Aim: Studies conducted at a global scale are necessary to make general conclusions on the effect ... more Aim: Studies conducted at a global scale are necessary to make general conclusions on the effect of urbanization on trophic interactions and explore how these effects change along latitudinal gradients. Since biotic interactions are more intense at lower latitudes, we predict that the intensity of trophic interactions will decrease in response to urbanization (quantified by human population density), with higher impacts of urbanization at higher latitudes.Location: Global (881 study sites).Time period: 2000-2021.Major taxa studied: Birds, arthropods and woody plants.Methods: We compiled global data on insect herbivory and bird predation from individual studies using similar methodologies, and fitted generalized linear mixed models to test the effect of human population density, latitude and their interaction on these two response variables.Results: The intensity of herbivory and predation decreased with the increase of human population density at lower latitudes, remained unaffected...
Although variation in effect sizes and predicted values among studies of similar phenomena is ine... more Although variation in effect sizes and predicted values among studies of similar phenomena is inevitable, such variation far exceeds what might be produced by sampling error alone. One possible explanation for variation among results is differences among researchers in the decisions they make regarding statistical analyses. A growing array of studies has explored this analytical variability in different (mostly social science) fields, and has found substantial variability among results, despite analysts having the same data and research question. We implemented an analogous study in ecology and evolutionary biology, fields in which there have been no empirical exploration of the variation in effect sizes or model predictions generated by the analytical decisions of different researchers. We used two unpublished datasets, one from evolutionary ecology (blue tit, Cyanistes caeruleus, to compare sibling number and nestling growth) and one from conservation ecology (Eucalyptus, to compa...
Migratory birds generally have tightly scheduled annual cycles, in which delays can have carry-ov... more Migratory birds generally have tightly scheduled annual cycles, in which delays can have carry-over effects on the timing of later events, ultimately impacting reproductive output. We tested whether migration schedules among eight breeding populations of Arctic SkuasStercorarius parasiticusbetween Greenland and Siberia differ as a function of their use of seven widely divergent wintering areas across the Atlantic Ocean, Mediterranean Sea and Indian Ocean, and predicted that migrations over larger distances, with tighter schedules, would provide less room for compensating delays. Breeding at higher latitudes led not only to later reproduction and migration, but also faster spring migration and shorter time between return to the breeding area and clutch initiation. Wintering area was consistent within individuals among years; and more distant areas were associated with more time spent on migration and less time in the wintering areas. Skuas adjusted the period spent in the non-breedin...
This is an open access article under the terms of the Creative Commons Attribution License, which... more This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
According to the Latitudinal Biotic Interaction Hypothesis (LBIH), the general increase in biodiv... more According to the Latitudinal Biotic Interaction Hypothesis (LBIH), the general increase in biodiversity towards lower latitudes can be partially explained by an increase in the intensity of biotic interactions. While LBIH received some support for plant-herbivores interactions, much less is known about how higher trophic levels may contribute to shape biotic interactions across latitudinal gradients. We hypothesized that the diversity of insectivorous birds increases towards lower latitude, leading to higher predation rates on insect herbivores. We deployed plasticine caterpillars in 138 oak trees in 47 sites along a 19 degree latitudinal gradient in Europe to quantify bird insectivory through predation attempts. In addition, we used passive acoustic monitoring to (i) characterize the acoustic diversity of surrounding soundscapes; and (ii) infer both taxonomic and functional diversity of insectivorous birds from recordings. The functional diversity of insectivorous birds increased t...
Several studies have shown that insectivorous birds are attracted to herbivore-damaged trees even... more Several studies have shown that insectivorous birds are attracted to herbivore-damaged trees even when they cannot see or smell the actual herbivores or their faeces. However, it often remained an open question whether birds are attracted by herbivore-induced changes in leaf odour or in leaf light reflectance or by both types of changes. Our study addressed this question by investigating the response of great tits (Parus major) and blue tits (Cyanistes caeruleus) to Scots pine (Pinus sylvestris) damaged by pine sawfly larvae (Diprion pini). We released the birds individually to a study booth, where they were simultaneously offered a systemically herbivore-induced and a non-infested control pine branch. In the first experiment, the birds could see the branches, but not smell them, because each branch was kept inside a transparent, air-tight cylinder. In the second experiment, the birds could smell the branches, but not see them, because each branch was placed inside a non-transparent...
