Morphology of the Hooded crow Corvus corone cornix in relation to age, sex and latitude

The morphological variation of the Hooded crow at Trondheim, Norway, was studied, based on a sample of 734 birds collected during a six year period. Mouth colour, plumage colour, skull thickness and feather length were found to be characters which could readily be used to separate juveniles from adults. Females aged 15–19 months had a thinner skull roof than older female birds. Low coefficients of variation were found for the lengths of the third primary and of the tail feathers. A discriminant analysis showed that of the various body dimensions studied bill height and bill length distinguished the sexes most precisely. A high degree of sexual dimorphism was also found to exist in body weight and in the thickness of the skull roof.
Those body structures which develop at about the same stage during the juvenile growth period were associated with the same principal component, viz. the lengths of bony structures, parts that develop early on in life, were intercorrelated (wing bones, tarsus, bill basis and the width of foramen magnum). The lengths of the primaries and of tail feathers were also intercorrelated, structures which develop late.
The mean body weight of the Hooded crow population studied in Norway was intermediate between that of the Hooded crow in Germany and of the Carrion and Hooded crows in England and Scotland. No such differences were found in wing length. Norwegian Hooded crows have shorter tails than German ones, but their bills are much larger, in particular for the females. Therefore, the degree of sexual dimorphism in bill size seems to be reduced at high latitudes.     

Genetic variation of carrion and hooded crows and their hybrids based on RAPD-PCR data

RAPD-PCR analysis of genetic variability of carrion crows, hooded crows and their phenotypic hybrids from the zones of overlap of their areals and hybridization of parental forms in Siberia has been carried out. According to RAPD variability the following parameters were estimated for both species and their hybrids: genetical distances (DN), polymorphism (P95), mean expected heterozygosity (H(e)), gene fixation coefficient (Fst) and the portion of interpopulation differentiation (Gst). It was shown that the rate of genetic variability in carrion crows and hooded crows is less than in the hybrids. It turned out that the hybrids were genetically closer to each other (DN = 0.295) than to both parental species (DN = 0.441 and 0.397 with C. corone and C. cornix, correspondingly). The studied individuals of C. cornix show the minimal level of heterozygosity and polymorphism (H(e) = 0.12 and P95 = = 27.8%) in comparison with C. corone and the crows of hybrid zone (H(e) = 0.18 and 0.20; P95 = 42.6% and 50.4%, correspondingly). Right's indexes of inbreeding (Fst = 0.285) and population spliting (Gst = 0.352) indicate low genetic variability in all the analysed birds. Nevertheless UPGMA and NJ dendrogrammes differentiate the individuals from the areals of carrion crows, hooded crows and from hybrid zone to different clusters. Our data correlate with molecular-genetic investigations of Corvidae carried out with other methods, and do not contradict the conception of carrion crows and hooded crows as semispecies in the context of the conception of overspecies.     

The Control of Color in Birds

SYNOPSIS. The colors of birds result from deposition of pigments—mainlymelanins and carotenoids—in integumentary structures,chiefly the feathers. The plumages of birds indicate their age,sex, and mode of living, and play important roles in camouflage,mating, and establishment of territories. Since feathers aredead structures, change of color of feathers is effected throughdivestment (molt) and replacement. The color and pattern ofa feather are determined by the interplay of genetic and hormonalinfluences prevailing in its base during regeneration. Mostbirds replace their feathers at least once annually. Some wearthe same kind of basic plumage all the time butothers alternatea basic and breeding plumage, either in one (the male) or bothsexes. Still others may have more than two molts, adding supplementalplumage at certain times in the plumage cycle. The varietiesof patterns of molt, the kinds of plumage, and the colors andpatterns of feathers among birds apparently are the result ofseveral kinds of selection pressures working through evolution.     

