Crossbills Research Articles

Using long-term citizen science data to understand distribution and habitat use of an irruptive species

Unpredictable movement patterns of irruptive migrants make them difficult to study through structured survey methods. We used citizen science data to assess the distribution and habitat usage of an irruptive migrant outside its core area of occurrence. We curated 50 years of citizen science data on Red crossbills (Loxia curvirostra) in Arkansas and field observations to 1) assess their distribution and habitat use outside their core area of occurrence, 2) determine if Red crossbills breed in Arkansas, 3) characterize the occurrence and abundance of Red crossbills in Arkansas, and 4) identify “call types” or ecotypes of Red crossbills present in Arkansas during 2017–18 irruption. We constructed a sample bias-corrected species distribution model using elevation, land cover type, and monthly temperature and precipitation as predictors for MaxEnt algorithm. The most significant predictors of Red crossbill distribution were land cover type, elevation, and precipitation of October. The probability of Red Crossbill occurrence was high in developed areas, areas with large water bodies, open areas, and evergreen forests. Red crossbills were found to breed in Arkansas based on field observations and historical records. The time-series captures the major irruption years within last 50 years in Arkansas. It also depicts months when the probability of finding Crossbills in Arkansas is highest. In addition, we documented four “call types” or ecotypes of Red crossbills in Arkansas, including type 1 and 4 that were previously unreported. Our study illustrates utility of citizen science observations for understanding the occurrence, distribution, and habitat use of difficult to study nomadic species.

Seasonal Patterns of Fat Deposits in Relation to Migratory Strategy in Facultative Migrants

Physiological preparations for migration generally reflect migratory strategy. Migrant birds fuel long-distance flight primarily with lipids, but carrying excess fuel is costly; thus, the amount of fat deposited prior to departure often reflects the anticipated flight duration or distance between refueling bouts. Seasonal pre-migratory deposition of fat is well documented in regular seasonal migrants, but is less described for more facultative species. We analyze fat deposits of free-living birds across several taxa of facultative migrants in the songbird subfamily Carduelinae, including house finches (Haemorhous mexicanus), American goldfinches (Spinus tristis), pine siskins (Spinus pinus) and four different North American ecotypes of red crossbills (Loxia curvirostra), to evaluate seasonal fat deposition during facultative migratory periods. Our data suggest that the extent of seasonal fat deposits corresponds with migratory tendency in these facultative taxa. Specifically, nomadic red crossbills with a seasonally predictable annual movement demonstrated relatively large seasonal fat deposits coincident with the migratory periods. In contrast, pine siskins, thought to be more variable in timing and initiation of nomadic movements, had smaller peaks in fat deposits during the migratory season, and the partial migrant American goldfinch and the resident house finch showed no peaks coincident with migratory periods. Within the red crossbills, those ecotypes that are closely associated with pine habitats showed larger peaks in fat deposits coincident with autumn migratory periods and had higher wing loading, whereas those ecotypes associated with spruces, Douglas-fir and hemlocks showed larger peaks coincident with spring migratory periods and lower wing loading. We conclude that population averages of fat deposits do reflect facultative migration strategies in these species, as well as the winter thermogenic challenges at the study locations. A difference in seasonal fattening and wing loading among red crossbill ecotypes is consistent with the possibility that they differ in their migratory biology, and we discuss these differences in light of crossbill reproductive schedules and phenologies of different conifer species.

The effects of food limitation on behavior, corticosterone, and the use of social information in the red crossbill (Loxia curvirostra).

Meeting metabolic demands through foraging is a basic animal need that drives the evolution of foraging adaptations. The use of social information is one adaptation that could improve foraging success and fitness if it helps animals locate food when conditions are challenging. It is unknown if food limitation-or the glucocorticoid hormones that are often released when food is limited-can influence the extent to which animals use social information or their ability to learn novel foraging techniques. We explored the effects of limited access to food on activity levels, corticosterone secretion, and social information use in red crossbills, a highly social songbird species adapted to cope with high degrees of resource unpredictability. Using an observer/demonstrator paradigm, food limited or well fed observers were allowed to watch demonstrators solve a novel feeding puzzle before being allowed to attempt the puzzle themselves across repeated trials. Our findings suggest that food limitation transiently increased activity levels but did not result in long-term elevations of corticosterone and did not increase the speed at which red crossbills utilized social information to solve the novel foraging task. However, food limitation may have increased the value of using socially acquired information, as foraging technique performance improved faster in food limited birds relative to controls. Social learning was further demonstrated by the red crossbills in this study when naïve observers overwhelmingly learned a socially-demonstrated task over an undemonstrated task when tested on a two-task foraging board.

