Ittero alirugginose

Identificazione

The common name for the red-winged blackbird is taken from the mainly black adult male's distinctive red shoulder patches, or epaulets, which are visible when the bird is flying or displaying. At rest, the male also shows a pale yellow wingbar. The spots of males less than one year old, generally subordinate, are smaller and more orange than those of adults. The female is blackish-brown and paler below. The female is smaller than the male, at 17–18 cm (6.7–7.1 in) long and weighing 41.5 g (1.46 oz), against his length of 22–24 cm (8.7–9.4 in) and weight of 64 g (2.3 oz). The smallest females may weigh as little as 29 g (1.0 oz) whereas the largest males can weigh up to 82 g (2.9 oz). Each wing can range from 8.1–14.4 cm (3.2–5.7 in), the tail measures 6.1–10.9 cm (2.4–4.3 in), the culmen measures 1.3–3.2 cm (0.51–1.26 in) and the tarsus measures 2.1 cm (0.83 in). The upper parts of the female are brown, while the lower parts are covered by an intense white and dark veining; also presents a whitish superciliary list. According to Crawford (1977), females exhibit a salmon-pink stain on the shoulders and a light pink color on the face and below this when they are a year old, while older females show a stain usually more crimson on the shoulders and a darker pink hue on and under the face. Observations in females in captivity indicate that small amounts of yellow pigment are present on the shoulders of these after leaving the nest, that the concentration of the pigment increases with the first winter plumage after the change of the feathers and that the passage from yellow to orange generally takes place in the second summer with the acquisition of the second winter plumage, after which no further changes in feather color occur. The colored area on the wing increases in surface with the age of the female, and varies in intensity from brown to a bright red-orange similar to that of the males in their first year.<p>Young birds resemble the female, but are paler below and have buff feather fringes. Both sexes have a sharply pointed bill. The tail is of medium length and is rounded. The eyes, bill, and feet are all black. Unlike most North American passerines which develop their adult plumage in their first year of life so that the one-year-old and the oldest individual are indistinguishable in the breeding season, the red-winged blackbird does not. It acquires its adult plumage only after the breeding season of the year following its birth when it is between thirteen and fifteen months of age. Young males go through a transition stage in which the wing spots have an orange coloration before acquiring the most intense tone typical of adults.<p>The male measures between 22 and 24 cm (8.7 and 9.4 in) in length, while the female measures 17 or 18 cm (6.7 or 7.1 in). Its wingspan is between 31 and 40 cm (12 and 16 in) approximately. Both the peak male and the legs, the claws and the eyes are black; in the female beak is dark brown and clear in the upper half at the bottom, and the tail is medium in length and rounded. As in other species polygyny exists, the red-winged blackbird exhibit considerable sexual dimorphism both in plumage and size, males weighing between 65 and 80g the females about 35g. Males are 50% heavier than females, 20% larger in its linear dimensions, and 20% larger compared to the length of their wings. The trend towards greater dimorphism in the size of non-monogamous ichterid species indicates that the larger size of males has evolved due to sexual selection.<p>The male is unmistakable except in the far west of the US, where the tricolored blackbird occurs. Males of that species have a darker red epaulet edged with white, not yellow. Females of tricolored, bicolored, red-shouldered and red-winged blackbirds can be difficult to identify in areas where more than one form occurs. In flight, when the field marks are not easily seen, the red-winged blackbird can be distinguished from less closely related icterids such as common grackle and brown-headed cowbird by its different silhouette and undulating flight.