Abstract The arctic skua (Stercorarius parasiticus) is one of the most long‐lived bird species. I... more Abstract The arctic skua (Stercorarius parasiticus) is one of the most long‐lived bird species. In 2010, we captured in Finland an adult, female arctic skua which had been ringed as a nestling in 1987. We tagged it also with a color ring. The bird has last been seen in July 2020 at the age of 33 years, making it most likely the oldest known arctic skua of the world. In 2010–2011 the bird carried a light‐level measuring geolocator, the data of which revealed that the bird had spent the nonbreeding season in the Canary Current area on the west coast of Africa. Breeding populations of arctic skuas have declined recently especially in British Isles, thus it is useful to get longevity data of this species with a high breeding site fidelity.
Plants infested by arthropod herbivores have evolved a plethora of defensive responses (Howe & Ja... more Plants infested by arthropod herbivores have evolved a plethora of defensive responses (Howe & Jander, 2008; Walling, 2000). They defend themselves by responses which directly harm the herbivores by, for example, feeding-induced reduction of the nutritive value of plant tissue or damaged-induced production of harmful chemicals (reviewed by Chen, 2008). Other feeding-induced plant responses indirectly exert a negative effect on the herbivores by attracting predators or parasitoids of the plant-infesting arthropods (e.g.,
Plants infested by arthropod herbivores have evolved a plethora of defensive responses (Howe & Ja... more Plants infested by arthropod herbivores have evolved a plethora of defensive responses (Howe & Jander, 2008; Walling, 2000). They defend themselves by responses which directly harm the herbivores by, for example, feeding-induced reduction of the nutritive value of plant tissue or damaged-induced production of harmful chemicals (reviewed by Chen, 2008). Other feeding-induced plant responses indirectly exert a negative effect on the herbivores by attracting predators or parasitoids of the plant-infesting arthropods (e.g.,
Data from the defensive behaviour and potency of the chemical defende during the group rearing st... more Data from the defensive behaviour and potency of the chemical defende during the group rearing stage of the experiment
Cry for help' hypothesis predicts that attraction of predators with chemical or visual cues can d... more Cry for help' hypothesis predicts that attraction of predators with chemical or visual cues can decrease insect damage of plants. Visual cues involve changes in photosynthetic activity and the reflectance of leaves, and there is some evidence that birds may use these changes as foraging cues. However, changes in the visual properties of leaves have not been quantified and it is not known how birds see these changes. We also presented and tested a new 'reduction in camouflage' hypothesis (not mutually exclusive with 'cry for help') stating that herbivore-mediated changes in leaf colour can increase the conspicuousness of herbivore against leaves. To define changes in the visual properties of leaves, their detectability to birds, and whether these changes affect the conspicuousness of herbivore, we manipulated the level of herbivory in silver birch trees (Betula pendula) with autumnal moth (Epirrita autumnata) larvae, and used blue tit (Cyanistes caeruleus) vision models to images of leaves and larvae. Hue, luminance (lightness), contrast, light transmission, chlorophyll content, photosynthetic activity and water content of the leaves were compared between herbivore-damaged and control trees. The leaves of herbivore-damaged trees had a decreased chlorophyll a concentration, increased contrast and they reflected more longer wavelengths. However, these changes are likely not obvious to birds. In contrast to our expectation, there were only minor differences in conspicuousness of larvae against the leaves of damaged trees, which may be very subtle to predator vision. Nevertheless, according to visual models, larvae should be easily detectable to birds from both herbivore-damaged and control trees. Keywords: trophic interactions, avian vision model, background matching, herbivory, camouflage Significance statement Herbivory affects photosynthetic machinery and light reflectance of leaves, and may thus provide visual foraging cues to birds, although it is not known how these changes appear to birds. We also hypothesised that the changes in leaves may reduce the camouflage of the herbivore. After applying herbivoretreatment and using the avian vision models, we found that the leaves of herbivore-damage may cause the leaves to appear to birds with higher contrast and greener or a more yellowish colour than control leaves. In addition, although the herbivore was visible to birds, it was slightly less conspicuous when on damaged trees, indicating that the herbivore can be adapted to changes in the food plant. Our results indicate that herbivory causes changes visual properties of leaves, but these changes are likely not obvious to birds. 