Trends in Nestling Body Size in a Declining Population of American Kestrels

American Kestrels (Falco sparverius) are small falcons that feed primarily on large insects but also small mammals and birds. During recent decades kestrel populations have been declining in North America, for reasons still unknown. One plausible explanation is that nestlings are becoming malnourished over the years from lack of food resources and are fledging at smaller sizes. Fledging at a smaller size puts kestrel nestlings at greater risk of post-fledging mortality due to starvation or predation. Here we examined long-term trends in the morphological development of American Kestrel nestlings to see if there have been any significant changes over time. We used about thirty years of measurement data from New Jersey and Pennsylvania nest box programs to explore this possibility. In New Jersey, wing length for females increased and wing length for males decreased. Body weight and body condition (wing length/body weight) did not change significantly over time in New Jersey. However, in Pennsylvania nestlings decreased in size, both in wing length and body weight. Moreover, body weight in Pennsylvania decreased disproportionately such that there also was a significant decrease in body condition. Decrease in body weight and body condition possibly could be caused by malnutrition from declines in insect populations, such as grasshoppers. The decrease also may be from exposure to contaminates such as pesticides. Evidence of American Kestrel nestlings decreasing in size could bring us closer to finding out why populations have been declining