House sparrows do not exhibit a preference for the scent of potential partners with different MHC-I diversity and genetic distances.

MHC genes play a fundamental role in immune recognition of pathogens and parasites. Therefore, females may increase offspring heterozygosity and genetic diversity by selecting males with genetically compatible or heterozygous MHC. In birds, several studies suggest that MHC genes play a role in mate choice, and recent evidence suggests that olfaction may play a role in the MHC-II discrimination. However, whether olfaction is involved in MHC-I discrimination in birds remains unknown. Previous studies indicate that house sparrow females with low allelic diversity prefer males with higher diversity in MHC-I alleles. Here, we directly explored whether female and male house sparrows (Passer domesticus) could estimate by scent MHC-I diversity and/or dissimilarity of potential partners. Our results show that neither females nor males exhibit a preference related to MHC-I diversity or dissimilarity of potential partners, suggesting that MHC-I is not detected through olfaction. Further studies are needed to understand the mechanisms responsible for mate discrimination based on MHC-I in birds.

Assessing behavioral sex differences to chemical cues of predation risk while provisioning nestlings in a hole-nesting bird.

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.

Exogenous Application of Methyl Jasmonate Increases Emissions of Volatile Organic Compounds in Pyrenean Oak Trees, Quercus pyrenaica.

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.

Attraction to Smelly Food in Birds: Insectivorous Birds Discriminate between the Pheromones of Their Prey and Those of Non-Prey Insects.

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.

What do we know about birds' use of plant volatile cues in tritrophic interactions?

The first study showing that birds can smell herbivore-induced plant volatiles was published ten years ago. Since then, only 12 studies have been published, showing contradictory results. This review evaluates the role of birds in relation to the crying for help hypothesis and their use of olfactory cues. In accordance with the methodologies used in previous studies, we herein provide a summary of experimental approaches and describe the advantages and disadvantages of experiments conducted in nature versus aviaries. Moreover, we recommend experimental methodologies which lead to a deeper knowledge of the topic, including reflection on the induction of plant defenses and adaptations of birds. Finally, we propose some interesting questions for future research to direct further studies towards a thorough and accurate description of birds' roles in tritrophic interactions.

Wild great and blue tits do not avoid chemical cues of predators when selecting cavities for roosting.

Small birds use cavities for roosting to decrease the thermoregulatory costs during the winter nights. The ability of birds to detect and escape from an approaching predator is impaired during roosting and thus the selection of such cavities should take into account the risk that a predator will find the cavity. Previous evidence suggested that birds in captivity are able to detect predator scent and avoid roosting in nest-boxes containing such predator chemical cues. Here, we tested whether birds also show this avoidance response under natural conditions. We performed three studies in three populations of blue and great tits. We added predator scent, a pungency scent or an odourless control to nest-boxes and compared the use of these nest-boxes for roosting. We found no differences between the scent treatments in the use of nest-boxes. Therefore, chemical cues indicating the potential presence of a predator are not enough for birds to avoid roosting in nest-boxes under natural conditions.

Insectivorous birds eavesdrop on the pheromones of their prey.

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.

Covariation and phenotypic integration in chemical communication displays: biosynthetic constraints and eco-evolutionary implications.

Chemical communication is ubiquitous. The identification of conserved structural elements in visual and acoustic communication is well established, but comparable information on chemical communication displays (CCDs) is lacking. We assessed the phenotypic integration of CCDs in a meta-analysis to characterize patterns of covariation in CCDs and identified functional or biosynthetically constrained modules. Poorly integrated plant CCDs (i.e. low covariation between scent compounds) support the notion that plants often utilize one or few key compounds to repel antagonists or to attract pollinators and enemies of herbivores. Animal CCDs (mostly insect pheromones) were usually more integrated than those of plants (i.e. stronger covariation), suggesting that animals communicate via fixed proportions among compounds. Both plant and animal CCDs were composed of modules, which are groups of strongly covarying compounds. Biosynthetic similarity of compounds revealed biosynthetic constraints in the covariation patterns of plant CCDs. We provide a novel perspective on chemical communication and a basis for future investigations on structural properties of CCDs. This will facilitate identifying modules and biosynthetic constraints that may affect the outcome of selection and thus provide a predictive framework for evolutionary trajectories of CCDs in plants and animals.