Recovery of hematocrit and fat deposits varies by cage size in food‐restricted captive red crossbills (Loxia curvirostra)

Hematocrit-or the percent volume of red blood cells in whole blood-is thought to fluctuate adaptively in response to changing oxygen demands that occur during different life activities and in different environments. Because red blood cells are made from materials that can be limiting, however, it is thought that hematocrit may also reflect general body condition and access to resources. We tested the effect of hydration state, resource restriction (i.e., time available to forage), and activity (i.e., different cage sizes) on hematocrit in captive red crossbills (Loxia curvirostra). We found no evidence that a mild dehydration protocol impacts hematocrit and only weak support that mild food restriction impacts hematocrit. Food restriction did, however, reduce fat deposits and fat loss was more significant in birds that were also sampled for hematocrit. Furthermore, food-restricted birds housed in flight aviaries recovered hematocrit but not fat stores following repeated blood sampling, whereas birds housed in small cages lost additional hematocrit but mitigated fat loss following successive bleeds. Together these results suggest that different flight demands may determine response to blood loss during food restriction, potentially revealing a trade-off between fat storage and red blood cell development. Our results also demonstrate the need for scientists to carefully record hematocrit data and the time course across which multiple tubes of blood are collected to avoid confounding real patterns with variation generated by sampling protocol.

On the relative importance of ecology and geographic isolation as drivers for differentiation of call types of red crossbill Loxia curvirostra in the Palearctic

Efforts to understand the process of speciation have been central to the research of biologists since the origin of evolutionary biology as a discipline. While it is well established that geographic isolation has played a key role in many speciation events, particularly in birds, there is ongoing debate about how frequent speciation is in the partial or complete absence of geographical isolation. In the red crossbill Loxia curvirostra, good arguments do exist for sympatric speciation processes. In this species, several classes of calls are clustered in distinct groups, so‐called ‘call types', which mate assortatively. Often, several call types can be found at a single site, breeding and feeding next to each other. It has been hypothesized that red crossbill call types evolved by specialising in extracting seeds from cones of different conifer species. Alternatively, call types might have evolved in temporal geographic isolation. Within Europe, little is known about the distribution of the various call types and preferences for distinct food resources. In this study, we analysed the temporal and spatial occurrence of red crossbill call types in the Palearctic, investigated potential reasons for call‐type composition at a site, and compared the occurrence of call types with the fructification of conifers. Call‐type composition changed with site and season but hardly with conifer species. With our data, we could localise range areas of twelve different call types, which cannot be explained by conifer species occurrence. Therefore, we suggest that call types evolved in parapatry in most of the northern Palearctic region, and, although contradictory results exist from Iberia, we argue that differentiation might be driven by the same drivers there as well. Additionally, we discuss the potential influence of anthropogenic changes of forest composition and distribution on call types, which offers a unique possibility for future studies.

Testing the shared-pathway hypothesis in the carotenoid-based coloration of red crossbills.