MOULT OF BUDGERIGARS MELOPSZTTACUS UNDULATUS

In captive Budgerigars Melopsitticus undulatus moult of primaries started in the middle of the tract and moved progressively inwards and outwards, the inner feathers being replaced faster than the outer ones. Full replacement of primaries took six to eight months and a new cycle of moult usually started before completion of the old cycle. Moult of secondaries followed no clear pattern and occurred less frequently than moult of primaries. Moult of rectrices started with the middle pair and moved progressively outwards on both sides. Complete moult of rectrices took about six months and a new cycle often started before completion of the old. Moult of the head and body occurred intermittently throughout the year. Birds fledged in juvenal plumage, they passed into first basic plumage with a partial moult (head and body feathers) and into definitive basic plumage with a moult of all contour feathers.
In the field in inland mid-eastern Australia, there were some birds replacing feathers and some with complete plumage in most months of the year. Birds with complete plumage may have been between moults or within a moult and between replacement of feathers. The proportion of birds in moult did not increase in intensity after breeding, or cease during breeding or before movements. Some birds of both sexes with gonads in a reproductive condition were replacing feathers. Rirds that were replacing feathers had similar lipid deposits to birds that had a complete plumage.     

Interpopulation variation in contour feather structure is environmentally determined in great tits

Background

The plumage of birds is important for flying, insulation and social communication. Contour feathers cover most of the avian body and among other functions they provide a critical insulation layer against heat loss. Feather structure and composition are known to vary among individuals, which in turn determines variation in the insulation properties of the feather. However, the extent and the proximate mechanisms underlying this variation remain unexplored.

Methodology/Principal Findings

We analyzed contour feather structure from two different great tit populations adapted to different winter regimes, one northern population in Oulu (Finland) and one southern population in Lund (Sweden). Great tits from the two populations differed significantly in feather structure. Birds from the northern population had a denser plumage but consisting of shorter feathers with a smaller proportion containing plumulaceous barbs, compared with conspecifics from the southern population. However, differences disappeared when birds originating from the two populations were raised and moulted in identical conditions in a common-garden experiment located in Oulu, under ad libitum nutritional conditions. All birds raised in the aviaries, including adult foster parents moulting in the same captive conditions, developed a similar feather structure. These feathers were different from that of wild birds in Oulu but similar to wild birds in Lund, the latter moulting in more benign conditions than those of Oulu.

Conclusions/Significance

Wild populations exposed to different conditions develop contour feather differences either due to plastic responses or constraints. Environmental conditions, such as nutrient availability during feather growth play a crucial role in determining such differences in plumage structure among populations.     

Distribution of unique red feather pigments in parrots

In many birds, red, orange and yellow feathers are coloured by carotenoid pigments, but parrots are an exception. For over a century, biochemists have known that parrots use an unusual set of pigments to produce their rainbow of plumage colours, but their biochemical identity has remained elusive until recently. Here, we use high-performance liquid chromatography to survey the pigments present in the red feathers of 44 species of parrots representing each of the three psittaciform families. We found that all species used the same suite of five polyenal lipochromes (or psittacofulvins) to colour their plumage red, indicating that this unique system of pigmentation is remarkably conserved evolutionarily in parrots. Species with redder feathers had higher concentrations of psittacofulvins in their plumage, but neither feather colouration nor historical relatedness predicted the ratios in which the different pigments appeared. These polyenes were absent from blood at the time when birds were replacing their colourful feathers, suggesting that parrots do not acquire red plumage pigments from the diet, but instead manufacture them endogenously at growing feathers.     

Is the denser contour feather structure in pale grey than in pheomelanic brown tawny owls Strix aluco an adaptation to cold environments?

In colour polymorphic species morphs are considered to be adaptations to different environments, where they have evolved and are maintained because of their differential sensitivity to the environment. In cold environments the plumage insulation capacity is essential for survival and it has been proposed that plumage colour is associated with feather structure and thereby the insulation capacity of the plumage. We studied the structure of contour feathers in the colour polymorphic tawny owl Strix aluco. A previous study of tawny owls in the same population has found strong selection against the brown morph in cold and snowy winters whereas this selection pressure is absent in mild winters. We predicted that grey morphs have a denser and more insulative plumage, enabling them to survive better in cold climate compared to brown ones. The insulative plumulaceous part of the dorsal contour feathers was larger and the fine structure of the plumulaceous part of the feather was denser in grey tawny owls than in brown ones. In the ventral contour feathers the plumulaceous part of the feather was denser in females than in males and in older birds without any differences between morphs. Our study suggests that insulative microscopical feather structures differ between colour morphs and we propose that feather structure may be a trait associated with morph‐specific survival in cold environments.     

