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Environmental noise from anthropogenic and other sources affects many aspects of animal ecology and behaviour, including acoustic communication. Acoustic masking is often assumed in field studies to be the cause of compromised communication in noise, but other mechanisms could have similar effects. We tested experimentally how background noise disrupted the response to conspecific alarm calls in wild superb fairy‐wrens, Malurus cyaneus, assessing the effects of acoustic masking, distraction and changes in vigilance. We first examined the birds' response to alarm‐call playbacks accompanied by different amplitudes of background noise that overlapped the calls in acoustic frequency. We then scored and videoed their response to alarm calls in two types of background noise, that did or did not overlap call frequency, but were broadcast at a constant amplitude. Birds were less likely to flee to alarm calls in higher amplitudes of overlapping noise, demonstrating that noise itself compromised communication independently of environmental correlates. Background noise affected the response only if it overlapped in frequency with the alarm calls, implying that the effect was not due to distraction. Further, birds were equally vigilant during background noise of overlapping or non‐overlapping frequency, indicating that the lack of response to alarm calls in overlapping noise was not due to enhanced vigilance and awareness that there was no predator. We conclude that alarm‐call reception was compromised by masking, a mechanism that is often assumed but rarely tested in an ecological context. Masking compromised reception of high‐frequency “aerial” alarm calls and so could reduce survival in background noise of similar frequency. While anthropogenic noise, which is often of lower frequency, is unlikely to affect communication with these calls, it could affect reception of acoustic cues of danger, or other conspecific or heterospecific alarm calls. A plain language summary is available for this article. Plain Language Summary

Aim In many species, density‐dependent effects on reproduction are an important driver of population dynamics. However, it is rarely considered that the direction of density dependence is expected to vary over space and time depending on anti‐predator behaviour and predator community. Aggregation may allow for effective group mobbing against avian nest predators while aggregation may also attract mammalian predators, causing negative density dependence. We aim to quantify spatial variation in the effect of conspecific breeding density on nest survival in a mobbing bird species (Eurasian oystercatcher; Haematopus ostralegus) and identify whether this variation in density dependence can be explained by the predator community. Location Country‐wide (The Netherlands). Methods We integrated reproductive data with breeding territory maps of Eurasian oystercatchers and occupancy maps of avian and mammalian predator species across the Netherlands for a 10‐year period. Results Spatial variation in the composition of the predator community explained the effects of neighbour density, showing decreasing nest survival when both conspecific density and mammalian dominance increased. Also, heterospecific density (from breeding godwits and lapwing) has an additional effect on the oystercatcher nest survival. Strikingly, this pattern did not extend to mammal‐free island populations. Main conclusions Our study provides evidence that both the strength and sign of density dependence can vary spatially within species, implying that it is dangerous to generalize results from a single local population to large‐scale management implications and modelling exercises. The study also suggests that conservation actions that aim to attract breeding birds should be prioritized in areas with fewer mammalian predators, but this idea requires further testing on island populations.

1. Breeding territory choice constitutes a crucial antipredator behaviour for animals that determines reproductive success and survival during the breeding season. On arrival to breeding grounds migrant prey face a multitude of 'waiting' predators already settled within the landscape. 2. We studied territory selection and reproductive investment of migrant pied flycatcher (Ficedula hypoleuca) relative to breeding pygmy owls (POs) (Glaucidium passerinum) and Tengmalm's owls (TOs) (Aegolius funereus). Diurnal POs present a greater predation threat to adult flycatchers (up to 80% songbirds in diet) compared with nocturnal TOs (up to 36%). 3. During territory selection, pied flycatchers strongly avoided POs (occupation: 42% in presence vs. 92% in absence of owl nest) but not TOs (80% vs. 75%). This suggests that flycatchers are able to distinguish between two potential predators, avoiding dangerous POs but not obviously responding to the less risky TOs. 4. Flycatchers responded to presence of PO nests with c. 4-day delay in the start of egg-laying. A significantly prolonged nest building period contributed to this potentially costly breeding delay. Flycatchers further significantly reduced initial reproductive investment in presence of POs by laying 8·2% smaller clutch sizes, even if laying date was controlled. No breeding delay and clutch size reduction was found relative to TO presence. 5. Our results highlight flexibility in breeding territory selection and reproductive strategies as antipredator responses to perceived risk in a multi-predator environment. This supports the idea that for prey, not all predators are equal.

