Annual abundance of common Kestrels (Falco tinnunculus) is negatively associated with second generation anticoagulant rodenticides.

Rats and mice can damage food and agricultural products as well as transmit diseases, thereby requiring control of their numbers. Application of Second Generation Anticoagulant Rodenticides (SGARs) often reduces rodent numbers locally. However, predators eating rodents, including non-target species, that have consumed SGARs may be secondarily exposed and potentially lethally poisoned. Here we study whether SGARs may have contributed to the widespread population declines of a rodent-eating raptor, the Common Kestrel (Falco tinnunculus) in the UK. We show that 161 (66.8%) of the 241 Kestrels submitted for ecotoxicology tests between 1997 and 2012 had detectable levels of at least one SGAR in their livers. Adult Kestrels had significantly higher prevalence of SGARs than juveniles, suggesting accumulation of SGARs through time. The prevalence and concentrations of individual SGARs in Kestrels were significantly higher in England than in Scotland. SGAR prevalence in Kestrels were positively associated with some land cover types, primarily arable cereals and broad-leaved woodland, and negatively associated with mainly mean elevation, probably reflecting variation in SGAR usage across land cover types. By using volunteer-collected data on national Kestrel abundance 1997-2012, we show that there is a negative correlation between the Kestrel population index in a specific year and the concentration of bromadialone as well as the total SGAR concentration in the same year. Although correlative, this is the first study to provide evidence for a potential population-limiting effect of SGARs on a raptor.

Sex-specific patterns of reproductive senescence in a long-lived reintroduced raptor.

For many species, there is evidence that breeding performance changes as an individual ages. In iteroparous species, breeding performance often increases through early life and is expected to level out or even decline (senesce) later in life. An individual's sex and conditions experienced in early life may also affect breeding performance and how this changes with age. Long-term monitoring of individuals from reintroduced populations can provide unique opportunities to explore age-related trends in breeding performance that might otherwise be logistically challenging. We used a unique dataset from a reintroduced population of white-tailed eagles Haliaeetus albicilla in Scotland, which has been intensively monitored since their initial reintroduction in 1975, to study age- and sex-specific trends in two measures of breeding performance. This monitoring provided data on the breeding performance of known individuals ranging in age from 3 to 26 years. We also explored changes in breeding performance in relation to early life experience (i.e., whether they were released or fledged in the wild). Breeding performance increased with age in early life in a similar manner for both sexes. We found stronger evidence for senescence in breeding performance in males than females. However, late-life female breeding success was associated with early life experience, while male senescent trends were not apparently impacted by conditions experienced during early life. Sexual differences in senescence mean that older males are less likely to breed successfully compared to older females, and this may influence females' mate changes later in life. This difference may suggest a linked sexual difference in survival rates or the possibility of proactive partner change by females in later life in this typically monogamous biparental species.

A review of predation as a limiting factor for bird populations in mesopredator-rich landscapes: a case study of the UK.

The impact of increasing vertebrate predator numbers on bird populations is widely debated among the general public, game managers and conservationists across Europe. However, there are few systematic reviews of whether predation limits the population sizes of European bird species. Views on the impacts of predation are particularly polarised in the UK, probably because the UK has a globally exceptional culture of intensive, high-yield gamebird management where predator removal is the norm. In addition, most apex predators have been exterminated or much depleted in numbers, contributing to a widely held perception that the UK has high numbers of mesopredators. This has resulted in many high-quality studies of mesopredator impacts over several decades. Here we present results from a systematic review of predator trends and abundance, and assess whether predation limits the population sizes of 90 bird species in the UK. Our results confirm that the generalist predators Red Fox (Vulpes vulpes) and Crows (Corvus corone and C. cornix) occur at high densities in the UK compared with other European countries. In addition, some avian and mammalian predators have increased numerically in the UK during recent decades. Despite these high and increasing densities of predators, we found little evidence that predation limits populations of pigeons, woodpeckers and passerines, whereas evidence suggests that ground-nesting seabirds, waders and gamebirds can be limited by predation. Using life-history characteristics of prey species, we found that mainly long-lived species with high adult survival and late onset of breeding were limited by predation. Single-brooded species were also more likely to be limited by predation than multi-brooded species. Predators that depredate prey species during all life stages (i.e. from nest to adult stages) limited prey numbers more than predators that depredated only specific life stages (e.g. solely during the nest phase). The Red Fox and non-native mammals (e.g. the American Mink Neovison vison) were frequently identified as numerically limiting their prey species. Our review has identified predator-prey interactions that are particularly likely to result in population declines of prey species. In the short term, traditional predator-management techniques (e.g. lethal control or fencing to reduce predation by a small number of predator species) could be used to protect these vulnerable species. However, as these techniques are costly and time-consuming, we advocate that future research should identify land-use practices and landscape configurations that would reduce predator numbers and predation rates.

Functional response, seasonal decline and landscape differences in nest predation risk.

Previous studies investigating density-dependent nest predation risk have suffered from unnaturally high nest densities (artificial nests) or lack of controls for nest predator densities (observational data on real nests). I designed an experiment with artificial nests within the natural range of nest densities using a Latin square design (to control for differences in densities of nest predators in time and space) to avoid the shortcomings of previous studies. I incorporated confounding factors, such as time in the breeding season and landscape type, in order to study whether they affected nest predation risk. The experiment was performed in dry semi-natural shrub-rich grasslands in Sweden, using artificial shrub-nests containing two quail (Coturnix coturnix) eggs and a plasticine egg. Finally, I validated my results obtained from the artificial nest experiment by comparing relative nest predation risk on artificial nests with nest predation risk on real nests of red-backed shrikes (Lanius collurio), a shrub-nesting passerine. Corvids were the major nest predators on artificial shrub nests as revealed by marks in plasticine eggs. Within the natural variation in densities of simultaneously active shrub nests, corvids increased their rates of predation with increasing densities of artificial nests, indicating a functional response. Nest predation risk decreased with time in the season and differed between grassland plots in farmland-dominated (high risk), farmland-forest mosaic (low risk), and forest-dominated (low risk) landscape surroundings. Furthermore, predation risk on artificial nests increased with decreasing distance to nests of at least one corvid species. Breeding red-backed shrikes selected grasslands with a low nest predation risk on artificial nests and reproductive success of shrikes was positively related to success of artificial nests. Moreover, the probability of success for both artificial and real red-backed shrike nests increased with increasing distance from the nearest corvid nest. Thus, results from the artificial nest experiment were validated by the results from the red-backed shrike population study. My results therefore suggest that predation risk measured on artificial nests can be used as a relative index of spatial and temporal variation in nest predation risk. My results also indicate that grasslands in forested landscapes should be given extra attention for conservation aspects, since nest predation risk is lower there compared to grasslands in more open habitats.