Color expression in experimentally regrown feathers of an overwintering migratory bird: implications for signaling and seasonal interactions

Plumage coloration in birds plays a critical role in communication and can be under selection throughout the annual cycle as a sexual and social signal. However, for migratory birds, little is known about the acquisition and maintenance of colorful plumage during the nonbreeding period. Winter habitat could influence the quality of colorful plumage, ultimately carrying over to influence sexual selection and social interactions during the breeding period. In addition to the annual growth of colorful feathers, feather loss from agonistic interactions or predator avoidance could require birds to replace colorful feathers in winter or experience plumage degradation. We hypothesized that conditions on the wintering grounds of migratory birds influence the quality of colorful plumage. We predicted that the quality of American redstart (Setophaga ruticilla) tail feathers regrown after experimental removal in Jamaica, West Indies, would be positively associated with habitat quality, body condition, and testosterone. Both yearling (SY) and adult (ASY) males regrew feathers with lower red chroma, suggesting reduced carotenoid content. While we did not observe a change in hue in ASY males, SY males shifted from yellow to orange plumage resembling experimentally regrown ASY feathers. We did not observe any effects of habitat, testosterone, or mass change. Our results demonstrate that redstarts are limited in their ability to adequately replace colorful plumage, regardless of habitat, in winter. Thus, feather loss on the nonbreeding grounds can affect social signals, potentially negatively carrying over to the breeding period.     

Quantifying structural variation in contour feathers to address functional variation and life history trade‐offs

Body feathers are important to many interactions birds have with their physical and social environments, such as streamlining the body for flight, thermoregulation, and social signaling. Birds differ dramatically in the texture of their body plumage depending on species and age class, likely reflecting different functional demands and age‐related trade‐offs in feather production. Despite the important insights potentially offered by studying variation in the structure of body feathers, there is no clear system for quantifying this variation. We present methods for quantifying age and species differences in the structure of body feathers. Most variation in our measures is due to species and age‐class differences, with little variance attributable to individual birds or to differences among feathers sampled from the same bird. We use our measures to test the hypothesis that the loosely‐textured plumage characteristic of many juvenile passerines reflects a trade‐off between investment in feather quality and rapid body growth that promotes early fledging. The structure of juvenile feathers was negatively correlated with duration of the nestling period among ten species of New World warblers (Parulidae), suggesting a trade‐off between investment in feathers and investment in rapid somatic development promoting fledging. Systematic studies of variation in the structure of body feathers will likely offer numerous other insights into avian biology.     

Prebreeding moult, plumage and evidence for a presupplemental moult in the Great Knot Calidris tenuirostris

We studied the prebreeding moult and resulting plumage in a long-distance migrant sandpiper (Scolopacidae), the Great Knot Calidris tenuirostris , on the non-breeding grounds (northwest Australia), on arrival at the staging grounds after the first migratory flight (eastern China) and on or near the Russian breeding grounds (Russian data from museum specimens). We show that breeding plumage scores and breast blackness were affected not only by the increase in moulted feathers but also in the wearing down of overlaying pale tips of fresh feathers. Birds migrating from Australia and arriving in China had completed or suspended moult, but more moult must occur in Asia as Russian specimens had moulted more of their mantle and scapular feathers. Russian birds had significantly more red feathering on their upperparts than had birds in Australia or those arriving in China. The increase in reddish feathers cannot by accounted for simply by continuation of the prealternate moult. Instead, a third, presupplemental moult must occur, in which red-marked feathers replace some scapular and especially mantle feathers that were acquired in a prealternate moult only 1–3 months earlier. Great Knot sexes show little size and plumage dimorphism, whereas two other sandpipers that have supplemental plumages (Ruff Philomachus pugnax and Bar-tailed Godwit Limosa lapponica ) are thought to be highly sexually selected. Bidirectional sexual selection may therefore be involved in the evolution of a supplemental plumage in Great Knots.     