The mechanisms involved in the production of red carotenoid-based ornaments of vertebrates are still poorly understood. These colorations often depend on enzymatic transformations (ketolation) of dietary yellow carotenoids, which could occur in the inner mitochondrial membrane (IMM). Thus, carotenoid ketolation and cell respiration could share biochemical pathways, favoring the evolution of ketocarotenoid-based ornaments as reliable indices of individual quality under sexual selection. Captive male red crossbills (Loxia curvirostra Linnaeus) were exposed to redox-active compounds designed to penetrate and act in the IMM: an ubiquinone (mitoQ) or a superoxide dismutase mimetic (mitoTEMPO). MitoQ can act as an antioxidant but also distort the IMM structure, increasing mitochondrial free radical production. MitoQ decreased yellow carotenoids and tocopherol levels in blood, perhaps by being consumed as antioxidants. Contrarily, mitoTEMPO-treated birds rose circulating levels of the second most abundant ketocarotenoid in crossbills (i.e., canthaxanthin). It also increased feather total red ketocarotenoid concentration and redness, but only among those birds exhibiting a redder plumage at the start of the study, that is, supposedly high-quality individuals. The fact that mitoTEMPO effects depended on original plumage color suggests that the red-ketocarotenoid-based ornaments indicate individual quality as mitochondrial function efficiency. The findings would thus support the shared pathway hypothesis.

Patterns of annual and seasonal immune investment in a temporal reproductive opportunist.

Historically, investigations of how organismal investments in immunity fluctuate in response to environmental and physiological changes have focused on seasonally breeding organisms that confine reproduction to seasons with relatively unchallenging environmental conditions and abundant resources. The red crossbill, Loxia curvirostra, is a songbird that can breed opportunistically if conifer seeds are abundant, on both short, cold, and long, warm days, providing an ideal system to investigate environmental and reproductive effects on immunity. In this study, we measured inter- and intra-annual variation in complement, natural antibodies, PIT54 and leucocytes in crossbills across four summers (2010–2013) and multiple seasons within 1 year (summer 2011–spring 2012). Overall, we observed substantial changes in crossbill immune investment among summers, with interannual variation driven largely by food resources, while variation across multiple seasons within a single cone year was less pronounced and lacked a dominant predictor of immune investment. However, we found weak evidence that physiological processes (e.g. reproductive condition, moult) or abiotic factors (e.g. temperature, precipitation) affect immune investment. Collectively, this study suggests that a reproductively flexible organism may be able to invest in both reproduction and survival-related processes, potentially by exploiting rich patches with abundant resources. More broadly, these results emphasize the need for more longitudinal studies of trade-offs associated with immune investment.

The influence of carbon-containing greenhouse gases on the dynamics of radial increment of Pinus cembra and Picea abies in the conditions of the Gorgany Nature Reserve

Based on the tree-ring chronology, we studied the radial increment of Swiss stone pine (Pinus cembra L.) and common spruce, or Norway spruce (Picea abies L.) as indicators of dendrochronological studies and, based on tree-ring chronology, reconstructed the annual changes of stem biomass and the amount of carbon accumulated in it by P. cembra and P. abies in the Gorgany Nature Reserve. Swiss stone pine stands are of great zoological importance as biotopes of nutcracker (Nucifraga caryocatactes L.), crossbill (Loxia curvirostra L.), capercaillie (Tetrao urogallus L.) and other representatives of rare boreal fauna. Samples were taken from trees of the Playa forest district of the Brusturyan Forest Hunting Range State Enterprise of the Transcarpathian Regional Forestry Administration (P. cembra and P. abies) from trees that did not have visual signs of damage in the Gorgany Nature Reserve. By cross-dating the radial increments of two radii for each tree, individual chronological series were drawn up. Carbon sequestration in P. cembra and P. abies stem wood was evaluated on the basis of dendrochronological analysis of radial increment. The accuracy of cross-dating of dendrochronological series of individual trees, individual chronological series, and representativeness of the generalized series were verified by conventional methods. It was found that the average annual radial increment of the trees under study is 11.48 mm in P. cembra and 14.39 in P. abies, and varies in the range of 1.86–5.49 mm. The data obtained indicate an increase in the ability of P. cembra and P. abies to accumulate carbon with increasing age. The study of carbon sequestration in the stem wood of P. cembra and P. abies, based on dendrochronological analysis, made it possible to reconstruct the annual variation in stem biomass increment and accumulated carbon. The analysis of interdependence of the radial increment of P. cembra and P. abies in the Gorgany Nature Reserve and the value of carbon-containing greenhouse gases in the atmosphere, carried out by the augmented data sample, made it possible to specify the periods most significant for the formation of annual rings. The reaction in radial increment is most pronounced from June to September, with the annual values of carbon-containing greenhouse gases being maximally reflected in radial increment for the period from October to April.

