Michael Clinchy

Balancing food and predator pressure induces chronic stress in songbirds

The never–ending tension between finding food and avoiding predators may be the most universal natural stressor wild animals experience. The ‘chronic stress’ hypothesis predicts: (i) an animals stress profile will be a simultaneous function of food and predator pressures given the aforesaid tension; and (ii) these inseparable effects on physiology will produce inseparable effects on demography because of the resulting adverse health effects. This hypothesis was originally proposed to explain synergistic (inseparable) food and predator effects on demography in snowshoe hares (Lepus americanus). We conducted a 2 × 2, manipulative food addition plus natural predator reduction experiment on song sparrows (Melospiza melodia) that was, to our knowledge, the first to demonstrate comparable synergistic effects in a bird: added food and lower predator pressure in combination produced an increase in annual reproductive success almost double that expected from an additive model. Here we report the predicted simultaneous food and predator effects on measures of chronic stress in the context of the same experiment: birds at unfed, high predator pressure (HPP) sites had the highest stress levels; those at either unfed or HPP sites showed intermediate levels; and fed birds at low predator pressure sites had the lowest stress levels.

Pattern Does Not Equal Process: What Does Patch Occupancy Really Tell Us about Metapopulation Dynamics?

Patch occupancy surveys are commonly used to parameterize metapopulation models. If isolation predicts patch occupancy, this is generally attributed to a balance between distance‐dependent recolonization and spatially independent extinctions. We investigated whether similar patterns could also be generated by a process of spatially correlated extinctions following a unique colonization event (analogous to nonequilibrium processes in island biogeography). We simulated effects of spatially correlated extinctions on patterns of patch occupancy among pikas (Ochotona princeps) at Bodie, California, using randomly located extinction disks to represent the likely effects of predation. Our simulations produced similar patterns to those cited as evidence of balanced metapopulation dynamics. Simulations using a variety of disk sizes and patch configurations confirmed that our results are potentially applicable to a broad range of species and sites. Analyses of the observed patterns of patch occupancy at Bodie revealed little evidence of rescue effects and strong evidence that most recolonizations are ephemeral in nature. Persistence will be overestimated if static or declining patterns of patch occupancy are mistakenly attributed to dynamically stable metapopulation processes. Consequently, simple patch occupancy surveys should not be considered as substitutes for detailed experimental tests of hypothesized population processes, particularly when conservation concerns are involved.

BROWN‐HEADED COWBIRDS SKEW HOST OFFSPRING SEX RATIOS

Predators (or parasites) can have both direct and indirect effects on prey (or host) demography. Recent theory suggests that the impact of either effect may be greater if predators and parasites skew the sex ratio of survivors. Brown-headed Cowbirds (Molothrus ater) are obligate brood parasites. Many studies have shown that adult cowbirds directly cause host offspring mortalities. Cowbird young may indirectly cause additional mortalities by increasing competition in the nest. The sex ratio of surviving host offspring will be skewed if one sex is a poorer competitor and thus more likely to die. We studied whether and how cowbird parasitism affects host offspring sex ratios in Song Sparrow (Melospiza melodia) nests. The proportion of female sparrows in parasitized nests (0.28 ± 0.01) was half that in unparasitized nests (0.57 ± 0.05) at six days post-hatch and at fledging (0.27 ± 0.06 vs. 0.52 ± 0.11). Consistent with their being at a competitive disadvantage, female sparrows were smaller and lighter than male sparrows in parasitized mixed-sex (both sexes present) nests. This indirect effect of cowbirds on the sex ratio of surviving host offspring may dramatically affect host demography. We suggest that predator- or parasite-mediated sex ratio biases could be important to the demography of many prey or host species.

A new Automated Behavioural Response system to integrate playback experiments into camera trap studies

