Demography of a widespread raptor across disparate regions

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.

A two‐sex integrated population model reveals intersexual differences in life history strategies in Cooper's hawks

“…For males, however, density dependence was manifested in several parameters, most notably AFY survival. Density dependence in survival rates has been reported or suspected in some other raptor species, including bald eagles ( Haliaeetus leucocephalus ; Elliott et al, 2011), Mauritius kestrels ( Falco punctatus ; Nicoll et al, 2003), American kestrels ( Falco sparverius ; McClure et al, 2021), and peregrine falcons ( Falco peregrinus ; Bruggeman et al, 2015), but to our knowledge, this is the first study to show that mechanisms of density dependence can differ between sexes. We were surprised to find no evidence for the relationship between density and fecundity in our study population because previous studies suggest adjustments in reproductive rates are the most frequent indication of density dependence in predatory birds (Bretagnolle et al, 2008; Chambert et al, 2020; Fasce et al, 2011).…”

“…One of the most important mechanisms of population regulation is density dependence, which typically manifests as an inverse relationship between abundance and the population growth rate (Hanski, 1990). Density dependence is difficult to measure directly, but IPMs have proven to be effective in detecting density dependence in various demographic parameters and the population growth rate (Abadi et al, 2012; McClure et al, 2021; Schaub et al, 2013).…”

Multi-species prey dynamics influence local survival in resident and wintering generalist predators

“…Proxies of food availability, such as global climatic and vegetation cover indexes, have been also occasionally used to test their influence on survival (Grande et al 2009 ; Mihoub et al 2010 ). Furthermore, stochasticity in prey densities may occur over time (seasonality in prey fluctuations) and space (spatial heterogeneity in prey densities) and this may influence spatio-temporal variability in raptor survival (McClure et al 2020 ). A paradigmatic example is cycles in small mammal densities and their influence on survival of predators in high-latitude ecosystems (Brommer et al 2002 ; Karell et al 2009 ; Millon et al 2014 ).…”

Falcons of North America, Second Edition

“…For example, an increase in the Cooper's Hawk (Accipiter cooperii) is listed as a possible hypothesis, though Smallwood et al (2009) suggest evidence contradicting this hypothesis from nest box studies. Similarly, newer research suggests declines are unlikely to be caused by low reproductive rates (McClure et al 2021, albeit published after Falcons of North America Second Edition was printed) or lack of suitable nesting sites (McClure et al 2017). Instead, declines may be the result of low fledgling or adult survival, mortality outside of the breeding season, or population differences in immigration and emigration (McClure et al 2021).…”

“…Similarly, newer research suggests declines are unlikely to be caused by low reproductive rates (McClure et al 2021, albeit published after Falcons of North America Second Edition was printed) or lack of suitable nesting sites (McClure et al 2017). Instead, declines may be the result of low fledgling or adult survival, mortality outside of the breeding season, or population differences in immigration and emigration (McClure et al 2021). Much work remains to be accomplished and debated to discern why American Kestrels have declined, but we feel that the book fairly reviews possible causes of declines.…”