Theoretical analysis of an iron mineral-based magnetoreceptor model in birds

Sensing the magnetic field has been established as an essential part of navigation and orientation of various animals for many years. Only recently has the first detailed receptor concept for magnetoreception been published based on histological and physical results. The considered mechanism involves two types of iron minerals (magnetite and maghemite) that were found in subcellular compartments within sensory dendrites of the upper beak of several bird species. But so far a quantitative evaluation of the proposed receptor is missing. In this article, we develop a theoretical model to quantitatively and qualitatively describe the magnetic field effects among particles containing iron minerals. The analysis of forces acting between these subcellular compartments shows a particular dependence on the orientation of the external magnetic field. The iron minerals in the beak are found in the form of crystalline maghemite platelets and assemblies of magnetite nanoparticles. We demonstrate that the pull or push to the magnetite assemblies, which are connected to the cell membrane, may reach a value of 0.2 pN -- sufficient to excite specific mechanoreceptive membrane channels in the nerve cell. The theoretical analysis of the assumed magnetoreceptor system in the avian beak skin clearly shows that it might indeed be a sensitive biological magnetometer providing an essential part of the magnetic map for navigation.     

Avian orientation: the pulse effect is mediated by the magnetite receptors in the upper beak

Migratory silvereyes treated with a strong magnetic pulse shift their headings by approximately 90°, indicating an involvement of magnetite-based receptors in the orientation process. Structures containing superparamagnetic magnetite have been described in the inner skin at the edges of the upper beak of birds, while single-domain magnetite particles are indicated in the nasal cavity. To test which of these structures mediate the pulse effect, we subjected migratory silvereyes, Zosterops l. lateralis, to a strong pulse, and then tested their orientation, while the skin of their upper beak was anaesthetized with a local anaesthetic to temporarily deactivate the magnetite-containing structures there. After the pulse, birds without anaesthesia showed the typical shift, whereas when their beak was anaesthetized, they maintained their original headings. This indicates that the superparamagnetic magnetite-containing structures in the skin of the upper beak are most likely the magnetoreceptors that cause the change in headings observed after pulse treatment.     

Sexual and seasonal differences in the frequency of beak marks on the wings of two Lethe butterflies

To assess bird predation pressure on butterflies, I investigated beak marks on the wings of two Lethe butterflies for 3 years in secondary temperate forests. If bird predation had significant effects on average longevity of butterflies, and if the number of specimens preyed upon was proportionate to the number of beak-marked specimens, the beak mark frequency would be negatively correlated with average longevity of a butterfly. Bird predation pressure is generally thought to influence average longevity of butterflies. Therefore, if there is a negative correlation between beak mark frequency and average longevity, bird predation pressure would be reflected in beak mark frequency. Beak mark frequency was negatively correlated with longevity in Lethe diana (Butler), the more abundant of the two species; thus, the beak mark frequency was considered to be a suitable index of bird predation pressure on the butterflies investigated in this study. In both Lethe species, beak mark frequency was higher in females than in males. Because female butterflies have a relatively smaller thorax and flight muscles and a larger abdomen that contains eggs, they are presumably weaker or less agile fliers than males, and are probably attacked more easily by birds. In autumn, butterflies were heavily attacked by birds irrespective of sex and species. Because the numbers of lepidopteran larvae, which are the preferred prey of many birds, decreased in autumn, birds were thought to shift their diets to alternative prey such as adult butterflies.     

Mechanical stress,fracture risk and beak evolution in Darwin's ground finches (Geospiza)

Darwin''s finches have radiated from a common ancestor into 14 descendent species, each specializing on distinct food resources and evolving divergent beak forms. Beak morphology in the ground finches (Geospiza) has been shown to evolve via natural selection in response to variation in food type, food availability and interspecific competition for food. From a mechanical perspective, however, beak size and shape are only indirectly related to birds'' abilities to crack seeds, and beak form is hypothesized to evolve mainly under selection for fracture avoidance. Here, we test the fracture-avoidance hypothesis using finite-element modelling. We find that across species, mechanical loading is similar and approaches reported values of bone strength, thus suggesting pervasive selection on fracture avoidance. Additionally, deep and wide beaks are better suited for dissipating stress than are more elongate beaks when scaled to common sizes and loadings. Our results illustrate that deep and wide beaks in ground finches enable reduction of areas with high stress and peak stress magnitudes, allowing birds to crack hard seeds while limiting the risk of beak failure. These results may explain strong selection on beak depth and width in natural populations of Darwin''s finches.     