<p>Two keto-carotenoids – carotenoid with a ketone group – reds synthesized by the birds themselves – namely astaxanthin and canthaxanthin – are responsible for the bright red color of the wing spots, but two yellow dietary precursor pigments – lutein and zeaxanthin – are also present in moderately high concentrations in red feathers. Astaxanthin is the carotenoid more abundant (35% of the total), followed by lutein (28%), canthaxanthin (23%) and zeaxanthin (12%). Such a balanced combination of dietary precursors and metabolic derivatives in colored feathers is quite unusual, not only within a species as a whole but also in individuals and particular feathers.<p>After removing the carotenoids in an experiment, the red feathers acquired a deep brown coloration. This is because the feather barbles of the colored spot contain melanin pigments – mainly eumelanin, which was equivalent to 83% of all melanins, but also pheomelanin at a concentration approximately equal to that of carotenoids, which seems to be a rare trait for carotenoid-based ornamental plumage. On the other hand, the feathers of the yellow stripes of the males are devoid of carotenoids —except occasionally when they appear tinged with a pink coloration derived from small amounts of said pigments— and present high concentrations of pheomelanin —82% of all melanins. Melanin concentrations in the yellow band are even higher than in the red spot.<p>These markings are vital in the defense of the territory. Males with larger spots are more effective at chasing away their non-territorial rivals and are more successful in contests within aviaries. When the red shoulder patches were dyed black as part of an experiment, 64% of males lost their territories, while only 8% of control subjects did. However, males whose wings had been dyed before they had mated could still attract females and successfully reproduce. In the red-winged blackbird, the spots on the wings are a sign of threat among males and have an unimportant role, if any, in intersex encounters. Therefore, the spots are likely to have evolved in response to pressures linked to intrasexual selection. Additionally, neither the size nor the coloration of the same are linked to the reproductive success of the males with those females that are not their mates, that is, those with which they eventually mate. It has been suggested that also in the case of females, the best explanation for the evolution of a variable coloration in the shoulder spots is that their intensity indicates their physical condition in aggressive encounters between them.<p>The fact that female red-winged blackbirds do not appear to consistently use variability in the size and color of male wing spots when choosing a mate runs counter to the classic role of carotenoid pigmentation ornamental feathers, mostly used in the attraction of a couple. In turn, its use as a sign of aggressiveness and social status against rival males is not a common trait in carotenoid ornaments. On the other hand, ornaments with a preponderance of melanin do tend to have an important role as an indicator of status in avian populations, so the spots of red-winged blackbirds seem to work more as melanin ornaments than as carotenoids.<p>The calls of the red-winged blackbird are a throaty check and a high slurred whistle, terrr-eeee. The male's song, accompanied by a display of his red shoulder patches, is a scratchy oak-a-lee, except that in many western birds, including bicolored blackbirds, it is ooPREEEEEom. The female also sings, typically a scolding chatter chit chit chit chit chit chit cheer teer teer teerr.

