Demography of a widespread raptor across disparate regions

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Abstract

Demographic differences between stable and declining populations can lend insight into drivers of population decline. The American Kestrel Falco sparverius is a widespread and often‐studied falcon, yet its demography is poorly understood, and the causes of observed population declines across much of North America remain unknown. Using integrated population models and sensitivity analysis, we examine how vital rates drive growth in population levels of American Kestrels at four discrete study sites – Florida, Idaho and Pennsylvania with stable nestbox occupancy, and New Jersey, where occupancy is declining. Population growth was most sensitive to changes in adult survival, yet was most correlated with immigration, in all populations. Additionally, population growth was positively correlated with survival rates of juveniles in Pennsylvania. We found evidence for density‐dependence in at least one vital rate for all populations except Florida. Fecundity was density‐dependent in New Jersey and Idaho, and the population growth rate was density‐dependent in Idaho. Adult survival, immigration and the population growth rate were density‐dependent in Pennsylvania. The New Jersey population had the highest rate of fecundity, suggesting that declines there are probably not caused by reproductive failure. Our study demonstrates that two principal demographic processes, adult survival and immigration, drive population dynamics of American Kestrels – both of which are understudied.

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