'Reduction in camouflage' could thus work independently from 'cry for help', although they can also share plant physiological mechanisms, if the cry for help is mediated by vision. To study the effects of herbivory on the visual properties of leaves, we manipulated herbivoredamage in silver birches (Betula pendula) by adding leaf-chewing autumnal moth larvae (Epirrita autumnata) on the experimental trees. We applied an avian vision model of blue tit (Cyanistes caeruleus) to investigate whether herbivory causes optical changes in the leaves that may be visible to birds and whether these changes make the larvae potentially more detectable. In addition to using visual models, chlorophyll concentration, fluorescence induction (i.e. indicators of photosynthesis), light transmission, and water content of the intact leaves of herbivore-damaged and undamaged control trees were measured. Methods Study system Silver birch is one of the most common deciduous trees in Finland and it is a host to several insect species (Koponen 1983; Heimonen et al. 2015), including the autumnal moth. Because the autumnal moth is a common generalist (Silvonen et al. 2014), its camouflage may provide intermediate matching to several backgrounds, instead of close matching to a single host plant species (Merilaita et al. 1999; Houston et al. 2007), although to human eyes the larvae closely resemble the leaf colour of many deciduous species
It is often assumed that birds are able to choose a breeding territory that will later on have th... more It is often assumed that birds are able to choose a breeding territory that will later on have the most food for nestlings. Studies on this essential question are, however, few. We studied territory choice of a long-distance migrant, the pied flycatcher Ficedula hypoleuca in southwestern Finland. In one study area, we monitored the territory choice of males via the order of territory settlement. Female territory choice was studied experimentally in another study area where the correlation between male and territory quality was removed by not allowing males a free choice of territory. We defined several habitat characteristics and estimated the abundances of invertebrate prey from air, ground and trees with appropriate traps from the surroundings of the nest sites in both study areas. Against the expectation that parent birds would choose an arthropod-rich territory, neither males nor females seemed to choose those territories that later had the most food for nestlings. There was, ho...
Some insectivorous birds orient towards insect-defoliated trees even when they do not see the fol... more Some insectivorous birds orient towards insect-defoliated trees even when they do not see the foliar damage or the herbivores. There are, however, only a few studies that have examined the mechanisms behind this foraging behaviour. Previous studies suggest that birds can use olfactory foraging cues (e.g. volatile organic compounds (VOCs) emitted by defoliated plants), indirect visual cues or a combination of the two sensory cues. VOCs from insect-defoliated plants are known to attract natural enemies of herbivores, and researchers have hypothesized that VOCs could also act as olfactory foraging cues for birds. We conducted three experiments across a range of spatial scales to test this hypothesis. In each experiment, birds were presented with olfactory cues and their behavioural responses or foraging outcomes were observed. In the first experiment, two different VOC blends, designed to simulate the volatile emissions of mountain birch (Betula pubescens ssp. czerepanovii) after defoliation by autumnal moth (Epirrita autumnata) larvae, were used in behavioural experiments in aviaries with pied flycatchers (Ficedula hypoleuca). The second experiment was a field-based trial of bird foraging efficiency; the same VOC blends were applied to mountain birches, silver birches (B. pendula) and European white birches (B. pubescens) with plasticine larvae attached to the trees to serve as artificial prey for birds and provide a means to monitor predation rate. In the third experiment, the attractiveness of silver birch saplings defoliated by autumnal moth larvae versus intact controls was tested with great tits (Parus major) and blue tits (Cyanistes caeruleus) in an aviary. Birds did not orient towards either artificial or real trees with VOC supplements or towards herbivore-damaged saplings when these saplings and undamaged alternatives were hidden from view. These findings do not support the hypothesis that olfactory foraging cues are necessary in the attraction of birds to herbivore-damaged trees.