A small, “wader-like” bird from the Early Eocene of Messel (Germany)

A small bird with a shorebird-like beak and a very long hind toe is described from the Early Eocene of Messel in Germany. Vanolimicola longihallucis, gen. et sp. nov. is one of the few candidate species for a “wading bird” from Messel, that is, a bird that may have foraged along the shoreline of the ancient lake. A few features indicate a relationship to charadriiform birds and Jacanidae (jacanas) in particular, but charadriiform affinities are only weakly supported. In the preserved skeletal elements, V. longihallucis also resembles the taxon Songzia from the Early Eocene of China, which is considered to be a representative of the Ralloidea (rails and allies). A potential biogeographic significance of the new Messel fossil is strengthened by similar fossils from the North American Green River Formation.     

Non-redundancy in seed dispersal and germination by native and introduced frugivorous birds: implications of invasive bird impact on native plant communities

Seed dispersal by vertebrate animals is important for the establishment of many fleshy-fruited plant species. Different frugivorous species can provide different seed dispersal services according to their specific dietary preferences as well as behaviour and body traits (e.g. body size and beak size of birds). Our aim was to study redundancies and complementarities in seed dispersal and germination between the two main native seed disperser birds and the introduced silver pheasant Lophura nycthemera in the temperate Patagonian forests. For this, we collected fresh droppings from the studied species and analyzed seed content. We conducted germination trials for four plant species common in bird droppings; two native species (Aristotelia chilensis and Rhaphithamnus spinosus) and two invasive non-native species (Rubus ulmifolius and Rosa rubiginosa). Both native frugivorous birds and the silver pheasant dispersed fruits of non- native fleshy-fruited plants, but their roles were non-redundant in terms of species dispersed and effect on seed germination. The silver pheasant dispersed a proportionally high number of non-native seeds, while native birds dispersed a high number of native seeds. In addition, the effect of gut treatment in seed germination differed between seed dispersers. Native birds promoted the germination for the two native plant species studied, while the silver pheasant promoted the germination of one non-native plant. This suggests that seed dispersal by the silver pheasant may contribute to the spread of some invasive fleshy-fruited plants in the ecosystems that otherwise would not be dispersed by any other bird. The understanding of redundancies and complementarities on seed dispersal and germination between native and introduced birds will allow improving the management of fleshy-fruited non-native plants.     

Bats use magnetite to detect the earth's magnetic field

While the role of magnetic cues for compass orientation has been confirmed in numerous animals, the mechanism of detection is still debated. Two hypotheses have been proposed, one based on a light dependent mechanism, apparently used by birds and another based on a "compass organelle" containing the iron oxide particles magnetite (Fe(3)O(4)). Bats have recently been shown to use magnetic cues for compass orientation but the method by which they detect the Earth's magnetic field remains unknown. Here we use the classic "Kalmijn-Blakemore" pulse re-magnetization experiment, whereby the polarity of cellular magnetite is reversed. The results demonstrate that the big brown bat Eptesicus fuscus uses single domain magnetite to detect the Earths magnetic field and the response indicates a polarity based receptor. Polarity detection is a prerequisite for the use of magnetite as a compass and suggests that big brown bats use magnetite to detect the magnetic field as a compass. Our results indicate the possibility that sensory cells in bats contain freely rotating magnetite particles, which appears not to be the case in birds. It is crucial that the ultrastructure of the magnetite containing magnetoreceptors is described for our understanding of magnetoreception in animals.     

The magnetite-based receptors in the beak of birds and their role in avian navigation

Iron-rich structures have been described in the beak of homing pigeons, chickens and several species of migratory birds and interpreted as magnetoreceptors. Here, we will briefly review findings associated with these receptors that throw light on their nature, their function and their role in avian navigation. Electrophysiological recordings from the ophthalmic nerve, behavioral studies and a ZENK-study indicate that the trigeminal system, the nerves innervating the beak, mediate information on magnetic changes, with the electrophysiological study suggesting that these are changes in intensity. Behavioral studies support the involvement of magnetite and the trigeminal system in magnetoreception, but clearly show that the inclination compass normally used by birds represents a separate system. However, if this compass is disrupted by certain light conditions, migrating birds show ‘fixed direction’ responses to the magnetic field, which originate in the receptors in the beak. Together, these findings point out that there are magnetite-based magnetoreceptors located in the upper beak close to the skin. Their natural function appears to be recording magnetic intensity and thus providing one component of the multi-factorial ‘navigational map’ of birds.     