ALBINISM AND PHENOTYPE OF BARN SWALLOWS (HIRUNDO RUSTICA) FROM CHERNOBYL

Abstract The effects of mutation on phenotypic expression are supposed to be mainly deleterious because mutations disrupt the expression of genes that function relatively well under current environmental conditions. Thus, mutations are assumed to give rise to deviant phenotypes that are generally selected against. Radioactive contamination in the Chernobyl region of Ukraine is associated with a significant increase by a factor two to 10 in mutation rate in microsatellite markers of the barn swallow, Hirundo rustica. Barn swallows from Chernobyl had a temporally constant, elevated frequency of partial albinism compared to the situation before radioactive contamination and compared to birds from a control area. Albinism disproportionately affected the carotenoid‐based plumage of the head, suggesting that carotenoid metabolism is particularly susceptible to the effects of radiation. Individuals with partially albinistic plumage had, on average, lower mean phenotypic values than other birds, and this was particularly the case for males. Furthermore, differences in phenotypic variation, as determined using Levene's test, were significantly larger in partial albinos compared to nonalbinos in males, but not in females, even though the null expectation would be the opposite due to the lower mean phenotypic values of partial albinos. Although small phenotypes were commonly associated with germline mutations, there was no general decrease in overall body size during the period 1991–2000, implying that small individuals were selected against. Because partial albinism is disfavored by natural selection, the effects of mutations are deleterious, giving rise to a balance between mutation and selection.     

Partial albinism in a semi-isolated population of great reed warblers

Albinism in birds is thought to result from the expression of recessive alleles that disrupt melanin pigmentation at feather development. We have studied great reed warblers Acrocephalus arundinaceus in a recently founded and increasing population in Sweden during 15 years for the presence of birds with albinistic feathers. The study population was founded in 1978 and the few cases of albinism was exclusively recorded during the first five years of our study (1985-1989). This fits to the expected pattern if albinism is governed by recessive alleles; we have previously demonstrated that the population suffered from inbreeding during the first years of our study. The albinistic birds experienced a similar lifetime reproductive success as normally coloured birds.     

Sexual dichromatism in the yellow-breasted chat Icteria virens: spectrophotometric analysis and biochemical basis

Sexual dimorphism or dichromatism has long been considered the result of sexual selection. However, for many organisms the degree to which sexual dichromatism occurs has been determined within the confines of human perception. For birds, objective measures of plumage color have revealed previously unappreciated sexual dichromatism for several species. Here we present an unbiased assessment of plumage dichromatism in the yellow-breasted chat Icteria virens . Chats exhibit yellow to orange throat and breast plumage that to the unaided human observer differs only subtly in color. Spectrophotometric analyses revealed that chat throat and breast feathers exhibited reflective curves with two peaks, one in the ultraviolet and one in the yellow end of the spectrum. We found differences in both the shape and magnitude of reflectance curves between males and females. Moreover, for feathers collected from the lower edge and middle of the breast patch, male plumage reflected more light in the ultraviolet and yellow wavelengths compared to females, whereas male throat feathers appeared brighter than those of females only in the ultraviolet. Biochemical analyses indicated that the plumage pigmentation consisted solely of the carotenoid all- trans lutein and we found that males have higher concentrations of plumage carotenoids than females. Feathers that were naturally unpigmented reflected more UV light than yellow feathers, suggesting a potential role of feather microstructure in UV reflectance.     

Feather mite abundance increases with uropygial gland size and plumage yellowness in Great Tits Parus major

Plumicolous feather mites are ectosymbiotic organisms that live on bird feathers. Despite their abundance and prevalence among birds, the ecology of the interaction between these organisms and their hosts is poorly known. As feather mites feed on oil that birds spread from their uropygial gland, it has been hypothesized, but never tested, that the number of feather mites increases with the size of the uropygial gland of their hosts. In this study the number of feather mites is considered with respect to uropygial gland size in a breeding population of Great Tits Parus major in order to test this hypothesis. As predicted, the number of feather mites correlated positively with the uropygial gland size of their hosts, showing for the first time that uropygial gland size can explain the variance in feather mite load among conspecifics. Previous studies relating feather mite load to plumage colour have suggested that feather mites may be parasitic or neutral. To confirm this, the yellowness of breast feathers was also assessed. However, the results ran in the opposite direction to that expected, showing a positive correlation between mite load and plumage yellowness, which suggests that further work is needed to give clear evidence for a specific nature of feather mites. However, Great Tits with higher mite loads had lower hatching and breeding success, which may support the idea that feather mites are parasites, although this effect must be taken with caution because it was only found in males. Age or sex effects were not found on the number of feather mites, and it is proposed that hormonal levels may not be sufficient to explain the variation in feather mite loads. Interestingly, a positive correlation was detected between uropygial gland size and plumage brightness, which could be a novel factor to take into account in studies of plumage colour.     