Morphological divergence among Spanish Common Crossbill populations and adaptations to different pine species

Crossbills (Loxia spp.) provide a classical avian model of ecological specialization on food resources. Previous studies have suggested that morphometric, genetic and vocal diversification among Common Crossbill Loxia curvirostra populations is better explained by ecological distance (use of different conifers) than by geographical distance, indicating that populations have diverged adaptatively. We tested for adaptive divergence in Iberian crossbills using bill and body size measurements of 6082 crossbills from 27 sites, each consisting of a dominant or single pine (Pinus) of four possible species. Crossbills using different pines differed significantly in body size and bill size and shape. There was no correlation between geographical and morphological distance among sampling sites, consistent with the hypothesis that the morphological divergence of Iberian crossbills is shaped by their ecological differences (foraging on alternative conifers) rather than geographical distance. However, for unknown reasons, Common Crossbills foraging on Pinus sylvestris in Iberia have on average much smaller bills than Parrot Crossbills Loxia pytyopsittacus feeding on the same pine species in northern Europe. The extent to which crossbills specialize on Iberian P. sylvestris remains to be established. Specialization on conifers with overlapping geographical distributions may be facilitated by matching habitat choice of crossbills as a function of their local intake rates.

Differential gene expression of Plasmodium homocircumflexum (lineage pCOLL4) across two experimentally infected passerine bird species

Plasmodium parasites are present in a wide range of host species, some of which tend to be more susceptible than others, potentially as an outcome of evolved tolerance or resistance. Common starlings seem to cope with malaria infection while common crossbills are more susceptible to the same infections. That raises the question if the parasites rely on the same molecular mechanisms regardless of host species or do Plasmodium parasites change gene-expressions in accordance to the environment different hosts might provide? We used RNA-sequencing from starlings and crossbills, experimentally infected with Plasmodium homocircumflexum (lineage pCOLL4). The assembled transcriptome contained a total of 26,733 contigs. Parasite expression patterns differed between bird species. Parasites had higher expression of cell-invasion genes when infecting crossbills compared to starlings whereas in starlings genes related to apoptosis or/and oxidative stress showed higher expression levels. This article reveals how a Plasmodium parasite might adjust its expression and gene function depending on the host species infected.

Biodiversity and Structure of Undisturbed Mountain Siberian Pine Taiga of the Idarsky Belogorye Ridge (East Sayan)

The structure of flora and fauna, which is a standard of the state and dynamics of the taiga forests in humid climatic facies of the Altai-Sayan mountain region, has been analyzed based on the example of the undisturbed mountain Siberian Pine taiga (Eastern Sayan mountains). The ecological–phytocenotic features and floristic and faunistic composition of the late succession stage are studied. The dominance of the green moss group of forest types is revealed across the topographic profiles. The forest stands have a complex age structure with the dominance of Pinus sibirica Du Tour. and are characterized by low productivity (quality classes IV–V prevail), high class of normality (from 0.5 to 1.0), and sufficient regeneration. A floristic, ecological–coenotic analysis of the herb–dwarf-shrub layer, the elements of undergrowth, and the moss cover indicates the dominance of humid taiga flora. The species of the taiga ecological–coenotic group (ECG) (Vaccinium myrtillus, Carex iljinii, Calamagrostis obtusata, Gymnocarpium dryopteris, Phegopteris connectilis, Oxalis acetosella, Aegopodium alpestre, Trientalis europaea, Linnaea borealis, Maianthemum bifolium, Stellaria bungeana, et al.), moss–bog ECG (Ledum palustre, Vaccinium uliginosum, Carex globularis et al.), and bor-taiga ECG (Vaccinium vitis-idaea, Pyrola rotundifolia, Lycopodium annotinum et al.) form the herb–dwarf-shrub layer in the prevailing forest types. Green mosses (Hylocomium splendens and Pleurozium schreberi with a mixture of Ptilium crista-castrensis and Rhytidiadelphus triquetrus) dominate in the moss layer. Polytrichum commune, P. strictum, Dicranum scoparium, D. polysetum, and Sphagnum sp. mosses are found in some sites. The increased role of higher spore plants characterizes the flora as well-preserved archaic features flora. The Siberian pine forest coenoflora consists of 224 species and belongs to the Cyperaceae type. In the structure of the geographical elements of the flora, the leading role belongs to the Palaearctic, European, and Siberian elements with an increased role of endemics. The species typical for the Siberia taiga complex compose the core of the fauna. The most diverse avifauna is represented by 102 species (Tarsiger cyanurus, Parus montanus, Parus ater, Luscinia calliope, Sitta europaea, Loxia curvirostra, Coccothraustes coccothraustes, Tetrastes bonasia, Phylloscopus proregulus, etc.).