鸟类利用地磁场定向的机制

地磁场和生物体问的相互作用关系是一个很有趣的未解之迷.虽然对地磁场的生物学作用至今还知之不多,为过近来有关鸟类利用地磁场信息定向的研究取得了较大的进展.很多鸟类能够对地磁场和外加磁场信息做出反应,这些反应可能通过磁场一生物化学过程介导.对此,目前有两种被广为接受的解释,一种认为在鸟喙上方存在一个磁场信息感受器,另一种认为通过视觉成像系统感受磁场信息.另外,最近研究发现磁场信息的感知分析功能有明显的单侧优势特征.虽然目前有关鸟类利用磁场信息定向的研究取得了很多进展,但是要想解释并利用鸟类的磁场定向功能还有很多工作要做.本文结合最近的研究发现对这一有趣的问题进行了综述.     

陕西秦岭东段南坡繁殖鸟类群落组成的30年变化

于2008和2010年每年5～7月在秦岭东段南坡的商洛地区商南、山阳、柞水和洛南4县根据海拔和植被类型选择样点,并采用样线法和网捕法相结合的方法对繁殖鸟类群落进行了调查。共观察记述鸟类159种,农田和栽培植物带(215～650 m)有鸟类96种,其中优势种是山麻雀(Passer rutilans),低山阔叶林带(650～1 200 m)鸟类种数最多,有135种,其中优势种是绿背山雀(Parus monticolus),针阔混交林带(1 200～2 200 m)有73种,其中优势种是橙翅噪鹛(Garrulax elliotii);秦岭东段南坡鸟类群落与中段南坡以及西段南坡和巴山北坡鸟类群落有较高的相似性,但是和秦岭北坡、巴山南坡的鸟类群落相似性较小,反映出秦岭和巴山对鸟类物种组成的阻隔作用和该地区鸟类区系组成的过渡性。和30年前相比,秦岭东段南坡鸟类物种增加了64种,但是有8种鸟类在2008和2010年的调查中未见到。总体上,秦岭东段南坡的繁殖鸟类群落组成在过去的30年里已经发生了明显的变化。     

Avian magnetic compass: Its functional properties and physical basis

The avian magnetic compass was analyzed in bird species of three different orders - Passeriforms, Columbiforms and Galliforms - and in three different behavioral contexts, namely migratory orientation, homing and directional conditioning. The respective findings indicate similar functional properties: it is an inclination compass that works only within a functional window around the ambient magnetic field intensity, it tends to be lateralized in favor of the right eye, and it is wavelength-dependent, requiring light from the short-wavelength range of the spectrum. The underlying physical mechanisms have been identified as radical pair processes, spin-chemical reactions in specialized photopigments. The iron-based receptors in the upper beak do not seem to be involved. The existence of the same type of magnetic compass in only very distantly related bird species suggests that it may have been present already in the common ancestors of all modern birds, where it evolved as an all-purpose compass mechanism for orientation within the home range.     

Interaction of magnetite-based receptors in the beak with the visual system underlying 'fixed direction' responses in birds

Background

European robins, Erithacus rubecula, show two types of directional responses to the magnetic field: (1) compass orientation that is based on radical pair processes and lateralized in favor of the right eye and (2) so-called 'fixed direction' responses that originate in the magnetite-based receptors in the upper beak. Both responses are light-dependent. Lateralization of the 'fixed direction' responses would suggest an interaction between the two magnetoreception systems.

Results

Robins were tested with either the right or the left eye covered or with both eyes uncovered for their orientation under different light conditions. With 502 nm turquoise light, the birds showed normal compass orientation, whereas they displayed an easterly 'fixed direction' response under a combination of 502 nm turquoise with 590 nm yellow light. Monocularly right-eyed birds with their left eye covered were oriented just as they were binocularly as controls: under turquoise in their northerly migratory direction, under turquoise-and-yellow towards east. The response of monocularly left-eyed birds differed: under turquoise light, they were disoriented, reflecting a lateralization of the magnetic compass system in favor of the right eye, whereas they continued to head eastward under turquoise-and-yellow light.

Conclusion

'Fixed direction' responses are not lateralized. Hence the interactions between the magnetite-receptors in the beak and the visual system do not seem to involve the magnetoreception system based on radical pair processes, but rather other, non-lateralized components of the visual system.     