Environmental Pollution Affects the Plumage Color of Great Tit Nestlings through Carotenoid Availability

Birds need to acquire carotenoids for their feather pigmentation from their diet, which means that their plumage color may change as a consequence of human impact on their environment. For example, the carotenoid-based plumage coloration of Great tit, Parus major, nestlings is associated with the degree of environmental pollution. Breast feathers of birds in territories exposed to heavy metals are less yellow than those in unpolluted environments. Here we tested two hypotheses that could explain the observed pattern: (I) deficiency of carotenoids in diet, and (II) pollution-related changes in transfer of carotenoids to feathers. We manipulated dietary carotenoid levels of nestlings and measured the responses in plumage color and tissue concentrations. Our carotenoid supplementation produced the same response in tissue carotenoid concentrations and plumage color in polluted and unpolluted environments. Variation in heavy metal levels did not explain the variation in tissue (yolk, plasma, and feathers) carotenoid concentrations and was not related to plumage coloration. Instead, the variation in plumage yellowness was associated with the availability of carotenoid-rich caterpillars in territories. Our results support the hypothesis that the primary reason for pollution-related variation in plumage color is carotenoid deficiency in the diet.     

Plumage colour in nestling blue tits: sexual dichromatism,condition dependence and genetic effects

Sexual-selection theory assumes that there are costs associated with ornamental plumage coloration. While pigment-based ornaments have repeatedly been shown to be condition dependent, this has been more difficult to demonstrate for structural colours. We present evidence for condition dependence of both types of plumage colour in nestling blue tits (Parus caeruleus). Using reflectance spectrometry, we show that blue tit nestlings are sexually dichromatic, with males having more chromatic (more 'saturated') and ultraviolet (UV)-shifted tail coloration and more chromatic yellow breast coloration. The sexual dimorphism in nestling tail coloration is qualitatively similar to that of chick-feeding adults from the same population. By contrast, the breast plumage of adult birds is not sexually dichromatic in terms of chroma. In nestlings, the chroma of both tail and breast feathers is positively associated with condition (body mass on day 14). The UV/blue hue of the tail feathers is influenced by paternally inherited genes, as indicated by a maternal half-sibling comparison. We conclude that the expression of both carotenoid-based and structural coloration seems to be condition dependent in blue tit nestlings, and that there are additional genetic effects on the hue of the UV/blue tail feathers. The signalling or other functions of sexual dichromatism in nestlings remain obscure. Our study shows that nestling blue tits are suitable model organisms for the study of ontogenetic costs and heritability of both carotenoid-based and structural colour in birds.     

Differential moult patterns in relation to antipredator behaviour during incubation in four tundra plovers

Golden plovers and Grey Plovers Pluvialis spp. all have very distinct breeding plumage rich in contrast, with a conspicuous black belly and breast bordered by a bright white fringe. Eurasian Golden Plovers are known partly to replace their breeding plumage with striped yellow feathers during incubation, different from both breeding and non-breeding plumages. In this study a similar partial breeding moult was observed in Pacific Golden Plovers and American Golden Plovers caught on the nest or collected during incubation, although the feathers did not differ clearly from those of non-breeders. This moult starts during incubation and precedes the post-breeding moult into non-breeding plumage. Because the lighter feathers reduce the contrast between the black belly and the white flanks, we suggest that during incubation the plumage characteristic that plays an important role in mate choice is no longer important; at this stage it is better for the bird to be inconspicuous. Additional information on museum skins of golden plovers and of Grey Plovers indicated that only the three golden plovers undergo this partial moult, but that Grey Plovers in general retain full breeding plumage throughout incubation. The three golden plovers also resemble each other in their generally very passive nest defence strategies. In contrast, the larger Grey Plovers actively chase and attack aerial and ground predators. Thus, a reduced conspicuousness of the body plumage during incubation is likely to benefit the golden plovers more than the Grey Plover. We suggest that nest defence behaviour, plumage characteristics and perhaps size have co-evolved as a response to different selection pressures in golden plovers and Grey Plover, but alternative hypotheses are also discussed.     