ABSTRACT Earlier studies have suggested that insectivorous birds, similar to invertebrate predato... more ABSTRACT Earlier studies have suggested that insectivorous birds, similar to invertebrate predators and parasitoids, may be guided by herbivore-induced plant volatiles (HIPVs) to damaged, herbivore-rich trees. Recent studies have also shown that birds use olfaction more than previously thought, underlying the potential for HIPVs to be sensed by insectivorous birds and utilised during foraging for prey. The HIPV production in plants is mediated, at least partly, by the jasmonic acid signalling pathway, and similar HIPVs to those induced by herbivores can often be induced by exposing plants to methyl jasmonate (MeJa). We studied the effects of MeJa on volatile emission and bird attraction using mature mountain birches (Betula pubescens ssp. czerepanovii) under natural conditions in northern Finland. Experimental trees were assigned to four treatment groups: herbivore-damaged [autumnal moth (Epirrita autumnata)], higher dose of MeJa (30 mM), lower dose of MeJa (15 mM) and control. All trees had three branches covered with mesh bags, but there were larvae inside the bags only of the herbivore-damage treatment. Bird predation rate was monitored with artificial plasticine larvae which were checked daily for peck marks. Birds most often pecked the larvae in the herbivore-damaged trees, but the attractiveness of MeJa-treated trees did not differ from the control. High within-treatment variation in systemic HIPV emissions probably masked MeJa treatment effects. The bird predation rate was high in birches that emitted large amounts of α-pinene. Thus, α-pinene may be one cue used by birds to find herbivore-rich birches.
Proceedings of the Royal Society B: Biological Sciences, 2018
The evolution of cooperation and social behaviour is often studied in isolation from the ecology ... more The evolution of cooperation and social behaviour is often studied in isolation from the ecology of organisms. Yet, the selective environment under which individuals evolve is much more complex in nature, consisting of ecological and abiotic interactions in addition to social ones. Here, we measured the life-history costs of cooperative chemical defence in a gregarious social herbivore, Diprion pini pine sawfly larvae, and how these costs vary under different ecological conditions. We ran a rearing experiment where we manipulated diet (resin content) and attack intensity by repeatedly harassing larvae to produce a chemical defence. We show that forcing individuals to allocate more to cooperative defence (high attack intensity) incurred a clear cost by decreasing individual survival and potency of chemical defence. Cooperative behaviour and the magnitude of its costs were further shaped by host plant quality. The number of individuals participating in group defence, immune responses ...
The tritrophic interactions between plants, herbivores and avian predators are complex and prone ... more The tritrophic interactions between plants, herbivores and avian predators are complex and prone to trophic cascades. We conducted a meta-analysis of original articles that have studied birds as predators of invertebrate herbivores, to compare top-down trophic cascades with diVerent plant responses from diVerent environments and climatic areas. Our search found 29 suitable articles, with a total of 81 separate experimental study set-ups. The meta-analysis revealed that plants beneWted from the presence of birds. A signiWcant reduction was observed in the level of leaf damage and plant mortality. The presence of birds also positively aVected the amount of plant biomass, whereas eVects on plant growth were negligible. There were no diVerences in the eVects between agricultural and natural environments. Similarly, plants performed better in all cli-matic areas (tropical, temperate and boreal) when birds were present. Moreover, both mature plants and saplings gained beneWts from the pre...