Are female starlings able to recognize the scent of their offspring?

BACKGROUND: Although there is growing evidence that birds may have individual chemical profiles that can function in several social contexts, offspring recognition based on olfactory cues has never been explored. This ability should be more likely evolved in colonial birds and/or species suffering brood parasitism, in which the risk of being engaged in costly misdirected parental care is high. METHODOLOGY/PRINCIPAL FINDINGS: We performed a choice experiment to examine whether females of the spotless starling, Sturnus unicolor, a species that is colonial, and where a fraction of the population is exposed to intraspecific brood parasitism, can discriminate between the scent of their offspring and that of unrelated nestlings. We also explored whether the development of the uropygial gland secretion may play a role in such olfactory discrimination by performing the choice experiments to females rearing nestlings of two different ages, that is, without and with developed uropygial glands. Results showed that female starlings did not preferentially choose the scent of their offspring, independently of whether the gland of nestlings was developed or not. CONCLUSIONS/SIGNIFICANCE: Our results suggest that female starlings do not have or do not show the ability to distinguish their offspring based on olfaction, at least up to 12-14 days of nestling age. Further research is needed to examine whether odour-based discrimination may function when fledgling starlings leave the nest and the risk of costly misidentification is likely to increase.

Birds exploit herbivore-induced plant volatiles to locate herbivorous prey.

Arthropod herbivory induces plant volatiles that can be used by natural enemies of the herbivores to find their prey. This has been studied mainly for arthropods that prey upon or parasitise herbivorous arthropods but rarely for insectivorous birds, one of the main groups of predators of herbivorous insects such as lepidopteran larvae. Here, we show that great tits (Parus major) discriminate between caterpillar-infested and uninfested trees. Birds were attracted to infested trees, even when they could not see the larvae or their feeding damage. We furthermore show that infested and uninfested trees differ in volatile emissions and visual characteristics. Finally, we show, for the first time, that birds smell which tree is infested with their prey based on differences in volatile profiles emitted by infested and uninfested trees. Volatiles emitted by plants in response to herbivory by lepidopteran larvae thus not only attract predatory insects but also vertebrate predators.

Sex recognition by odour and variation in the uropygial gland secretion in starlings.

1. Although a growing body of evidence supports that olfaction based on chemical compounds emitted by birds may play a role in individual recognition, the possible role of chemical cues in sexual selection of birds has been only preliminarily studied. 2. We investigated for the first time whether a passerine bird, the spotless starling Sturnus unicolor, was able to discriminate the sex of conspecifics by using olfactory cues and whether the size and secretion composition of the uropygial gland convey information on sex, age and reproductive status in this species. 3. We performed a blind choice experiment during mating, and we found that starlings were able to discriminate the sex of conspecifics by using chemical cues alone. Both male and female starlings preferred male scents. Furthermore, the analysis of the chemical composition of the uropygial gland secretion by using gas chromatography-mass spectrometry (GC-MS) revealed differences between sexes, ages and reproductive status. 4. In conclusion, our study reveals for first time that a passerine species can discriminate the sex of conspecifics by relying on chemical cues and suggests that the uropygial gland secretion may potentially function as a chemical signal used in mate choice and/or intrasexual competition in this species.

Sleeping birds do not respond to predator odour.

