Ecology of the Southeastern American Kestrel
AuthorBrown, Jessica Leah
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Life history theory predicts that fitness will be maximized by balancing production of offspring with the parents' residual reproductive value. Whether this balance is achieved at the expense of parental or nestling condition is not clear for species with intermediate life-history characteristics, such as the southeastern American kestrel (Falco sparverius paulus). We provided food supplements to 61 nesting kestrel pairs that were matched with 63 control pairs in 2008 and 2009 in north-central Florida, USA. We analyzed between-year effects on reproductive decisions for the next year's first nest, such as timing of incubation, clutch size, and apparent nest success, along with annual adult female survival and nestling mass at time of fledging, with Bayesian hierarchical or capture-mark-recapture models. Treatment effects varied by year: in 2008, nestlings were similar in mass regardless of treatment, but food-supplemented adult females survived at very high rates. However, in 2009, food-supplemented nestlings were heavier than their control counterparts, and survival of supplemented adult females decreased. Weather and changes in nesting phenology, regardless of treatment groups, suggested that 2009 was more energetically demanding than 2008. We interpret the variable response of kestrels to our food supplement as evidence for a fixed investment in nestlings, such that in challenging years, adult females were unwilling to sacrifice their own condition for their nestlings.Reproduction is thought to be costly, leading to a hypothesized tradeoff between investment in a current reproductive attempt and investment in self-maintenance and future reproduction. The outcome of reproductive attempts is frequently assessed by short-term measurements of investment, such as number of offspring at time of independence or body condition of offspring; however, a more appropriate measure in the context of lifetime reproductive strategy is the eventual recruitment of offspring to the breeding population. We used Bayesian mark-recapture models to evaluate the effects of a food supplementation experiment on the long-term survival of American kestrels that had fledged from experimental nests. Although nestlings that had received the food supplement tended to fledge with higher body conditions than control counterparts, their higher rate of survival was better explained by treatment group independent of the difference in body condition. The food supplementation process likely affected nestling survival and recruitment through mechanisms not quantified here, such as changes in parental behavior during the post-fledging period or nutritional effects not measured by a size-corrected mass index. Our study demonstrates how food supplementation can result in a long-term demographic consequence for offspring. Moreover, this difference was greatest in a year when breeding adult survival was not positively affected by the food supplement, providing support for the hypothesized tradeoff.Integrated population models (IPMs) offer enhanced abilities to explore population dynamics, especially when considering sparse data sets commonly associated with threatened or endangered populations. We analyzed the recent population trends of a population of Southeastern American Kestrels associated with a network of nest boxes in north-central Florida, USA. Although the subspecies is considered of conservation concern, little is known about demographic vital rates or population trajectories. We used Bayesian IPMs that simultaneously considered mark-recapture data sets, fledgling production, and population surveys to assess recent population growth rates, productivity, and demography. We further evaluated the potential of the nest box population to serve as a source to the surrounding population that used natural cavities by comparing local and overall population growth rates, apparent survival probabilities, and recapture probabilities between an IPM that explicitly modeled immigration and one that did not. Overall population growth rates suggested that the population was stable, even though immigration was apparently important with approximately 0.3 female immigrants per resident female kestrel each year. Explicitly modeling immigration resulted in lower estimates of juvenile kestrel apparent survival probability, suggesting that a large proportion of locally produced juveniles emigrated rather than recruited locally. These results emphasize the utility of data sets from the monitoring of nest boxes, as well as the potential contribution of juveniles fledged from nest boxes to the regional population. The IPM approach allowed effective modeling of real-world data sets, including sparse data that are typically characteristic of threatened populations.Despite the recent rapid decline of many grassland bird species, the relative importance of open habitat extent versus habitat configuration for population persistence is unclear. Although many studies have attempted to address the effects of habitat area, edge, or landscape context on open habitat passerine birds, few have considered other taxa such as raptors. Most studies also consider static assessments of bird populations, such as abundance indices or raw presence estimates, rather than dynamic processes of population expansion or contraction. We used southeastern American kestrels in north-central Florida as a model system to explore the relative influence of landscape metrics on site occupancy patterns at two spatial scales, and for two different time periods. We modeled the occupancy of kestrel nest boxes with Bayesian state-space models that separated the latent and partially observed process of true occupancy probability from the detection probability. Results from reversible jump Markov chain Monte Carlo (RJMCMC) algorithms suggested that the continued occupancy of nest boxes, or Phi, was negatively influenced by disaggregation of open habitats rather than the extent of that land cover during the later time period (2008-2010). None of the landscape metrics appeared to influence Phi during the early time period (1992-93) or Gamma3, the probability of colonization of nest boxes between time periods. Our results indicate that continued fragmentation of open habitat would be deleterious for this threatened subspecies, but that many of the recommended land cover management practices, such as frequent low-intensity controlled burns, may help conservation attempts.