they routinely nest and feed in close proximity to humans. They are among the most well-studied raptors in North America, although very little research has been conducted in Montana. As widespread and abundant as they are, however, two facts about kestrels might surprise the average birder. First, their numbers have been dropping steadily in much of North America for more than 50 years. Second, despite decades of field work, kestrel researchers are not exactly sure what is causing the decline.

American Kestrels are obligate cavity nesters that lay their eggs in old woodpecker holes and natural cavities in trees, cavities in cliffs, holes in abandoned buildings, and old stick nests built by Black-billed Magpies. They also readily occupy artificial nest boxes. They feed on a variety of small mammals, small birds, reptiles, and invertebrates. Kestrels occur in Montana throughout the year, but like several other so-called permanent residents (e.g., Mallard, Red-tailed Hawk), most individuals are migratory, and many local breeders are replaced in winter by birds that come from farther north in Canada or Alaska. Their presence in winter at northern latitudes is strongly tied to the availability of Microtus voles, whose numbers can fluctuate widely from one year to the next. The scant amount of data from kestrels banded in Montana suggests that their main wintering area is in Mexico, which is consistent with recoveries of kestrels banded at nests in Idaho, Washington, and Oregon (Henny and Brady 1994). Occurring over a wide range and crossing international borders make it difficult to identify factors that might contribute to population declines.

Evidence for declining numbers of kestrels comes from every conceivable source, namely the North American Breeding Bird Survey (BBS), the Christmas Bird Count (CBC), counts at migration watch sites, and studies of birds nesting in boxes. BBS data from 1966–2017 (Sauer et al. 2017) indicate that kestrel numbers declined significantly survey-wide (Fig. 1) and in 26 of 58 states and provinces for which good data were available. The highest declines were in the Northeast, including drops of 3.6% per year in New Jersey, 4.3% in Connecticut, and 4.9% in Massachusetts over the 52-year period. Out West, population declines were highest in British Columbia (2.2% per year), Idaho (2.1%), and Utah (2.1%). Numbers dropped nonsignificantly by 0.7% per year in Montana and increased significantly in only two states, Tennessee (1.3% per year) and Missouri (1.8%). CBC data analyzed from 1975–2011, using counts that had been conducted over long periods of time, also showed a strong negative population trend (Paprocki et al. 2014). An analysis of fall counts at seven migration watch sites with at least 20 years of data revealed that kestrel numbers declined significantly at every site, the drops beginning as early as the mid- to late 1970s in New Jersey and Pennsylvania and as late as the early 2000s in Minnesota and California (Ely et al. 2018; see also Farmer and Smith 2009). Finally, kestrel numbers declined significantly from 1984–2007 in each of eight studies of birds nesting in boxes, the study areas occurring in Florida, Georgia, Virginia and Maryland, New Jersey, Massachusetts, Pennsylvania, Saskatchewan, and Yukon Territory (Smallwood et al. 2009). The mean annual decline in box occupancy for these eight studies was 3.0% and ranged from a low 0.6% in Pennsylvania to a high of 4.7% in New Jersey.

Hypotheses put forth to explain these declines include reductions in food supply (Ely et al. 2018); habitat loss and degradation (Smallwood and Bird 2002, Farmer et al. 2008); predation by increasing numbers of Cooper’s Hawks (Farmer et al. 2008); chemical pollution and pesticides (Rattner et al. 2015); pathogens, such as West Nile virus (Nemeth et al. 2006); and climate change. Regarding the latter, a recent study that examined morphological data from migrating kestrels captured at seven locations found that some birds were getting smaller on the basis of a significant drop in body mass at three of the study sites and a significant decline in wing length at five of the study sites (Ely et al. 2018). One possible explanation for the shift toward smaller birds is reduced food supply owing to a warming climate. Moreover, from 1992–2015, laying dates of kestrels nesting in boxes in southwestern Idaho advanced by 15 days, and the earlier nesting corresponded with an earlier growing season that resulted from warming temperatures over the 23-year study period (Smith et al. 2017). Despite the appeal of these hypotheses, so far none of them has held up to scrutiny. For starters, the decline in kestrel numbers began before West Nile virus arrived in North America in the 1990s, and the population trend has been steadily downward (Fig. 1) and did not undergo a steeper drop after Cooper’s Hawks began increasing in the 1970s (Smallwood et al. 2009, McClure et al. 2017). Habitat degradation, especially loss of snags that provided nesting cavities, has been a widespread problem for many secondary cavity nesters, and indeed, multiple studies found that kestrel numbers increased after nest boxes were provided (e.g., Toland and Elder 1987, Smallwood and Collopy 2009). More recently, however, occupancy rates of boxes have declined in several areas, suggesting that kestrel populations are not limited by the availability of nesting cavities (McClure et al. 2017).