The figs of winter: Seasonal importance of fruiting fig trees (Ficus: Moraceae) for urban birds

Birds and figs are conspicuous members of the tropical and subtropical ecosystems. Because they are easily observed and very speciose, their relationships have been well studied in many areas, and the figs are considered a keystone resource for many bird species which are efficient fig seed dispersers. Taiwan has a relatively high endemism rate for many taxa (17% of bird species) but because of its high human population density, most lowland habitats are heavily developed, of which much of it covered by dense urban habitation. To establish the importance of urban figs for birds, we focused our surveys mostly on three common urban fig species (Ficus caulocarpa, F. microcarpa and F. subpisocarpa). We observed trees with ripening figs from July 2013 to December 2016 in order to determine the composition of the fig-consuming bird community. In addition, we added all the information available in the scientific literature and birdwatchers' observations which we could find. In total, we observed 42 bird species consuming 18 fig species. The bird diversity in urban areas was non-negligible even during winter. Therefore, there are two reasons why figs are important for Taiwan's bird avifauna: in cities, the tree diversity is generally low so that figs provide a stable food resource; and since figs are fruiting all year-round, they are one of the few reliable resources available during winter when many migrant birds overwinter in Taiwan. Already crucial for many species in tropical and subtropical forests, fig trees may also be essential for urban birds in tropical and subtropical regions.     

Evolution of tooth crown shape in Mesozoic birds,and its adaptive significance with respect to diet

Both the evolution of tooth morphology and the relationship between dental features and diet in toothed birds have long been studied. Here we quantify variation in tooth crown shape in 28 key Mesozoic bird species, and examine differences in dental morphology among birds belonging to different taxonomic groupings and inferred to have had different diets. Using geometric morphometric methods (GMM) and phylogenetic comparative methods (PCM), we found few clear differences in tooth crown shape between different taxonomic and ecological categories, and our analysis provides little support for many dietary inferences drawn in previous studies. However, the Solnhofen Archaeopteryx, Jeholornis, Protopteryx, Pengornis, Longipteryx, Tianyuornis, Mengciusornis, Ichthyornis and Hesperornis all were found to possess relatively specialized tooth crown shapes, perhaps reflecting specialized diets such as insectivory, granivory, piscivory and consumption of soft-shelled arthropods. Similarity in tooth crown shape across many Mesozoic birds may indicate the lack of dietary specialization, and the association between tooth form and diet may have been weakened in any case by ‘functional replacement’ of the dentition by a horny beak and, in many cases, gastroliths.     

Juvenile Osprey Navigation during Trans-Oceanic Migration

To compensate for drift, an animal migrating through air or sea must be able to navigate. Although some species of bird, fish, insect, mammal, and reptile are capable of drift compensation, our understanding of the spatial reference frame, and associated coordinate space, in which these navigational behaviors occur remains limited. Using high resolution satellite-monitored GPS track data, we show that juvenile ospreys (Pandion haliaetus) are capable of non-stop constant course movements over open ocean spanning distances in excess of 1500 km despite the perturbing effects of winds and the lack of obvious landmarks. These results are best explained by extreme navigational precision in an exogenous spatio-temporal reference frame, such as positional orientation relative to Earth''s magnetic field and pacing relative to an exogenous mechanism of keeping time. Given the age (<1 year-old) of these birds and knowledge of their hatching site locations, we were able to transform Enhanced Magnetic Model coordinate locations such that the origin of the magnetic coordinate space corresponded with each bird''s nest. Our analyses show that trans-oceanic juvenile osprey movements are consistent with bicoordinate positional orientation in transformed magnetic coordinate or geographic space. Through integration of movement and meteorological data, we propose a new theoretical framework, chord and clock navigation, capable of explaining the precise spatial orientation and temporal pacing performed by juvenile ospreys during their long-distance migrations over open ocean.     

Activation changes in zebra finch (Taeniopygia guttata) brain areas evoked by alterations of the earth magnetic field

Many animals are able to perceive the earth magnetic field and to use it for orientation and navigation within the environment. The mechanisms underlying the perception and processing of magnetic field information within the brain have been thoroughly studied, especially in birds, but are still obscure. Three hypotheses are currently discussed, dealing with ferromagnetic particles in the beak of birds, with the same sort of particles within the lagena organs, or describing magnetically influenced radical-pair processes within retinal photopigments. Each hypothesis is related to a well-known sensory organ and claims parallel processing of magnetic field information with somatosensory, vestibular and visual input, respectively. Changes in activation within nuclei of the respective sensory systems have been shown previously. Most of these previous experiments employed intensity enhanced magnetic stimuli or lesions. We here exposed unrestrained zebra finches to either a stationary or a rotating magnetic field of the local intensity and inclination. C-Fos was used as an activity marker to examine whether the two treatments led to differences in fourteen brain areas including nuclei of the somatosensory, vestibular and visual system. An ANOVA revealed an overall effect of treatment, indicating that the magnetic field change was perceived by the birds. While the differences were too small to be significant in most areas, a significant enhancement of activation by the rotating stimulus was found in a hippocampal subdivision. Part of the hyperpallium showed a strong, nearly significant, increase. Our results are compatible with previous studies demonstrating an involvement of at least three different sensory systems in earth magnetic field perception and suggest that these systems, probably less elaborated, may also be found in nonmigrating birds.     