Late Pleistocene and Earliest Holocene avifauna from the Loutra Almopias Cave (Macedonia, Greece)

The bird fauna of the Loutra Almopias Cave comprised eight orders, 22 families, 41 genera and at least 47 species (68 different taxa) established based on 551 avian bone finds. The Loutra Almopias Cave is a new (26th) Pleistocene avian locality in Greece. About 92.5% of the material was dated to the Earliest Holocene (11.230 ± 110 y. BP), coming from the upper chamber of the cave, whereas an older assemblage was dated to the Late Pleistocene (37,880 y. ± 370/360 BP), coming from the floor chambers of the cave. Here is provided the first fossil record of 13 species in Greece (Buteo lagopus, Francolinus francolinus, Lagopus lagopus, Bonasa bonasia, Dryocopus martius, Lulula arborea, Bombicylla garrulus, Cinclus cinclus, Sylvia borin, Carduelis carduelis, Loxia curvirostra, Plectrophenax nivalis, and Emberiza cirlus). Three genera (Francolinus, Anthus and Riparia/Ptyonoprogne) were established for the first time in Pleistocene deposits in the country. The record of Fr. francolinus was the first one for the Balkans and Eastern Europe. At both sites inside the cave, regardless of their different age, the habitat preferences of the identified bird species indicated that the surrounding paleoenvironment included both grassy openlands (dominated by gramineans) and woodland (coniferous or mixed woods). In addition, rocky habitats were represented in the surroundings of the locality. The presence of small species of Perdicinae (26%) and corvids (34%) indicated the former existence and prevelance of open grassy fields and rocky environments. The Alpine chough was the most abundant species at both localities. In the Earliest Holocene it comprised 29.5%, whereas in the Late Pleistocene it reached 25.6% of the material. Such a representation suggests a natural non-human accumulation of the material, probably due to the feeding behavior of Bubo bubo. The Late Pleistocene record (19 taxa; 41 finds) included Falco sp. cf. F. peregrinus, Perdix perdix, Perdix sp., Alectoris graeca, Alectoris sp., Lagopus cf. lagopus, cf. Bonasa bonasia, Columba livia, Columba palumbus, Bubo bubo, Melanocorhypha calandra, Anthus sp., Turdus sp., Pica pica, Pyrrhocorax graculus, Loxia curvirostra, Coccothraustes coccothraustes, Pyrrhula pyrrhula, Fringillidae gen. indet., non-Passeriformes indet. and also suggests the site has been used by the eagle owl. The climate probably used to be drier and cooler than in the Earliest Holocene. The record of 17 woodland species from the Earliest Holocene that were absent in the Late Pleistocene could be explained by the more humid and moderate climate at the very end of the Pleistocene.

Beak morphology predicts apparent survival of crossbills: due to selective survival or selective dispersal?