BACKGROUND: During sleep animals are relatively unresponsive and unaware of their environment, and therefore, more exposed to predation risk than alert and awake animals. This vulnerability might influence when, where and how animals sleep depending on the risk of predation perceived before going to sleep. Less clear is whether animals remain sensitive to predation cues when already asleep. METHODOLOGY/PRINCIPAL FINDINGS: We experimentally tested whether great tits are able to detect the chemical cues of a common nocturnal predator while sleeping. We predicted that birds exposed to the scent of a mammalian predator (mustelid) twice during the night would not go into torpor (which reduces their vigilance) and hence would not reduce their body temperature as much as control birds, exposed to the scent of another mammal that does not represent a danger for the birds (rabbit). As a consequence of the higher body temperature birds exposed to the scent of a predator are predicted to have a higher resting metabolic rate (RMR) and to lose more body mass. In the experiment, all birds decreased their body temperature during the night, but we did not find any influence of the treatment on body temperature, RMR, or body mass. CONCLUSIONS/SIGNIFICANCE: Our results suggest that birds are not able to detect predator chemical cues while sleeping. As a consequence, antipredatory strategies taken before sleep, such as roosting sites inspection, may be crucial to cope with the vulnerability to predation risk while sleeping.

Parasites and health affect multiple sexual signals in male common wall lizards, Podarcis muralis.

Multiple advertising sexual traits may either advertise different characteristics of male condition or be redundant to reinforce reliability of signals. Research has focused on multiple visual traits. However, in animals that use different multiple additional sensory systems, such as chemoreception, different types of traits might have evolved to signal similar characteristics of a male quality using different sensory channels. We examined whether ventral coloration and chemicals in femoral gland secretions of male common wall lizards, Podarcis muralis, are affected by their health state (blood-parasite load and cell-mediated immune response). Our results indicated that less parasitized lizards had brighter and more yellowish ventral colorations and also femoral secretions with higher proportions of two esters of octadecenoic acid. In addition, lizards with a greater immune response had more saturated coloration and secretions with higher proportions of octadecenoic acid methyl ester. We suggest that these signals would be reliable because only healthier males seemed able to allocate more carotenoids to coloration and presumably costly chemicals to secretions. The use of multiple sensory channels may provide more opportunities to signal a male quality under different circumstances, but also may reinforce the reliability of the signal when both types of traits may be perceived simultaneously.

Refuge use: a conflict between avoiding predation and losing mass in lizards.

Prey often respond to predation risk by increasing refuge use, but this strategy may entail a loss of body condition. Factors responsible for this loss of body condition remain unclear. Also how prey deal with refuge use to cope with predation risk without incurring costs of body condition, and how initial body condition affects refuge use remain barely known. We analyzed in the field whether adult Iberolacerta cyreni lizards modify their escape strategies and refuge use in areas with different levels of habitat deterioration and ecotourism pressure, which represent different levels of predation risk, and the consequences of changes in antipredator behavior to body condition. Lizards inhabiting deteriorated areas, where risk is higher, remained closer to refuges, but decreased time spent hidden in refuges after attacks, probably to maintain similar body condition than lizards inhabiting natural areas. We performed two laboratory experiments to isolate potential costs of refuge use that might affect the body condition of male lizards: a) a decrease of the efficiency of digestion due to low temperatures inside refuges and/or b) a reduction in food intake. Results suggest that refuge use is costly in terms of body condition due to reduced food intake, but the loss of efficiency of digestion seems unimportant. Lizards modified refuge use in relation to their body condition, with lizards with worse condition decreasing time hidden after predatory attacks. We concluded that lizards compensated for increased predation risk with flexible antipredatory strategies, coping with risk without incurring costs for body condition.

Pregnant female lizards Iberolacerta cyreni adjust refuge use to decrease thermal costs for their body condition and cell-mediated immune response.