Inappropriate anti‐predator responses (naiveté) towards introduced predators is a key factor contributing to the extinction and endangerment of prey species world‐wide and the failure of wildlife reintroductions. Here, we test the idea that success of reintroduction can be improved by exposing a predator naïve prey species to introduced predators under controlled conditions (in situ predation) prior to reintroduction, such that prey adopt increased wary behaviours to aid in survival. We exposed a population of a naïve marsupial, the greater bilby (Macrotis lagotis), to a controlled number of introduced predators (feral cats, Felis catus) for 2 years within a large fenced paddock and then compared the pre‐release behaviour and post‐translocation survival of predator‐exposed and predator‐naïve bilbies over 40 days. Behavioural assays conducted in a small pen prior to reintroduction suggested that predator‐exposed bilbies were warier as they spent less time moving and more time in cover than predator naïve bilbies. After translocation, predator‐exposed bilbies were more likely to survive to 40 days and were less likely to be preyed upon by cats than predator‐free bilbies. Synthesis and applications. Naiveté towards predators is a major problem thwarting successful reintroductions world‐wide. Our study demonstrates that exposure to predators under controlled conditions can increase survival of reintroduced prey and is a promising approach to overcome the problem of naiveté towards introduced predators and the global problem of prey naiveté. Future conservation of naïve prey species may depend on such training methods prior to releasing into areas where predators are present. Naiveté towards predators is a major problem thwarting successful reintroductions world‐wide. Our study demonstrates that exposure to predators under controlled conditions can increase survival of reintroduced prey and is a promising approach to overcome the problem of naiveté towards introduced predators and the global problem of prey naiveté. Future conservation of naïve prey species may depend on such training methods prior to releasing into areas where predators are present.

Animals frequently engage in anti-predator behaviour, but the intensity and choice of behaviour is likely dependent on the environment. We used a common-garden experiment to test whether Allenby’s gerbils (Gerbillus andersoni allenbyi) from different sites in the Negev Desert show different responses to the same level of risk. We tested whether animals from less covered (more risky) environments cope with predation risk better by foraging longer and faster in open microhabitats. To do so, we captured individuals from three areas that differ in vegetation cover and then quantified food consumption and apprehension in a standard outdoor vivarium (an enclosure). This vivarium simulated a natural environment, with natural moonlight, artificial cover, and two live owls. Animals from less vegetated environments consumed more food but foraged slower and with more apprehension. We also found that some populations prefer to forage more in the first quarter of the lunar cycle than the third, and others prefer the opposite, despite these moon phases having equal luminosity. That suggests an evolutionary coordination game might be at play. Our results show anti-predator behaviour is diverse among populations and similar within, which may hint the existence of different ecotypes if further differences are found.Significance statementIn this study, we analysed whether anti-predator behaviour is consistent within populations, and different among them. We found that gerbils have different anti-predator behaviour depending on their site of origin, and that difference continues even when placed in a common environment. Animals from environments poor in cover foraged slowly and more carefully but did so for a longer time. Interestingly, we also found that when animals from different locations are placed in the same environment together, the average behaviour of this mixed group is similar to the most cautious of the mix.

Predation risk imposes fundamental trade-offs between survival and reproduction, often leading to sex- and age-specific differences in anti-predator behaviour. We investigated these trade-offs in the jumping spider Saitis barbipes , a species in which cryptic females and juveniles contrast with conspicuously coloured males that engage in courtship displays. Using high-quality in-focus and blurred predator images, we tested first behavioural responses and reaction distances of adult males, adult females, and juveniles. While all groups reliably recognised predator cues, adult males consistently approached stimuli more closely before showing anti-predator behaviour and frequently responded with courtship rather than defensive behaviours. In contrast, females and juveniles more often employed passive strategies such as freezing. Stimulus clarity had no effect on the type of response, whereas distance strongly influenced behavioural responses. These results indicate that predator recognition is innate, but response thresholds are context-dependent and shaped by sex, age, and reproductive role. Conspicuous adult males appear to suppress or delay anti-predator responses in favour of courtship, highlighting an evolutionary trade-off between reproductive effort and survival.