Aim: Studies conducted at a global scale are necessary to make general conclusions on the effect ... more Aim: Studies conducted at a global scale are necessary to make general conclusions on the effect of urbanization on trophic interactions and explore how these effects change along latitudinal gradients. Since biotic interactions are more intense at lower latitudes, we predict that the intensity of trophic interactions will decrease in response to urbanization (quantified by human population density), with higher impacts of urbanization at higher latitudes.Location: Global (881 study sites).Time period: 2000-2021.Major taxa studied: Birds, arthropods and woody plants.Methods: We compiled global data on insect herbivory and bird predation from individual studies using similar methodologies, and fitted generalized linear mixed models to test the effect of human population density, latitude and their interaction on these two response variables.Results: The intensity of herbivory and predation decreased with the increase of human population density at lower latitudes, remained unaffected...
Although variation in effect sizes and predicted values among studies of similar phenomena is ine... more Although variation in effect sizes and predicted values among studies of similar phenomena is inevitable, such variation far exceeds what might be produced by sampling error alone. One possible explanation for variation among results is differences among researchers in the decisions they make regarding statistical analyses. A growing array of studies has explored this analytical variability in different (mostly social science) fields, and has found substantial variability among results, despite analysts having the same data and research question. We implemented an analogous study in ecology and evolutionary biology, fields in which there have been no empirical exploration of the variation in effect sizes or model predictions generated by the analytical decisions of different researchers. We used two unpublished datasets, one from evolutionary ecology (blue tit, Cyanistes caeruleus, to compare sibling number and nestling growth) and one from conservation ecology (Eucalyptus, to compa...
Migratory birds generally have tightly scheduled annual cycles, in which delays can have carry-ov... more Migratory birds generally have tightly scheduled annual cycles, in which delays can have carry-over effects on the timing of later events, ultimately impacting reproductive output. We tested whether migration schedules among eight breeding populations of Arctic SkuasStercorarius parasiticusbetween Greenland and Siberia differ as a function of their use of seven widely divergent wintering areas across the Atlantic Ocean, Mediterranean Sea and Indian Ocean, and predicted that migrations over larger distances, with tighter schedules, would provide less room for compensating delays. Breeding at higher latitudes led not only to later reproduction and migration, but also faster spring migration and shorter time between return to the breeding area and clutch initiation. Wintering area was consistent within individuals among years; and more distant areas were associated with more time spent on migration and less time in the wintering areas. Skuas adjusted the period spent in the non-breedin...
This is an open access article under the terms of the Creative Commons Attribution License, which... more This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
According to the Latitudinal Biotic Interaction Hypothesis (LBIH), the general increase in biodiv... more According to the Latitudinal Biotic Interaction Hypothesis (LBIH), the general increase in biodiversity towards lower latitudes can be partially explained by an increase in the intensity of biotic interactions. While LBIH received some support for plant-herbivores interactions, much less is known about how higher trophic levels may contribute to shape biotic interactions across latitudinal gradients. We hypothesized that the diversity of insectivorous birds increases towards lower latitude, leading to higher predation rates on insect herbivores. We deployed plasticine caterpillars in 138 oak trees in 47 sites along a 19 degree latitudinal gradient in Europe to quantify bird insectivory through predation attempts. In addition, we used passive acoustic monitoring to (i) characterize the acoustic diversity of surrounding soundscapes; and (ii) infer both taxonomic and functional diversity of insectivorous birds from recordings. The functional diversity of insectivorous birds increased t...
Several studies have shown that insectivorous birds are attracted to herbivore-damaged trees even... more Several studies have shown that insectivorous birds are attracted to herbivore-damaged trees even when they cannot see or smell the actual herbivores or their faeces. However, it often remained an open question whether birds are attracted by herbivore-induced changes in leaf odour or in leaf light reflectance or by both types of changes. Our study addressed this question by investigating the response of great tits (Parus major) and blue tits (Cyanistes caeruleus) to Scots pine (Pinus sylvestris) damaged by pine sawfly larvae (Diprion pini). We released the birds individually to a study booth, where they were simultaneously offered a systemically herbivore-induced and a non-infested control pine branch. In the first experiment, the birds could see the branches, but not smell them, because each branch was kept inside a transparent, air-tight cylinder. In the second experiment, the birds could smell the branches, but not see them, because each branch was placed inside a non-transparent...