Tassonomia

The red-winged blackbird is one of five species in the genus Agelaius and is included in the family Icteridae, which is made up of passerine birds found in North and South America. The red-winged blackbird was formally described as Oriolus phoeniceus by Carl Linnaeus in 1766 in the twelfth edition of his Systema Naturae, but was later moved with the other American blackbirds to the genus Agelaius by Louis Pierre Vieillot in 1816. Linnaeus specified the type location as "America" but this was restricted to Charleston, South Carolina in 1928. The genus name is derived from Ancient Greek agelaios, meaning "gregarious". The specific epithet, phoeniceus, is Latin meaning "crimson" or "red". The red-winged blackbird is a sister species to the red-shouldered blackbird (Agelaius assimilis) that is endemic to Cuba. These two species are together sister to the tricolored blackbird (Agelaius tricolor) that is found on the Pacific coast region of the California and upper Baja California in Mexico.<p>Depending on the authority, between 20 and 24 subspecies are recognised which are mostly quite similar in appearance. However, there are two isolated populations of bicolored blackbirds that are quite distinctive: A. p. californicus of California and A. p. gubernator of central Mexico. The taxonomy and relationships between these two populations and with red-winged blackbirds is still unclear. Despite the similarities in most forms of the red-winged blackbird, in the subspecies of Mexican Plateau, A. p. gubernator, the female's veining is greatly reduced and restricted to the throat; the rest of the plumage is very dark brown,[self-published source?] and also in a different family from the European redwing and the Old World common blackbird, which are thrushes (Turdidae). In the California subspecies, A. p. californicus and A. p. mailliardorum, the veining of the female specimens also covers a smaller surface and the plumage is dark brown, although not in the gubernator grade; and also its superciliary list is absent or poorly developed. The male subspecies mailliardorum, californicus, aciculatus, neutralis, and gubernator lack the yellow band on the wing that is present in most male members of the other subspecies. The red-shouldered blackbird was formerly considered as a subspecies of red-winged blackbird. They were split by the American Ornithologists' Union in 1997.<p>Listed below are the red-winged blackbird subspecies and groups of subspecies recognized as of January 2014 with their respective distribution areas and the location of their wintering quarters:

Nome scientifico

Agelaius phoeniceus

Autorità

(Linnaeus, 1766)

Ordine

Passeriformi

Famiglia

Icteridae

Genere

Agelaius

Areale e habitat

The red-winged blackbird is widely spread throughout North America, except in the arid desert, high mountain ranges, and arctic or dense afforestation regions. It breeds from central-eastern Alaska and Yukon in the northwest, and Newfoundland in the northeast, to northern Costa Rica in the south, and from the Atlantic to the Pacific. Northern populations migrate to the southern United States, but those that breed there, in Mexico, and in Central America are sedentary. Red-winged blackbirds in the northern reaches of the range are migratory, spending winters in the southern United States and Central America. Migration begins in September or October, but occasionally as early as August. In western and Central America, populations are generally non-migratory.<p>The red-winged blackbird inhabits open grassy areas. It generally prefers wetlands, and inhabits both freshwater and saltwater marshes, particularly if cattail is present. It is also found in dry upland areas, where it inhabits meadows, prairies, and old fields. In a large part of its distribution area, it constitutes the most abundant passerine bird in the swamps in which it nests. It is also present in areas without much water, where it inhabits open fields – often agricultural areas – and sparse deciduous forests.<p>In the winter of 1975–1976, near Milan in western Tennessee, red-winged blackbirds were observed resting in a mixed roost that came to house 11 million individuals in January and early February in a plantation of 4.5 hectares of yellow pine (Pinus taeda) with little undergrowth was seen in soybean fields during the day, being that these constituted only 21% of the habitat in the area and that the other bird species present in the roost were not commonly observed in these fields; they were also common in cornfields. the presences of the bird in feedlots has increased as winter progressed, but, accounting for less than 5% of the icterids and starlings recorded in both feedlots of cows and pigs, they were much rarer there than the brown-headed cowbird, common grackle and common starling.<p>During the breeding season, the density of breeding adults is much higher in swamps than in highland fields. Although the highest concentrations of nesting red-winged blackbird are found in swamps, most of them nest in upland habitats since they are much more abundant. In a study conducted in the Wood County, Ohio between 1964 and 1968, the density of territorial males in wetland habitat was found to be 2.89 times those in upland habitat. But, because of the small amount of wetland habitat, the total estimated upland population of territorial maleswas 2.14 times the wetland population. Alfalfa (Medicago sativa) and other legume crops (hay) were the principal habitat for breeding redwings in the county. Despite the marked preference for wetland habitats, the greater population in uplands reflects wetlands' scarcity.<p>In that same study, reproductive individuals in upland habitats demonstrated a slight preference for old and new grasses such as Phleum pratense, Dactylis glomerata, Poa spp., Festuca spp., and Bromus spp. In the early part of the breeding season and new non-graminoid herbaceous plants in the mid and late season. In wetlands, they consistently preferred old and new broadleaf monocotyledons, primarily Carex spp, broadleaf, and Typha spp, and consistently rejected old and new narrow-leaved monocots, primarily narrow-leaved Phalaris arundinacea and Calamagrostis canadensis, and non-graminoid herbaceous plants. Red-winged blackbirds breeding in the highlands favored older tall vegetation only in the early part of the breeding season on April to May, and higher new vegetation and dense vegetation throughout the breeding season. For their part, those who settled in wetlands seemed to have a slight predilection for taller old vegetation. During the breeding season, this species is attracted to tall vegetation that restricts visibility.<p>Early season preferences for old grassland in the highlands and old broadleaf monocotyledons in wetlands point to the importance of upright residual vegetation. Highland grasses and broadleaf monocots in wetlands stand partially vertically and are easily visible in early spring, unlike clovers, narrow-leaved monocotyledons in wetlands and most non-graminoid herbaceous plants. Old alfalfa plants are also partially upright in early spring, but they are not as consistently chosen a species as old grasses. The start of the territorial activity is earlier when the amount of residual vegetation is large. Vegetation structural strength also appears to be important for nesting as females tend to broadleaf monocotyledons in wetlands throughout the breeding season and new non-graminoid herbaceous plants in the mid to late season.<p>In southwestern Michigan, the density of cattail stems in different ponds was positively related to the concentration of reproductive adults. However, other studies detected a predilection for more scattered vegetation. One found that red-winged blackbirds avoided the marsh wren (Pantaneros chivirenes), a predator common in nests of this species, reproducing among more dispersed vegetation, which was more easily defended from the chivirines; conversely, the chivirines seemed to prefer the denser vegetation, where they were more likely to avoid the aggressiveness of the red-winged blackbird; These differences in habitat selection between one species and another resulted in spatial segregation of their breeding areas. On the other hand, red-winged blackbirds tend to opt for small lots of vegetation and plants with thick stems.