Flower‐dwelling predators make flowers dangerous foraging sites for pollinators, potentially affecting their anti‐predator behaviour. Moreover, predation vulnerability often varies among pollinators' body sizes with interspecific comparisons showing that smaller species are more vulnerable than larger ones. However, how intraspecific body size variation influences pollinator behaviour under predation risk is still unknown, especially under natural conditions. We hypothesized that bumblebee workers of different sizes will exhibit different foraging strategies under predation risk. We predict that (a) small workers should more often exhibit anti‐predator behaviours than larger workers. We also hypothesized that the anti‐predator behaviour should be influenced by predator size and reward availability; therefore, we expect (b) higher avoidance behaviour towards larger predator sizes and (c) more and longer visits to inflorescences with high nectar availability. Finally, we expect that (d) nectar availability should overcome the anti‐predator behaviour in less vulnerable, large, workers. We recorded flower visitation, time spent and rejection behaviours of different sizes of Bombus terrestris (Apidae) workers (large, medium and small) to inflorescences of Alstroemeria aurea (Alstroemeriaceae) with different treatments of artificial spiders (small and large) and nectar availability (with, without). Anti‐predator and foraging behaviour of bumblebees was affected by the size of the worker, the presence of artificial spiders and nectar availability. Large and medium size bumblebees strongly reduced flower visitation and time spent in the presence of artificial spiders, consistently avoiding flowers with spiders, regardless of spider size or nectar availability. Instead, small bumblebees seldom modified their behaviour when facing artificial spiders, only increasing their avoidance or decreasing their foraging time in nectarless flowers hosting large artificial spiders. This pattern of larger workers being more sensitive to predation risk than smaller ones at the intraspecific level in B. terrestris is contrary to the expected and acknowledged trend based on previous interspecific comparisons, but partially consistent with predictions of models of optimal foraging theory. Intraspecific behavioural variability was uncovered only when nectar was available, whereas artificial predator size rarely modified bumblebee anti‐predator and foraging behaviour. Therefore, our findings suggest that the trade‐off between maximizing resource intake and minimizing predation risk strongly varies across bumblebee worker body sizes. Translated Resumen Los depredadores que cazan sobre flores hacen que éstas sean sitios peligrosos de forrajeo para los polinizadores, pudiendo afectar su comportamiento. En general, la vulnerabilidad a la depredación varía con el tamaño del polinizador, siendo en comparaciones interespecíficas las especies más pequeñas las más vulnerables. Sin embargo, aún se desconoce cómo la variación intraespecífica del tamaño corporal influye en el comportamiento del polinizador bajo riesgo de depredación, especialmente en condiciones naturales.. Esperamos que abejorros de distinto tamaño exhiban diferentes estrategias de alimentación ante el riesgo de depredación. Predecimos que (i) obreras pequeñas manifiesten con mayor frecuencia comportamientos anti depredadores que obreras más grandes. Conjuntamente, esperamos (ii) una mayor evasión hacia tamaños de depredadores más grandes, y (iii) más visitas y mayor permanencia en inflorescencias con néctar. Finalmente, esperamos que (iv) la disponibilidad de néctar prevalezca a la manifestación del comportamiento anti depredador en obreras grandes, supuestamente menos vulnerables. Registramos a campo el número de visitas, tiempo de permanencia y el comportamiento de evasión de diferentes tamaños de obreras de Bombus terrestris (Apidae) (grandes, medianas y pequeñas) a inflorescencias de Alstroemeria aurea (Alstroemeriaceae) con arañas artificiales (pequeñas, grandes) y distinta disponibilidad de néctar (con, sin). El comportamiento de los abejorros se vio afectado por su tamaño corporal, la presencia de arañas artificiales y la disponibilidad de néctar. Abejorros grandes y medianos disminuyeron considerablemente las visitas y el tiempo de permanencia en inflorescencias, evadiendo consistentemente las flores que albergaron arañas artificiales, independientemente del tamaño del modelo o la cantidad de néctar. En cambio, abejorros pequeños rara vez modificaron su comportamiento, aumentando la evasión y disminuyendo el tiempo de permanencia solo en flores sin néctar y con arañas artificiales grandes. El patrón encontrado a nivel intraespecífico en B. terrestris resultó ser opuesto al observado en comparaciones interespecíficas, pero parcialmente consistente con modelos de teoría de forrajeo óptimo. Esta variabilidad intraespecífica se manifestó solo cuando había néctar disponible, mientras el tamaño del depredador apenas modificó el comportamiento. Nuestros hallazgos sugieren que el balance entre maximizar la ingesta de recursos y minimizar el riesgo de depredación varía fuertemente con el tamaño corporal de los abejorros. The authors' study shows how the risk of predation upon flowers can act as a strong selective force modulating pollinators' foraging and anti‐predator behaviour at the intraspecific level of a bumblebee species. They found that the trade‐off between maximizing resource intake and minimizing predation risk strongly varies across bumblebee worker sizes.