Summary How animals respond to anthropogenic disturbances is a core component of conservation biology and how they respond to predators and competitors is equally of central importance to wildlife ecology. Camera traps have rapidly become a critical tool in wildlife research by providing a fully automated means of observing animals without needing an observer present, permitting data to be collected on rare or elusive species and infrequent events. Snapshots from camera traps revealing a species’ presence have been the principal data used to date to gauge behaviour; but, lacking experimental controls, such data permit only correlational analyses potentially open to confounding effects. Playback experiments provide a powerful means to directly test the behavioural responses of animals, enabling strong inferences and rigorous conclusions not subject to the potential confounds affecting the interpretation of snapshot data; the principal factor to date limiting the use of playback experiments being the need to have an observer present. We developed an Automated Behavioural Response system (ABR) comprising a custom-built motion-sensitive speaker system that can be paired with any commercially available camera trap, providing the means to conduct playback experiments directly testing the behavioural responses of any species that can be ‘caught’ on a camera trap. We describe field tests in Uganda, Canada and the USA, experimentally testing the effects of anthropogenic disturbances and interactions among large carnivores, in species as diverse as elephants, black bears, chimpanzees and cougars; experiments that would be completely infeasible without the ABR. We evaluate factors affecting the rate of successful data collection in the experiments in Uganda and Canada, and detail how we maximized the systems performance in the USA experiment. By integrating the power playback experiments provide to directly and rigorously test behavioural responses with the capacity camera trapping affords to study virtually any animal anywhere, the ABR can both greatly expand the range of research questions addressed by conservation biologists and wildlife ecologists and qualitatively improve the rigour of the resulting conclusions. We discuss various ways to optimize the ABRs performance in any circumstance, and the many novel research opportunities made available by this new methodology.

Too important to tamper with: predation risk affects body mass and escape behaviour but not escape ability

Summary Escaping from a predator is a matter of life or death, and prey are expected to adaptively alter their physiology under chronic predation risk in ways that may affect escape. Theoretical models assume that escape performance is mass dependent, whereby scared prey strategically maintain an optimal body mass to enhance escape. Experiments testing the mass-dependent predation risk hypothesis have demonstrated that prior experience of predation risk can affect body mass, and the behavioural decisions about evasive actions to take. Other studies on natural changes in body mass indicate that mass can affect escape. No single experiment has tested if all of these components are indeed linked, which is a critical necessary condition underpinning the mass-dependent predation risk hypothesis. We tested all components of the mass-dependent predation risk hypothesis in a repeated measures experiment by presenting predator and non-predator cues to brown-headed cowbirds housed in semi-natural conditions. Exposure to predator cues affected body mass, fat, pectoral muscle thickness and evasive actions (take-off angle and speed), but not the physiological capacity to escape, as measured by flying ability. Examining individual variation revealed that flying ability was unrelated to mass loss in either sex, unrelated to mass gain in males, and only females that gained a very large amount of mass flew poorly. We next conducted a body mass manipulation in the laboratory to rigorously test whether small to large perturbations in mass can ever affect flying ability. We induced either no change in mass (control), a moderate reduction of 10% which the literature suggests should enhance flight. Flying ability was maintained regardless of treatment. Examining individual variation revealed the same precise patterns as in the first experiment. We conclude that prey may alter their mass and evasive actions in response to predation risk, but their escape ability remains robust and inelastic, presumably because disabling oneself is likely to lead to disastrous consequences. We suggest that animals may only face a mass-dependent predation risk trade-off in a narrow set of circumstances linked to life-history stages that require large amounts of mass gain, for example, parturition and migration. A lay summary is available for this article.

Fear affects parental care which predicts juvenile survival and exacerbates the total cost of fear on demography

Fear itself (perceived predation risk) can affect wildlife demography, but the cumulative impact of fear on population dynamics is not well understood. Parental care is arguably what most distinguishes birds and mammals from other taxa, yet only one experiment on wildlife has tested fear effects on parental food provisioning and the repercussions this has for the survival of dependent offspring, and only during early-stage care. We tested the effect of fear on late-stage parental care of mobile dependent offspring, by locating radio-tagged Song Sparrow fledglings and broadcasting predator or non-predator playbacks in their vicinity, measuring their parents behavior and their own, and tracking the offsprings survival to independence. Fear significantly reduced late-stage parental care, and parental fearfulness (as indexed by their reduction in provisioning when hearing predators) significantly predicted their offsprings condition and survival. Combining results from this experiment with that on early-stage care, we project that fear itself is powerful enough to reduce late-stage survival by 24%, and cumulatively reduce the number of young reaching independence by more than half, 53%. Experiments in invertebrate and aquatic systems demonstrate that fear is commonly as important as direct killing in affecting prey demography, and we suggest focusing more on fear effects and on offspring survival will reveal the same for wildlife.

Fearlessness towards extirpated large carnivores may exacerbate the impacts of naïve mesocarnivores

Lay Summary Large carnivores protect ecosystems by frightening mesocarnivores, and “fearless” mesocarnivores wreak havoc where large carnivores are lost. Species may become naive to potential threats (“tame” in Darwin’s words) when separated from predators, and we show that smaller “mesocarnivores” such as raccoons can be naive to large carnivores where these top predators have been driven to local extinction. Naivete in mesocarnivores may threaten biodiversity by leading to unchecked mesocarnivore foraging.Twitter: @JPSuraci