Migrazione

Resident species

Comportamento

The red-winged blackbird is territorial, polygynous, gregarious and a short-distance migratory bird. Its way of flying is characteristic, with rapid wing flaps punctuated by brief periods of gliding flight. The behavior of males makes their presence easily perceived: they perch in high places such as trees, bushes, fences, telephone lines, etc. Females tend to stay low, prowling through the vegetation and building their nests. They can be found in home gardens, particularly during their migration, if seeds have been scattered on the ground. The forest curtains serve as a resting place during the day.<p>For several weeks after their first appearance in early spring, red-winged blackbirds are generally seen in flocks made up entirely of males. During those days, they are seldom seen at their breeding sites, except early morning and late afternoon. In most of the remaining hours of the day, they frequent open and often elevated agricultural land, where they feed mainly on grain stubble and grassy fields. When disturbed while eating, they fly to the nearest deciduous trees and immediately after landing they begin to sing.<p>The red-winged blackbird is omnivorous. It feeds primarily on plant materials, including seeds from weeds and waste grain such as corn and rice, but about a quarter of its diet consists of insects and other small animals, and considerably more so during breeding season. It prefers insects, such as dragonflies, damselflies, butterflies, moths, and flies, but also consumes snails, frogs, eggs, carrion, worms, spiders and mollusks. The red-winged blackbird forages for insects by picking them from plants, or by catching them in flight. Sometimes insects are obtained by exploring the bases of aquatic plants with their small beaks, opening holes to reach the insects hidden inside. Aquatic insects, particularly emerging odonates, are of great importance in the diet of red-winged blackbirds that breed in swamps. These birds typically capture the odonates when the larvae climb up the stem of a plant from the water, get rid of their exuviae, and cling to the vegetation while their exoskeletons harden. The years of emergence of periodical cicadas provide an overabundant amount of food. According to Edward Howe Forbush, when red-winged blackbirds arrive north in the spring, they feed in the fields and meadows. Then, they follow the plows, collecting larvae, earthworms and caterpillars left exposed, and in case there is a plague of Paleacrita vernata caterpillars in a fruit orchard, these birds will fly a kilometer to get them for their chicks.<p>In season, red-winged blackbirds eat blueberries, blackberries, and other fruits. These birds can be lured to backyard bird feeders by bread and seed mixtures and suet. In late summer and in autumn, the red-winged blackbird will feed in open fields, mixed with grackles, cowbirds, and starlings in flocks which can number in the thousands. It feeds on corn while it is maturing; once the grain has hardened it is relatively safe from this bird, since its beak and digestive system are not adapted for the consumption of hard and whole corn grains, unlike the Common grackle, which has a longer and stronger beak. Studies of the stomachs of individuals of both sexes reveal that males consume higher proportions of crop grains, while females ingest a relatively larger amount of herb seeds and animal matter. In the winter of 1975–1976, near Milan, western Tennessee, corn and herb seeds were the main foods consumed by red-winged blackbird. Herbs whose seeds were commonly consumed were Sorghum halepense, Xanthium strumarium, Digitaria ischaemum, Sporobolus spp., Polygonum spp. and Amaranthus spp.<p>The red-winged blackbird nests in loose colonies. The nest is built in cattails, rushes, grasses, sedge, or alder or willow bushes. The nest is constructed entirely by the female over the course of three to six days. It is a basket of grasses, sedge, and mosses, lined with mud, and bound to surrounding grasses or branches. It is located 7.6 cm (3.0 in) to 4.3 m (14 ft) above water.<p>A clutch consists of three or four, rarely five, eggs. Eggs are oval, smooth and slightly glossy, and measure 24.8 mm × 17.55 mm (0.976 in × 0.691 in). They are pale bluish green, marked with brown, purple, and/or black, with most markings around the larger end of the egg. These are incubated by the female alone, and hatch in 11 to 12 days. Red-winged blackbirds are hatched blind and naked, but are ready to leave the nest 11 to 14 days after hatching.<p>Red-winged blackbirds are polygynous, with territorial males defending up to 10 females. However, females frequently copulate with males other than their social mate and often lay clutches of mixed paternity. Pairs raise two or three clutches per season, in a new nest for each clutch.<p>The reproductive season of the red-winged blackbird extends approximately from the end of April to the end of July. On the other hand, in different states has been estimated that the period in which the active nests contained eggs lay between beginning in late April and early late August; and in northern Louisiana nests were found to harbor chicks from late April to late July. The peak of the nesting season (the time with the highest number of active nests) has been recorded between the first half of May and the beginning of June in different places. A study in eastern Ontario found that although red-winged blackbirds began nesting earlier in years with warm springs, associated with low winter values in the North Atlantic Oscillation Index, egg laying dates remained unchanged. Male testosterone levels peak in the early part of the breeding season, but remain high throughout the season. Females reproduce for up to ten years.<p>Many aspects of territorialism (for example, the number of male songs and displays, and the number of intrusions into foreign territories) peak before mating. After this, the frequency of many of the territorial behaviors decreases and the males are mainly concerned with defending the females, eggs, and chicks against predation. Experiments in the systematic removal of birds from their territories suggest that the extra population of males that is present in swamps before copulations disappears after copulation.