Lizards often respond to increased predation risk by increasing refuge use, but this strategy may entail a loss of thermoregulatory opportunities, which may lead to a loss of body condition. This may be especially important for pregnant oviparous female lizards, because they need to maintain optimal body temperatures as long as possible to maximize developmental embryos rate until laying. However, little is known about how increased time spent at low temperatures in refuges affects body condition and health state of pregnant female lizards. Furthermore, it is not clear how initial body condition affects refuge use. Female Iberian rock lizards forced to increase time spent at low temperatures showed lower body condition and tended to show lower cell-mediated immune responses than control females. Therefore, the loss of thermoregulatory opportunities seems to be an important cost for pregnant females. Nevertheless, thereafter, when we simulated two repeated predatory attacks, females modified refuge use in relation to their body condition, with females with worse condition decreasing time hidden after attacks. In conclusion, female lizards seemed able to compensate increased predation risk with flexible antipredatory strategies, thus minimizing costs for body condition and health state.

Reliable signaling by chemical cues of male traits and health state in male lizards, Lacerta monticola.

In spite of the importance of chemoreception in social organization and sexual selection of lizards, there is a lack of general knowledge on how the characteristics of chemical signals mediate these behaviors. Moreover, it is unknown which are the mechanisms that might confer honesty to the information provided by chemical signals. We analyzed here whether characteristics of the lipophilic fraction of femoral gland secretions of Lacerta monticola male lizards can be related to the morphology, physical condition, and health state of the sender. Our results indicated that some male traits, such as body size, number of blue spots, and number of femoral pores and their level of fluctuating asymmetry, were related to variability in the relative proportions of some lipophilic chemical compounds found in secretions. Thus, conspecifics could obtain reliable information on the producer of a scent mark based on chemicals alone, and this might be the basis of female choice observed in this lizard. Moreover, only males with a greater T-cell immune response had higher proportions of two steroids (ergosterol and dehydrocholesterol) in their femoral secretions, which might suggest that the signal is honest and costly to produce. We suggest that only high-quality males could divert these compounds from metabolism to secretions in order to produce an exaggerated and honest "chemical ornament."

Thermal dependence of chemical assessment of predation risk affects the ability of wall lizards, Podarcis muralis, to avoid unsafe refuges.

Prey often respond to predators by increasing refuge use, but some refuges may expose prey to other types of predators. Because predators are not always visible inside refuges, their chemical stimuli may provide early warning of their presence. However, in ectotherms, chemosensory detection of predators may be thermally dependent. Lizards often hide in cold refuges, where their body temperature (Tb) may decrease, and this might affect their ability to detect chemicals of saurophagous snakes that ambush inside refuges. We examined the effects of Tb of wall lizards, Podarcis muralis, on their ability to detect chemicals of smooth snakes, Coronella austriaca. Differential tongue flick (TF) rates and behavioral patterns of lizards in response to scent of smooth snakes showed that lizards with optimal Tb discriminated and avoided the snake's scent, whereas lizards with suboptimal Tb did not. We also examined the importance of this effect on movement patterns and refuge use by lizards in terraria with safe (odorless) or unsafe refuges (snake-scented). While results of this experiment were variable, there was evidence of snake avoidance in that lizards at optimal Tb spent less time in snake-scented refuges relative to odorless refuges. Therefore, this study provides evidence that chemosensory discrimination of snake chemicals is thermally dependent, and, thus, suboptimal Tb impedes a lizard's ability to avoid snake-scented refuges.

Double gametocyte infections in apicomplexan parasites of birds and reptiles.

The simultaneous occurrence of male and female gametocytes inside a single host blood cell has been suggested to enhance apicomplexan transmission ["double gametocyte infection (DGI) hypothesis"]. We did a bibliographic search and a direct screen of blood smears from wild birds and reptiles to answer, for the first time, how common are these infections in the wild. Taking these two approaches together, we report here cases of DGIs in Plasmodium, Haemoproteus, Leucocytozoon and Hepatozoon, and cases of male-female DGIs in Haemoproteus of birds and reptiles and in Leucocytozoon of birds. Thus, we suggest that DGIs and male female DGIs are more widespread than previously thought, opening a new research avenue on apicomplexan transmission.