Group-living provides many fitness benefits for individual members, including improved foraging and predator vigilance. If such benefits are especially pronounced for sick members, group-living can act as a form of behavioural tolerance by offsetting mortality costs of infection. We experimentally tested this possibility by examining whether group-living impacts foraging and anti-predator behaviours in house finches (Haemorhous mexicanus) with or without conjunctivitis caused by Mycoplasma gallisepticum. We varied both group-living (single-housed or group-housed) and infection (M. gallisepticum-inoculated or sham-inoculated) and performed four behavioural assays at peak infection: two assessing how birds respond to foraging opportunities and two assessing responses to predation threats. Both social treatment and disease status influenced most measured behaviours, with single-housed, diseased birds consistently the least responsive to foraging opportunities and predation threats. While group-living also benefited healthy individuals (e.g. led to faster responsiveness) in most behavioural assays, our results suggest that diseased birds particularly benefit from group-living. Further, detected behavioural differences with group-living were not explained by effects of sociality on disease severity or pathogen load, which did not differ with group-living. By augmenting behaviours key to survival during infection, group-living may act as a form of behavioural tolerance for social species, with important implications for transmission dynamics.

1. Some types of flexible foraging behaviours were incorporated into ecological thought in the 1960s, but the population dynamical consequences of such behaviours are still poorly understood. 2. Flexible foraging-related traits can be classified as shifts in general and specific foraging effort, and shifts in general and specific defense. 3. Many flexible foraging behaviours suggested by theory have received very little empirical attention, and empirical techniques used to compare the magnitudes of behavioural and non-behavioural responses to predation are likely to have overestimated the behavioural components. 4. Adaptively flexible foraging in theory causes significant changes in the forms of consumer functional responses and generates a variety of indirect interactions. These can alter fundamental ecological processes, such as co-existence of competitors, and top-down or bottom-up effects in food webs. 5. Many aspects of flexible foraging are still largely unknown, including the issues of how to represent the dynamics of such phenotypically plastic traits, how flexible traits in multiple species interact, what types of adaptive movements occur in metacommunities, and how adaptive behaviours influence evolutionary change. 6. Population dynamics in large food webs may be less dependent on behavioural flexibility than in small webs because species replacement may preempt some potential types of behavioural change within species.

Inability to recognise and/or express effective anti-predator behaviour against novel predators as a result of ontogenetic and/or evolutionary isolation is known as ‘prey naiveté’. Natural selection favours prey species that are able to successfully detect, identify and appropriately respond to predators prior to their attack, increasing their probability of escape and/or avoidance of a predator. However, for many prey species, learning and experience are necessary to develop and perform appropriate antipredator behaviours. Here, we investigate how a remnant population of bilbies (Macrotis lagotis) in south-west Queensland responded to the scents of two predators, native dingoes (Canis familiaris) and introduced feral cats (Felis catus); a procedural control (rabbits; Oryctolagus cuniculus); and an experimental control (no scent). Bilbies in Queensland have shared more than 8000 years of co-evolutionary history with dingoes and less than 140 years with feral cats and less than 130 years with rabbits. Bilbies spent the greatest proportion of time investigating and the least amount of time digging when cat and dingo/dog faeces were present. Our results show that wild-living bilbies displayed anti-predator responses towards the olfactory cues of both a long-term predator (dingoes) and an evolutionary novel predator (cats). Our findings suggest that native species can develop antipredator responses towards introduced predators, providing support for the idea that predator naiveté can be overcome through learning and natural selection as a result of exposure to introduced predators.