Dozens of morphologically differentiated populations, subspecies and species of crossbills (genus Loxia) exist. It has been suggested that this divergence is due to variation in the conifer cones that each population specialises upon, requiring a specific beak size to efficiently separate the cone scales. If so, apparent survival should depend on beak size. To test this hypothesis, we undertook multievent capture–recapture modelling for 6844 individuals monitored during 27 years in a Pyrenean common crossbill L. curvirostra population in a forest of mountain pine Pinus uncinata. Apparent survival was indeed related to beak width, resulting in stabilizing selection around an optimum that was close to the observed mean beak width, indicating that local crossbill beak morphology is adapted to the conifer they feed upon. Both natural selection (selective mortality) and selective emigration of maladapted individuals may explain our findings. As is often the case in capture–recapture analyses but rarely recognised, we could not formally decompose apparent survival into selective mortality versus selective permanent emigration. Nonetheless, there are several indications that selective permanent emigration should not be fully excluded. First, natural selection by itself would have to be unusually strong compared to other empirical estimates to create the observed pattern of apparent survival. Second, the observed mean beak width was a bit lower than the estimated optimum beak width. This can be explained by immigration of crossbills with smaller beaks originating from southern populations, which may subsequently have left the study area permanently in response to low food intake. This is in line with a detected transient effect in the data, yet apparently little influx from crossbills from northern Europe. When permanent emigration is phenotypically selective this will have ecological and evolutionary consequences, so this possibility deserves more attention in general.

Estimation of moult duration in birds with suspended moults: the case of the Red Crossbill and its relation to reproduction

Suspended moult is a relatively common phenomenon in birds, but it has remained relatively ignored for a long time in studies dealing with moulting process. Furthermore, the new and increasing number of models used to estimate moult duration systematically omit the fact that suspended moults can occur and that they significantly alter parameter estimates. Taking suspended moults into account is critical to obtain reliable moult-associated parameter estimates, which is fundamental given the demographic and evolutionary consequences of moult in birds. The main goal of this work is to develop a standardised method useful for characterising the main moulting parameters (such as the duration and starting date) of bird species that perform suspended moults. Additionally, with the aim of delving into our understanding of when and why suspended moults happen, we study their relationship with the occurrence of second breeding attempts in summer. We used data obtained from a Red Crossbill population in the Pyrenees during a period of 17 years. We observed that the percentage of crossbills undergoing suspended moult was approximately 50% during summer (July–August) and that moult suspension ultimately gives rise to a lengthened moulting period. Underhill–Zucchini models assume a time-linear replacement of feathers, overestimating the moult duration. Based on these same modelling approaches, we proposed a method to calculate the real moult duration after removing the time during which the moult was suspended. We also obtained evidence supporting the idea that crossbills suspend their moult as a strategy that could increase their breeding output in summer. The method proposed here can be used as a tool for working with species that undergo suspended moults.

Diversification in trophic morphology and a mating signal are coupled in the early stages of sympatric divergence in crossbills

AbstractUnderstanding the mechanisms generating diversity in mating signals is critical to understanding the process of speciation. One mechanism of mating signal diversification occurs when phenotypes that experience divergent ecological selection also affect the production of mating signals, resulting in a coupling between ecological diversification and mating signal diversification. Here, we present evidence that rapid diversification in bill size has resulted in the diversification of some components of song structure in a young adaptive radiation of seed-eating finches (red crossbill, Loxia curvirostra complex). Specifically, we find that larger-billed ecotypes sing songs with lower minimum frequencies, lower syllable repetition rates and greater vocal deviation (i.e. lower performance) than smaller-billed ecotypes for pure tonal syllables. In contrast, bill size was not correlated with maximum frequency or frequency bandwidth, and we found no relationship between bill size and any song parameters in buzzy syllables. Furthermore, we found no evidence for a relationship between the degree of bill size divergence and the potential for song discrimination between sympatric ecotypes. Because bill size is correlated with some features of pure tonal syllables (which appear to be most important for courtship in crossbills) in crossbill song, our results suggest that there was an early-evolving link between ecological and mating signal diversification that may have influenced the rapid evolution of reproductive isolation between sympatric ecotypes.

