When I was a graduate student (in the 1970s), American Kestrels were one of the most common avian predators I saw in upstate New York on almost a daily basis, as they perched along roadsides hoping to spot an errant grasshopper or mouse in the grass.
Kestrels can be found throughout the Americas, from the most northern Alaskan tundra to the tip of South America (except the Amazon basin), including Caribbean islands, in a wide variety of habitats. Although still considered one of the most common raptors, their populations have declined 48% since the 1960s, as the magnitude of the red areas showing population declines in the Breeding Bird census data show.
While I rarely see kestrels in MN these days, I was pleased to see them frequently while we were in Puerto Rico, and even on its small island neighbor, Culebra, where they seem to be fairly common.
Kestrels prospered in the late 18th and 19th centuries in the Americas, when forests were being cleared for agriculture. The open grassy spaces made perfect hunting grounds for their insect, small bird and mammal diet, and forest borders provided the requisite number of tree holes for nesting. So why the decline in more recent times, and especially in the last couple of decades?
Are they vulnerable as prey of other raptors? Cooper’s Hawk populations are increasing as Kestrel populations are shrinking — coincidence or causation? A recent paper found no evidence for Cooper’s Hawk numbers or incidence of West Nile virus as a cause of Kestrel population decline, but suggested that nesting sites were a limiting factor.
Another explanation might be changes in farming practices in the U.S.: clearing the fields of hedge rows and forest edges where the birds might find nest holes, pesticides that eradicate potential insect prey, conversion of more pasture land to cropland, etc.
Progress for humans often take its toll on the wildlife, unfortunately.