Continued monitoring of kestrels throughout their annual cycle will be required to identify which factors have the strongest influence on population trends. Perhaps of great importance, kestrels have been very poorly studied on their wintering grounds. Clearly, creative monitoring strategies must be developed for breeding areas, migration routes, and the winter range (McClure et al. 2017). In any case, the next time you stop to admire this beautiful bird, keep in mind that your ability to find kestrels on a good day of birding will become increasingly difficult as their numbers continue to decline.

Literature Cited

Ely, T. E., C. W. Briggs, S. E. Hawks, G. S. Kaltenecker, D. L. Evans, F. J. Nicoletti, J.-F. Therrien, O. Allen, and J. P. DeLong. 2018. Morphological changes in American kestrels (Falco sparverius) at continental migration sites. Global Ecology and Conservation 15: e00400.

Farmer, C. J., L. J. Goodrich, E. Ruelas Inzunza, and J. P. Smith. 2008. Conservation status of North America’s birds of prey. Pages 303-420 in State of North America’s Birds of Prey (K. L. Bildstein, J. P. Smith, E. Ruelas Inzunsa, and R. R. Veit, Eds.). Series in Ornithology 3, Nuttall Ornithological Club, Cambridge, Massachusetts.

Farmer, C. J., and J. P. Smith. 2009. Migration monitoring indicates widespread declines of American Kestrels (Falco sparverius) in North America. Journal of Raptor Research 43: 263-273.

Henny, C.  J., and G.  L. Brady.  1994.  Partial migration and wintering localities of American Kestrels nesting in the Pacific Northwest. Northwestern Naturalist 75: 37-43.

Marks, J. S., P. Hendricks, and D. Casey. 2016. Birds of Montana. Buteo Books, Arrington, Virginia.

McClure, C. J. W., S. E. Schulwitz, R. Van Buskirk, B. P. Pauli, and J. A. Heath. 2017. Commentary: Research recommendations for understanding the decline of American Kestrels (Falco sparverius) across much of North America. Journal of Raptor Research 51: 455-464.

Nemeth, N., D. Gould, R. Bowen, and N. Komar. 2006. Natural and experimental West Nile virus infection in five raptor species. Journal of Wildlife Diseases 41: 1-13.

Paprocki, N., J. A. Heath, and S. J. Novak. 2014. Regional distribution shifts help explain local changes in wintering raptor abundance: Implications for interpreting population trends. PLoS ONE 9: e86814.

Rattner, B. A., K.E. Horak, R. S. Lazarus, S. L. Schultz, S. Knowles, B. G. Abbo, and S. F. Volker. 2015. Toxicity reference values for chlorophacinone and their application for assessing anticoagulant rodenticide risk to raptors. Ecotoxicology 24: 720-734.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski, Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link. 2017. The North American Breeding Bird Survey, Results and Analysis 1966–2015. Version 2.07.2017. Patuxent Wildlife Research Center, Laurel, Maryland.

Smallwood, J. A., and D. M. Bird. 2002. American Kestrel (Falco sparverius) in The birds of North America, No. 602 (A. Poole and F. Gill, Eds.). Academy of Natural Sciences of Philadelphia and American Ornithologists’ Union.

Smallwood, J. A., M. F. Causey, D. H. Mossop, and 11 other authors. 2009. Why are American Kestrel (Falco sparverius) populations declining in North America? Evidence from nest-box programs. Journal of Raptor Research 43: 274-282.

Smallwood, J. A., and M. W. Collopy. 2009. Southeastern American Kestrels respond to an increase in the availability of nest cavities in north-central Florida. Journal of Raptor Research 43: 291-300.

Smith, S. H., K. Steenhof, C. J. W. McClure, and J. A. Heath. 2017. Earlier nesting by generalist predatory bird is associated with human responses to climate change. Journal of Animal Ecology 86: 98-107.

Toland, B. R., and W. H. Elder. 1987. Influence of nest-box placement and density on abundance and productivity of American Kestrels in central Missouri. Wilson Bulletin 99: 712-717.