Attitudes toward and Acceptability of Management Strategies for a Population of Hooded Crows (Corvus cornix) in Slovenia

Recently, the number of hooded crows (Corvus cornix) in Slovenia has increased, which has resulted in a higher number of situations perceived as human–wildlife conflicts. The purpose of our survey was to provide a basis for population management measures of hooded crows that would be both feasible and acceptable to the public. An online survey based on snowball sampling received 1,042 responses, representing about 0.05% of the total and 0.13% of the active human population of the Republic of Slovenia. The major findings are that the majority of respondents were interested in learning more about problems caused by crows and think that crow numbers should be reduced, but by measures that would not cause their death. About one-third of respondents would be willing to participate in measures directed to controlling the crow population; however, about one-third would actively oppose such measures. They would even sign a petition regarding control measures. From the results, we conclude that the most suitable strategies in terms of acceptability would be passive, by educating people about how to avoid conflict with crows, and by better management of the resources affecting their foraging success.     

The association between plumage damage and feather-eating in free-range laying hens

Severe feather-pecking (SFP) persists as a highly prevalent and detrimental behavioural problem in laying hens (Gallus gallus domesticus) worldwide. The present experiment investigated the association between feather-eating and plumage damage, a consequence of SFP, in groups of free-range, ISA Brown laying hens. Single feathers were placed on the floor of the home pens. Feathers were sourced from seven different birds. A total of 50 birds in six pens with extensive plumage damage were compared with birds in six control pens with little plumage damage at 41 to 43 weeks of age (n=12 pens, 600 hens). Birds in pens with extensive plumage damage ingested more feathers (F=8.1, DF=1, 8, P=0.02), and also showed shorter latencies to peck at (χ²=54.5, DF=1, P<0.001), and ingest feathers (χ²=55.6, DF=1, P<0.001). Birds ingested feathers from a bird in the free-range facility, in which the testing took place, more quickly than from a bird housed in a separate cage facility (χ² = 39.0, DF=6, P<0.001). A second experiment investigated the predictive relationship between feather-eating and plumage damage. Feathers were presented to 16 pens of 50 pullets prior to the development of plumage damage, at 15 weeks of age, and then to the same hens after plumage damage had become prominent, at 40 weeks of age. Birds had a higher probability of ingesting feathers (F=142.0, DF=1, 231, P<0.001), pecked feathers more times (F=11.24, DF=1, 239, P<0.001), and also pecked (χ² = 127.3, DF=1, P<0.001) and ingested (χ²=189.3, DF=1, P<0.001) the feathers more quickly at 40 than 15 weeks of age. There was a trend for an interaction, where birds pecked feathers from the rump more times than feathers from the back at 40 weeks of age (F=3.46, DF=1, 237, P=0.06). However, a lack of variability in plumage damage between pens in this experiment precluded investigation of the predictive relationship. The results from the present study confirm the association between feather-eating and plumage damage, and suggest that birds may prefer feathers from particular body areas and from particular hens. Future experiments should focus on elucidating whether feather-eating may act as a predictor of SFP.     

An analysis of population genetic differentiation and genotype–phenotype association across the hybrid zone of carrion and hooded crows using microsatellites and MC1R

The all black carrion crow ( Corvus corone corone ) and the grey and black hooded crow ( Corvus   corone cornix ) meet in a narrow hybrid zone across Europe. To evaluate the degree of genetic differentiation over the hybrid zone, we genotyped crows from the centre and edges of the zone, and from allopatric populations in northern (Scotland–Denmark–Sweden) and southern Europe (western–central northern Italy), at 18 microsatellites and at a plumage candidate gene, the MC1R gene. Allopatric and edge populations were significantly differentiated on microsatellites, and populations were isolated by distance over the hybrid zone in Italy. Single-locus analyses showed that one locus, CmeH9, differentiated populations on different sides of the zone at the same time as showing only weak separation of populations on the same side of the zone. Within the hybrid zone there was no differentiation of phenotypes at CmeH9 or at the set of microsatellites, no excess of heterozygotes among hybrids and low levels of linkage disequilibrium between markers. We did not detect any association between phenotypes and nucleotide variation at MC1R , and the two most common haplotypes occurred in very similar frequencies in carrion and hooded crows. That we found a similar degree of genetic differentiation between allopatric and edge populations irrespectively of their location in relation to the hybrid zone, no differentiation between phenotypes within the hybrid zone, and neither heterozygote excess nor consistent linkage disequilibrium in the hybrid zone, is striking considering that carrion and hooded crows are phenotypically distinct and sometimes recognised as separate species.