Irene Saavedra

Global change is an important driver of the increase in emerging infectious diseases in recent decades. In parallel, interest in nature has increased, and different citizen science platforms have been developed to record wildlife observations from the general public. Some of these platforms also allow registering the observations of dead or sick birds. Here, we test the utility of live, sick and dead observations of birds recorded on the platform Observation.org for the early detection of highly pathogenic avian influenza virus (HPAIV) outbreaks in the wild in Belgium and The Netherlands. There were no significant differences in the morbidity/mortality rate through Observation.org one to four weeks in advance. However, the results show that the HPAIV outbreaks officially reported by the World Organisation for Animal Health (WOAH) overlapped in time with sudden increases in the records of sick and dead birds in the wild. In addition, in two of the five main HPAIV outbreaks recorded between 2016 and 2021, wild Anseriformes mortality increased one to two months before outbreak declaration. Although we cannot exclude that this increase was related to other causes such as other infectious diseases, we propose that Observation.org is a useful nature platform to complement animal health surveillance in wild birds. We propose possible approaches to improve the utility of the platform for pathogen surveillance in wildlife and discuss the potential for HPAIV outbreak detection systems based on citizen science to complement current surveillance programs of health authorities.

Birds can assess nest predation risk and adjust their parental activity accordingly. Risk taking behavior should be related to investment in reproduction as well as to confidence in parenthood that often differ between sexes. In those cases, sexual differences in risk taking behavior may be expected. For example, in blue tits, females invest more time and energy than males in nest-building, egg laying and incubation. Furthermore, confidence in parenthood is supposed to be higher for females, as extrapair paternity is common in this species. Therefore, the reproductive value of nestlings may be higher for females than for males and the former may assume greater risks to ensure nestling growth and maximize their reproductive success. We examined potential sexual differences in the risk assumed by parents in relation to perceived risk of predation inside the nest cavity, where predation risk perception may be higher. We increased perceived predation risk by adding predator chemical cues inside blue tit (Cyanistes caeruleus) nest-boxes, and we tested whether female and male parents differed in the risk assumed when taking care of nestlings. Females and males did not differ in the risk assumed in response to perceived predation risk. However, females reduced time devoted to nest sanitation activities when predator chemical cues were detected inside the nest-box, likely as an anti-predatory strategy to minimize their own risk of predation. Therefore, these results add to the evidence that birds can detect chemical cues of predators inside the nest cavity and suggest that the behavioral response to an increase in risk of predation perceived through olfactory cues is not sex-dependent in blue tit.

Birds can assess nest predation risk and adjust their parental activity accordingly. Risk taking behavior should be related to investment in reproduction as well as to confidence in parenthood that often differ between sexes. In those cases, sexual differences in risk taking behavior may be expected. For example, in blue tits, females invest more time and energy than males in nest-building, egg laying and incubation. Furthermore, confidence in parenthood is supposed to be higher for females, as extrapair paternity is common in this species. Therefore, the reproductive value of nestlings may be higher for females than for males and the former may assume greater risks to ensure nestling growth and maximize their reproductive success. We examined potential sexual differences in the risk assumed by parents in relation to perceived risk of predation inside the nest cavity, where predation risk perception may be higher. We increased perceived predation risk by adding predator chemical cues inside blue tit ( Cyanistes caeruleus ) nest-boxes, and we tested whether female and male parents differed in the risk assumed when taking care of nestlings. Females and males did not differ in the risk assumed in response to perceived predation risk. However, females reduced time devoted to nest sanitation activities when predator chemical cues were detected inside the nest-box, likely as an anti-predatory strategy to minimize their own risk of predation. Therefore, these results add to the evidence that birds can detect chemical cues of predators inside the nest cavity and suggest that the behavioral response to an increase in risk of predation perceived through olfactory cues is not sex-dependent in blue tit.

The tri-trophic interactions between plants, insects, and insect predators and parasitoids are often mediated by chemical cues. The attraction to herbivore-induced Plant Volatiles (HIPVs) has been well documented for arthropod predators and parasitoids, and more recently for insectivorous birds. The attraction to plant volatiles induced by the exogenous application of methyl jasmonate (MeJA), a phytohormone typically produced in response to an attack of chewing herbivores, has provided controversial results both in arthropod and avian predators. In this study, we examined whether potential differences in the composition of bouquets of volatiles produced by herbivore-induced and MeJA-treated Pyrenean oak trees (Quercus pyrenaica) were related to differential avian attraction, as results from a previous study suggested. Results showed that the overall emission of volatiles produced by MeJA-treated and herbivore-induced trees did not differ, and were higher than emissions of Control trees, although MeJA treatment showed a more significant reaction and released several specific compounds in contrast to herbivore-induced trees. These slight yet significant differences in the volatile composition may explain why avian predators were not so attracted to MeJA-treated trees, as observed in a previous study in this plant-herbivore system. Unfortunately, the lack of avian visits to the experimental trees in the current study did not allow us to confirm this result and points out the need to perform more robust predator studies.