Abstract The arctic skua (Stercorarius parasiticus) is one of the most long‐lived bird species. I... more Abstract The arctic skua (Stercorarius parasiticus) is one of the most long‐lived bird species. In 2010, we captured in Finland an adult, female arctic skua which had been ringed as a nestling in 1987. We tagged it also with a color ring. The bird has last been seen in July 2020 at the age of 33 years, making it most likely the oldest known arctic skua of the world. In 2010–2011 the bird carried a light‐level measuring geolocator, the data of which revealed that the bird had spent the nonbreeding season in the Canary Current area on the west coast of Africa. Breeding populations of arctic skuas have declined recently especially in British Isles, thus it is useful to get longevity data of this species with a high breeding site fidelity.
Plants infested by arthropod herbivores have evolved a plethora of defensive responses (Howe & Ja... more Plants infested by arthropod herbivores have evolved a plethora of defensive responses (Howe & Jander, 2008; Walling, 2000). They defend themselves by responses which directly harm the herbivores by, for example, feeding-induced reduction of the nutritive value of plant tissue or damaged-induced production of harmful chemicals (reviewed by Chen, 2008). Other feeding-induced plant responses indirectly exert a negative effect on the herbivores by attracting predators or parasitoids of the plant-infesting arthropods (e.g.,
Plants infested by arthropod herbivores have evolved a plethora of defensive responses (Howe & Ja... more Plants infested by arthropod herbivores have evolved a plethora of defensive responses (Howe & Jander, 2008; Walling, 2000). They defend themselves by responses which directly harm the herbivores by, for example, feeding-induced reduction of the nutritive value of plant tissue or damaged-induced production of harmful chemicals (reviewed by Chen, 2008). Other feeding-induced plant responses indirectly exert a negative effect on the herbivores by attracting predators or parasitoids of the plant-infesting arthropods (e.g.,
Data from the defensive behaviour and potency of the chemical defende during the group rearing st... more Data from the defensive behaviour and potency of the chemical defende during the group rearing stage of the experiment
Cry for help' hypothesis predicts that attraction of predators with chemical or visual cues can d... more Cry for help' hypothesis predicts that attraction of predators with chemical or visual cues can decrease insect damage of plants. Visual cues involve changes in photosynthetic activity and the reflectance of leaves, and there is some evidence that birds may use these changes as foraging cues. However, changes in the visual properties of leaves have not been quantified and it is not known how birds see these changes. We also presented and tested a new 'reduction in camouflage' hypothesis (not mutually exclusive with 'cry for help') stating that herbivore-mediated changes in leaf colour can increase the conspicuousness of herbivore against leaves. To define changes in the visual properties of leaves, their detectability to birds, and whether these changes affect the conspicuousness of herbivore, we manipulated the level of herbivory in silver birch trees (Betula pendula) with autumnal moth (Epirrita autumnata) larvae, and used blue tit (Cyanistes caeruleus) vision models to images of leaves and larvae. Hue, luminance (lightness), contrast, light transmission, chlorophyll content, photosynthetic activity and water content of the leaves were compared between herbivore-damaged and control trees. The leaves of herbivore-damaged trees had a decreased chlorophyll a concentration, increased contrast and they reflected more longer wavelengths. However, these changes are likely not obvious to birds. In contrast to our expectation, there were only minor differences in conspicuousness of larvae against the leaves of damaged trees, which may be very subtle to predator vision. Nevertheless, according to visual models, larvae should be easily detectable to birds from both herbivore-damaged and control trees. Keywords: trophic interactions, avian vision model, background matching, herbivory, camouflage Significance statement Herbivory affects photosynthetic machinery and light reflectance of leaves, and may thus provide visual foraging cues to birds, although it is not known how these changes appear to birds. We also hypothesised that the changes in leaves may reduce the camouflage of the herbivore. After applying herbivoretreatment and using the avian vision models, we found that the leaves of herbivore-damage may cause the leaves to appear to birds with higher contrast and greener or a more yellowish colour than control leaves. In addition, although the herbivore was visible to birds, it was slightly less conspicuous when on damaged trees, indicating that the herbivore can be adapted to changes in the food plant. Our results indicate that herbivory causes changes visual properties of leaves, but these changes are likely not obvious to birds. 