Vocal Tract Articulation in Zebra Finches

Background

Birdsong and human vocal communication are both complex behaviours which show striking similarities mainly thought to be present in the area of development and learning. Recent studies, however, suggest that there are also parallels in vocal production mechanisms. While it has been long thought that vocal tract filtering, as it occurs in human speech, only plays a minor role in birdsong there is an increasing number of studies indicating the presence of sound filtering mechanisms in bird vocalizations as well.

Methodology/Principal Findings

Correlating high-speed X-ray cinematographic imaging of singing zebra finches (Taeniopygia guttata) to song structures we identified beak gape and the expansion of the oropharyngeal-esophageal cavity (OEC) as potential articulators. We subsequently manipulated both structures in an experiment in which we played sound through the vocal tract of dead birds. Comparing acoustic input with acoustic output showed that OEC expansion causes an energy shift towards lower frequencies and an amplitude increase whereas a wide beak gape emphasizes frequencies around 5 kilohertz and above.

Conclusion

These findings confirm that birds can modulate their song by using vocal tract filtering and demonstrate how OEC and beak gape contribute to this modulation.     

繁殖期荒漠鸟类群落与自然植被的相关格局

为探讨植被结构和物种组成对鸟类群落多度分布的影响,于2010年5～8月,在阜康地区所涵盖的显域植被梯度带(梭梭、琵琶柴及蒿属荒漠)内分别布设3～4条样线,调查繁殖鸟类群落多度分布及植被参数.借助平均高度和盖度反映植被结构,平均物种数目反映植物组成.从鸟类群落中抽取显著正相关鸟种组合及狭分布鸟种组合,将不同的鸟类组合与植...     

Contrasting spatial patterns of taxonomic and functional richness offer insights into potential loss of ecosystem services

Functional and trophic perspectives on patterns of species occurrences have the potential to offer new and interesting insights into a range of spatially explicit problems in ecology and conservation. We present the function–area relationship (FAR) and explore linkages between functional and taxonomic species richness for South African birds. We first used beak morphology to classify a subset of 151 South African bird species into 18 functional groups and calculated both the species–area relationship and the FAR at quarter-degree resolution for South Africa. The relationship between functional and taxonomic richness by cell was quadratic rather than linear, with considerable scatter around the curve. We next looked at the spatial relationships between taxonomic diversity and response diversity (i.e. diversity within functional groups) using an a priori categorization of nearly all South African birds into nine functional groups. The spatial distribution of response richness also showed considerable variation in relation to taxonomic richness. Our results demonstrate a novel approach to linking taxonomic, functional and trophic patterns in space and suggest a way in which conservation planning, which has traditionally had a taxonomic focus, could formally incorporate a more functional and food-web-based approach.     

Floating bird nests provide resources for wildlife: ecological roles of vertebrates using <Emphasis Type="Italic">Aechmophorus</Emphasis> grebe nests at Clear Lake,California

Floating bird nests occur ephemerally in many wetlands, providing resources for wildlife, but the species using such nests and their ecological roles are poorly studied. Grebes (Podicipedidae) construct floating nests anchored to vegetation. During 2010–2017 we studied the vertebrates using nests of the colonially-nesting western grebe (Aechmophorus occidentalis) and Clark’s grebe (A. clarkii) at Clear Lake, California. In addition to both grebe species using each other’s nests for copulation and egg dumping, we identified 47 other species of vertebrates using Aechmophorus grebe nests, including one fish, one amphibian, two turtles, one snake, 39 birds, and three mammals. Ecological roles of vertebrates using grebe nests included: egg dumping (one bird); predation on grebe eggs, chicks, or adults (four birds and three mammals); scavenging on broken grebe eggs (five birds); resting (one amphibian, three reptiles, and 15 birds); foraging for non-avian prey on nests (13 birds) or in adjacent water (five birds); and incidental destruction of grebe eggs (one fish and two reptiles) and nests (many species). Floating grebe nests undergo a process of ecological succession. Egg dumping, predation, and scavenging occur during early stages of nest development and maintenance. Once abandoned, older nests are used for resting or foraging for prey in nearby water. Decomposing nests are colonized by arthropods which are preyed upon by birds. Because floating bird nests increase food web complexity by attracting aquatic, semiaquatic, and terrestrial organisms at all trophic levels, protecting the engineers and their nests should be considered a conservation priority by wetland managers.