The experimental study on susceptibility of common European songbirds to Plasmodium elongatum (lineage pGRW6), a widespread avian malaria parasite

BackgroundPlasmodium elongatum (cytochrome b lineage pGRW6) is a widespread avian malaria parasite, often causing severe disease in non-adapted hosts. This parasite lineage is of global distribution however, its virulence remains insufficiently understood, particularly in wild birds. Surprisingly, this infection has never been reported in Common starlings Sturnus vulgaris and Common crossbills Loxia curvirostra, common European songbirds which were extensively sampled across Europe. A hypothesis was proposed that these birds might be resistant to the pGRW6 infection. The aim of this study was to test this hypothesis.MethodsLineage pGRW6 was isolated from a naturally infected Eurasian reed warbler, multiplied in vivo and inoculated in Common starlings and Common crossbills. Experimental and control groups (8 birds in each) were maintained in controlled conditions and examined microscopically every 4 days. Haematocrit value and body mass were monitored in parallel. At the end of the experiment (44 days post exposure), samples of internal organs were collected and examined using histological methods for possible presence of phanerozoites.ResultsAll control birds remained uninfected. Experimental starlings were resistant. All exposed crossbills were susceptible and survived until the end of this study. Prepatent period was 12–16 days post exposure. Light parasitaemia (< 0.7%) developed in all birds, and only few phanerozoites were seen in bone marrow cells of 5 of 8 experimentally infected crossbills. Significant changes were reported only in haematocrit value but not body mass in the exposed crossbills compared to controls.ConclusionPlasmodium elongatum (pGRW6) is of low virulence in Common crossbills and is unable to develop in Common starlings, indicating innate resistance of the later bird species. Low virulence in Common crossbills is likely due to the inability or low ability of this parasite lineage to develop phanerozoites resulting in light (if at all) damage of stem bone marrow cells. This study suggests that susceptibility of different bird species to the lineage pGRW6 is markedly variable. The global distribution of this parasite might be due to low virulence in wild adapted avian hosts, which survive this infection and serve as reservoirs host for non-adapted birds in whom this infection is often lethal.

Character displacement of a learned behaviour and its implications for ecological speciation.

Cultural evolution may accelerate population divergence and speciation, though most support for this hypothesis is restricted to scenarios of allopatric speciation driven by random cultural drift. By contrast, the role of cultural evolution in non-allopatric speciation (i.e. speciation with gene flow) has received much less attention. One clade in which cultural evolution may have figured prominently in speciation with gene flow includes the conifer-seed-eating finches in the red crossbill (Loxia curvirostra) complex. Here we focus on Cassia crossbills (Loxia sinesciuris; an ecotype recently split taxonomically from red crossbills) that learn social contact calls from their parents. Previous work found that individuals modify their calls throughout life such that they become increasingly divergent from a closely related, sympatric red crossbill ecotype. This open-ended modification of calls could lead to character displacement if it causes population-level divergence in call structure that, in turn, reduces (maladaptive) heterospecific flocking. Heterospecific flocking is maladaptive because crossbills use public information from flockmates to assess resource quality, and feeding rates are depressed when flockmates differ in their ability to exploit a shared resource (i.e. when flockmates are heterospecifics). We confirm the predictions of character displacement by documenting substantial population-level divergence in Cassia crossbill call structure over just two decades and by using field experiments to demonstrate that Cassia and red crossbills differentially respond to these evolved differences in call structure, reducing heterospecific flock formation. Moreover, because crossbills choose mates from within flocks, a reduction in heterospecific flocking should increase assortative mating and may have been critical for speciation of Cassia crossbills in the face of ongoing gene flow in as few as 5000 years. Our results provide evidence for a largely neglected yet potentially widespread mechanism by which reproductive isolation can evolve between sympatric lineages as a byproduct of adaptive cultural evolution.

Patterns of Plasmodium homocircumflexum virulence in experimentally infected passerine birds