'Reduction in camouflage' could thus work independently from 'cry for help', although they can also share plant physiological mechanisms, if the cry for help is mediated by vision. To study the effects of herbivory on the visual properties of leaves, we manipulated herbivoredamage in silver birches (Betula pendula) by adding leaf-chewing autumnal moth larvae (Epirrita autumnata) on the experimental trees. We applied an avian vision model of blue tit (Cyanistes caeruleus) to investigate whether herbivory causes optical changes in the leaves that may be visible to birds and whether these changes make the larvae potentially more detectable. In addition to using visual models, chlorophyll concentration, fluorescence induction (i.e. indicators of photosynthesis), light transmission, and water content of the intact leaves of herbivore-damaged and undamaged control trees were measured. Methods Study system Silver birch is one of the most common deciduous trees in Finland and it is a host to several insect species (Koponen 1983; Heimonen et al. 2015), including the autumnal moth. Because the autumnal moth is a common generalist (Silvonen et al. 2014), its camouflage may provide intermediate matching to several backgrounds, instead of close matching to a single host plant species (Merilaita et al. 1999; Houston et al. 2007), although to human eyes the larvae closely resemble the leaf colour of many deciduous species
It is often assumed that birds are able to choose a breeding territory that will later on have th... more It is often assumed that birds are able to choose a breeding territory that will later on have the most food for nestlings. Studies on this essential question are, however, few. We studied territory choice of a long-distance migrant, the pied flycatcher Ficedula hypoleuca in southwestern Finland. In one study area, we monitored the territory choice of males via the order of territory settlement. Female territory choice was studied experimentally in another study area where the correlation between male and territory quality was removed by not allowing males a free choice of territory. We defined several habitat characteristics and estimated the abundances of invertebrate prey from air, ground and trees with appropriate traps from the surroundings of the nest sites in both study areas. Against the expectation that parent birds would choose an arthropod-rich territory, neither males nor females seemed to choose those territories that later had the most food for nestlings. There was, ho...
Some insectivorous birds orient towards insect-defoliated trees even when they do not see the fol... more Some insectivorous birds orient towards insect-defoliated trees even when they do not see the foliar damage or the herbivores. There are, however, only a few studies that have examined the mechanisms behind this foraging behaviour. Previous studies suggest that birds can use olfactory foraging cues (e.g. volatile organic compounds (VOCs) emitted by defoliated plants), indirect visual cues or a combination of the two sensory cues. VOCs from insect-defoliated plants are known to attract natural enemies of herbivores, and researchers have hypothesized that VOCs could also act as olfactory foraging cues for birds. We conducted three experiments across a range of spatial scales to test this hypothesis. In each experiment, birds were presented with olfactory cues and their behavioural responses or foraging outcomes were observed. In the first experiment, two different VOC blends, designed to simulate the volatile emissions of mountain birch (Betula pubescens ssp. czerepanovii) after defoliation by autumnal moth (Epirrita autumnata) larvae, were used in behavioural experiments in aviaries with pied flycatchers (Ficedula hypoleuca). The second experiment was a field-based trial of bird foraging efficiency; the same VOC blends were applied to mountain birches, silver birches (B. pendula) and European white birches (B. pubescens) with plasticine larvae attached to the trees to serve as artificial prey for birds and provide a means to monitor predation rate. In the third experiment, the attractiveness of silver birch saplings defoliated by autumnal moth larvae versus intact controls was tested with great tits (Parus major) and blue tits (Cyanistes caeruleus) in an aviary. Birds did not orient towards either artificial or real trees with VOC supplements or towards herbivore-damaged saplings when these saplings and undamaged alternatives were hidden from view. These findings do not support the hypothesis that olfactory foraging cues are necessary in the attraction of birds to herbivore-damaged trees.