Natural selection has favored the evolution of different capabilities that allow animals to obtain food—e.g., the development of senses for improving prey/food detection. Among these senses, chemical sense is possibly the most ancient mechanism used by organisms for environmental assessment. Comparative studies suggest the prime role of foraging ecology in the evolution of the olfactory apparatus of vertebrates, including birds. Here, we review empirical studies that have shown birds’ abilities to detect prey/food via olfaction and report the results of a study aiming to analyze the specificity of eavesdropping on prey pheromones in insectivorous birds. In a field study, we placed artificial larvae and a dispenser with one of three treatments—prey (Operopthera brumata) pheromones, non-prey (Rhynchophorus ferrugineus) pheromones, or a control unscented dispenser—on the branches of Pyrenean oak trees (Quercus pyrenaica). We then measured the predation rate of birds on artificial larvae. Our results show that more trees had larvae with signs of avian predation when they contained a prey pheromone dispenser than when they contained a non-prey pheromone dispenser or an unscented dispenser. Our results indicate that insectivorous birds can discriminate between the pheromones emitted by their prey and those emitted by non-prey insects and that they only exhibit attraction to prey pheromones. These results highlight the potential use of insectivorous birds in the biological control of insect pests.

Nesting mortality in birds is caused primarily by predation. The ability to use different predator cues to assess the level of predation risk could be essential to minimize the predation risk during the breeding period. Previous studies show that birds modified their parental behavior when they detected chemical or visual cues of predators in the surrounding of the nest. However, according to the threat sensitivity hypothesis, birds may use more than one predator cue in order to accurately assess the level of predation risk. In this assessment, the behavior of predators can provide important information to evaluate the motivation of the predator to launch an attack. However, there are no studies that examined the importance of chemical, visual and behavioral cues of predators on the antipredatory behavior of birds. We examined the antipredatory behavior of blue tit Cyanistes caeruleus parents when they were exposed to live predators or stuffed models of predators that also had predator chemical cues. Blue tits were able to detect the behavioral cues of predators and exhibited greater antipredatory behaviors when exposed to a live predator than to a stuffed model with chemical cues. The results suggest that birds use the chemical, visual and behavioral cues of the predators to assess the level of predation risk and adjust their antipredatory behavior accordingly.

Birds have developed different behavioural strategies to reduce the risk of predation during the breeding period. Bird species that nest in the open often cover their eggs to decrease the risk of predators detecting the clutches. However, in cavity nesters, the potential functions of egg covering have not been explored despite some bird species that nest in cavities also covering their eggs as open nesters do. We analysed whether egg covering is an antipredatory behaviour in the blue tit (Cyanistes caeruleus). We simulated an increase in the perceived risk of predation at experimental nests by adding predator scent inside the nest boxes during the egg‐laying period, whilst adding lemon essence or water to control nest boxes. Birds exposed to predator chemical cues in the nest of experimental pairs more frequently covered their eggs than birds exposed to an odorous control. These results suggest that egg covering may have evolved as an antipredatory behaviour also in cavity nesters to reduce the risk of egg predation and thus increase reproductive success in birds.

Chemical cues play a fundamental role in mate attraction and mate choice. Lepidopteran females, such as the winter moth (Operophtera brumata), emit pheromones to attract males in the reproductive period. However, these chemical cues could also be eavesdropped by predators. To our knowledge, no studies have examined whether birds can detect pheromones of their prey. O. brumata adults are part of the winter diet of some insectivorous tit species, such as the great tit (Parus major) and blue tit (Cyanistes caeruleus). We performed a field experiment aimed to disentangle whether insectivorous birds can exploit the pheromones emitted by their prey for prey location. We placed artificial larvae and a dispenser on branches of Pyrenean oak trees (Quercus pyrenaica). In half of the trees we placed an O. brumata pheromone dispenser and in the other half we placed a control dispenser. We measured the predation rate of birds on artificial larvae. Our results show that more trees had larvae with signs of avian predation when they contained an O. brumata pheromone than when they contained a control dispenser. Furthermore, the proportion of artificial larvae with signs of avian predation was greater in trees that contained the pheromone than in control trees. Our results indicate that insectivorous birds can exploit the pheromones emitted by moth females to attract males, as a method of prey detection. These results highlight the potential use of insectivorous birds in the biological control of insect pests.