BackgroundAvian malaria parasites (genus Plasmodium) are cosmopolitan and some species cause severe pathologies or even mortality in birds, yet their virulence remains fragmentally investigated. Understanding mechanisms and patterns of virulence during avian Plasmodium infections is crucial as these pathogens can severely affect bird populations in the wild and cause mortality in captive individuals. The goal of this study was to investigate the pathologies caused by the recently discovered malaria parasite Plasmodium homocircumflexum (lineage pCOLL4) in four species of European passeriform birds.MethodsOne cryopreserved P. homocircumflexum strain was multiplied and used for experimental infections. House sparrows (Passer domesticus), common chaffinches (Fringilla coelebs), common crossbills (Loxia curvirostra) and common starlings (Sturnus vulgaris) were exposed by subinoculation of infected blood. Experimental and control groups (8 individuals in each) were observed for over 1 month. Parasitaemia, haematocrit value and body mass were monitored. At the end of the experiment, samples of internal organs were collected and examined using histological and chromogenic in situ hybridization methods.ResultsAll exposed birds were susceptible, with similar average prepatent period and maximum parasitaemia, yet virulence was different in different bird species. Mortality due to malaria was reported in chaffinches, house sparrows and crossbills (7, 5 and 3 individuals died respectively), but not in starlings. Exoerythrocytic meronts (phanerozoites) were observed in the brain of all dead experimental birds. Blockage of blood vessels in the brain led to cerebral ischaemia, invariably causing brain damage, which is likely the main reason of mortality. Phanerozoites were observed in parenchymal organs, heart and muscles of all infected individuals, except starlings.ConclusionThis study shows that P. homocircumflexum is generalist and the same lineage caused similar parasitaemia-related pathologies in different host species. Additionally, the mode of exo-erythrocytic development is different in different birds, resulting in different mortality rates. This should be taken into consideration in studies addressing pathology during avian malaria infections.

Delimitation of call types of Red Crossbill (Loxia curvirostra) in the Western Palearctic

ABSTRACTVocal signals are important in many animal species for communication, coordination, and pair bonding and are especially well studied in birds. In the Red Crossbill (Loxia curvirostra) calls are an important trait for mate choice. In this species, calls having the same function (e.g., flight calls or excitement calls) are known to be clustered in distinct groups, so called ‘call types’. Each individual utters only calls of one call type. The driving force for the differentiation of Red Crossbill call types in the Palearctic remains unknown, as call types often overlap in space, and time and birds can be seen feeding on the same seeds. In this study, we investigated calls of crossbills, recorded within seven years in the Western Palearctic. We found at least 17 distinct call types of Red Crossbill and at least two call types of Parrot Crossbill (Loxia pytyopsittacus). There were obvious differences in call type delimitation between the northern and southern part of the study area. We argue this is in conflict with the ecological differentiation hypothesis and propose that there are other or further driving forces for this differentiation process.

How important are different non-native conifers in Britain to Common Crossbills Loxia curvirostra curvirostra?

ABSTRACTCapsulePines physically defend their seeds against seed-eating birds and mammals more than spruces or larches. Cone characteristics reflect the rate at which Common Crossbills Loxia curvirostra curvirostra extract seeds from different non-native conifers in Britain.AimsTo assess the profitability of different non-native conifers in Britain for Common Crossbills in winter.MethodsWe measured cone and seed parameters of conifers (Norway Spruce Picea abies, Sitka Spruce Picea sitchensis, Lodgepole Pine Pinus contorta and Japanese Larch Larix kaempferi) introduced into Britain and compared these with the native Scots Pine Pinus sylvestris. Feeding trials with captive Common Crossbills assessed intake rates.ResultsThe pines had thick and long scales, Japanese Larch had thin, short scales but thick seed coats and Sitka Spruce had thin, papery and short scales, and the thinnest seed coat. The two spruce species had more seeds per cone and the kernels had a higher energy content than the pines and larch. Feeding trials, simulating cones in winter, found that Common Crossbills failed to access seeds in closed Scots Pine cones. They also had difficulty in prising the scales of closed Lodgepole Pine cones but were able to forage on partially open cones. They took longer to extract seeds from large, open Lodgepole Pine cones than small ones, reflecting the effect of increasing scale thickness in larger pine cones. They also took longer to extract Lodgepole Pine seeds than Sitka Spruce and larch seeds. Although Common Crossbills could extract seeds quickly from open Sitka Spruce cones, the small seed size made the energy intake rate similar to Japanese Larch, if all seeds contained a kernel. However, after accounting for the proportion of seeds with a kernel, Sitka Spruce was the more profitable.ConclusionThe conifer food resource for crossbills in Britain has changed through the planting of non-native conifers. The physical properties of the cones and seeding phenology influence the rate at which Common Crossbills can extract seeds.