ABSTRACT Earlier studies have suggested that insectivorous birds, similar to invertebrate predato... more ABSTRACT Earlier studies have suggested that insectivorous birds, similar to invertebrate predators and parasitoids, may be guided by herbivore-induced plant volatiles (HIPVs) to damaged, herbivore-rich trees. Recent studies have also shown that birds use olfaction more than previously thought, underlying the potential for HIPVs to be sensed by insectivorous birds and utilised during foraging for prey. The HIPV production in plants is mediated, at least partly, by the jasmonic acid signalling pathway, and similar HIPVs to those induced by herbivores can often be induced by exposing plants to methyl jasmonate (MeJa). We studied the effects of MeJa on volatile emission and bird attraction using mature mountain birches (Betula pubescens ssp. czerepanovii) under natural conditions in northern Finland. Experimental trees were assigned to four treatment groups: herbivore-damaged [autumnal moth (Epirrita autumnata)], higher dose of MeJa (30 mM), lower dose of MeJa (15 mM) and control. All trees had three branches covered with mesh bags, but there were larvae inside the bags only of the herbivore-damage treatment. Bird predation rate was monitored with artificial plasticine larvae which were checked daily for peck marks. Birds most often pecked the larvae in the herbivore-damaged trees, but the attractiveness of MeJa-treated trees did not differ from the control. High within-treatment variation in systemic HIPV emissions probably masked MeJa treatment effects. The bird predation rate was high in birches that emitted large amounts of α-pinene. Thus, α-pinene may be one cue used by birds to find herbivore-rich birches.
Proceedings of the Royal Society B: Biological Sciences, 2018
The evolution of cooperation and social behaviour is often studied in isolation from the ecology ... more The evolution of cooperation and social behaviour is often studied in isolation from the ecology of organisms. Yet, the selective environment under which individuals evolve is much more complex in nature, consisting of ecological and abiotic interactions in addition to social ones. Here, we measured the life-history costs of cooperative chemical defence in a gregarious social herbivore, Diprion pini pine sawfly larvae, and how these costs vary under different ecological conditions. We ran a rearing experiment where we manipulated diet (resin content) and attack intensity by repeatedly harassing larvae to produce a chemical defence. We show that forcing individuals to allocate more to cooperative defence (high attack intensity) incurred a clear cost by decreasing individual survival and potency of chemical defence. Cooperative behaviour and the magnitude of its costs were further shaped by host plant quality. The number of individuals participating in group defence, immune responses ...
The tritrophic interactions between plants, herbivores and avian predators are complex and prone ... more The tritrophic interactions between plants, herbivores and avian predators are complex and prone to trophic cascades. We conducted a meta-analysis of original articles that have studied birds as predators of invertebrate herbivores, to compare top-down trophic cascades with diVerent plant responses from diVerent environments and climatic areas. Our search found 29 suitable articles, with a total of 81 separate experimental study set-ups. The meta-analysis revealed that plants beneWted from the presence of birds. A signiWcant reduction was observed in the level of leaf damage and plant mortality. The presence of birds also positively aVected the amount of plant biomass, whereas eVects on plant growth were negligible. There were no diVerences in the eVects between agricultural and natural environments. Similarly, plants performed better in all cli-matic areas (tropical, temperate and boreal) when birds were present. Moreover, both mature plants and saplings gained beneWts from the pre...
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