<p>Predation of eggs and nestlings is quite common. Nest predators include snakes, mink, raccoons, and other birds, even as small as marsh wrens. The red-winged blackbird is occasionally a victim of brood parasites, particularly brown-headed cowbirds. Since nest predation is common, several adaptations have evolved in this species. Group nesting is one such trait which reduces the risk of individual predation by increasing the number of alert parents. Nesting over water reduces the likelihood of predation, as do alarm calls. Nests, in particular, offer a strategic advantage over predators in that they are often well concealed in thick, waterside reeds and positioned at a height of one to two meters. Males often act as sentinels, employing a variety of calls to denote the kind and severity of danger. Mobbing, especially by males, is also used to scare off unwanted predators, although mobbing often targets large animals and man-made devices by mistake. The brownish coloration of the female may also serve as an anti-predator trait in that it may provide camouflage for her and her nest while she is incubating.<p>Predators of red-winged blackbirds include such species as raccoons, American mink, long-tailed weasels, black-billed magpies, common grackles, owls, red-tailed hawks, short-tailed hawks, and snakes (such as the northern water snake and the plains garter snake). Ravens and grazers such as marsh wrens feed on eggs (and even small chicks), if the nest is left unattended, destroying the eggs, occasionally drinking from them, and pecking the nestlings to death.<p>The relative importance of different nest predators varies by geographic region: the top predators in different regions include the marsh wren in British Columbia, the magpies in Washington, and the raccoons in Ontario. The incidence of avian predation in red-winged blackbird nests is higher in western populations than in eastern populations.<p>Due to high predation rates, especially of eggs and chicks, the red-winged blackbird has developed various adaptations to protect its nests. One of them consists of nesting in groups, which reduces the danger since there is a greater number of alert parents. Nesting over water also lowers the chances of an attack. Nests in particular offer a strategic advantage as they are often hidden among dense riparian reeds, at a height of one or two meters. males often act as sentinels, using a repertoire of calls. Males in particular hunt down potential predators to scare them away, even when dealing with much larger animals. Aggressiveness of the red-winged blackbird towards the marsh wren, which also nests in swamps, causes a partial interspecies territorialism. On the other hand, nocturnal predators such as raccoons and American mink are not attacked by adults. Coloration of the female could serve to camouflage it, protecting it and its nest when it is incubated.<p>The red-winged blackbird can accommodate ectoparasites such as various Phthiraptera, the Ischnocera Philopterus agelaii and Brueelia ornatissima, and hematophagous mites, like Ornithonyssus sylviarum, and endoparasites like Haemoproteus quiscalus, Leucocytozoon icteris, Plasmodium vaughani, nematodes, flukes and tapeworms.<p>The red-winged blackbird aggressively defends its territory from other animals, and will attack much larger birds. During breeding season, males will swoop at humans who encroach upon their nesting territory. Male red-winged blackbirds also exhibit important territorial behaviors, most of which provides them with the necessary fidelity for many years to come. A few important factors for male red-winged blackbirds’ adherence to territories include food, hiding spaces from predators, types of neighbors, and reactions towards predators. Additionally, a study was done on site fidelity and movement patterns by Les D. Beletsky and Gordon H. Orians in 1987 which explained much of the males’ territorial behaviors once migrated and settled onto a territory of their own. Sufficient evidence had shown that males are committed to staying in their territory over a long period of time and are not more likely to change territories at a younger age due to limited experience of knowledge for success. Studies also showed that most of the males that were first-time movers to a new territory were between two and three years old. The majority of males that moved were young and inexperienced. Later on they had moved towards more available territories. If males had chosen to leave their territory for reproductive success, as an example, they would do so within a short distance. Males who moved shorter distances were more successful in reproducing than those who moved longer distances. Further studies showed that when males moved further away from their territories there was a decrease in probability of successfully fledging.<p>The maximum longevity of the red-winged blackbird in the wild is 15.8 years.<p>Red-winged blackbirds that breed in the northern part of their range, i.e., Canada and border states in the United States, migrate south for the winter. However, populations near the Pacific and Gulf coasts of North America and those of Middle America are year-round resident. Red-winged blackbirds live in both Northern U.S. and Canada, ranging from Yucatan Peninsula in the south to the southern part of Alaska. These extensions account for the majority of the continent stretching from California's Pacific coast and Canada to the eastern seaboard. Much of the populations within Middle America are non-migratory. During the fall, populations begin migrating towards Southern U.S. Movement of red-winged blackbirds can begin as early as August through October. Spring migration begins anywhere between mid-February to mid-May. Numerous birds from northern parts of the U.S., particularly the Great lakes, migrate nearly 1,200 km (750 mi) between their breeding season and winter Winter territorial areas differ based on geographic location. Other populations that migrate year-round include those located in Middle America or in the western U.S. and Gulf Coast. Females typically migrate longer distances than males. These female populations located near the Great Lakes migrate nearly 230 km (140 mi) farther. Yearly-traveled females also migrate further than adult males, while also moving roughly the same distance as other adult females. Red-winged blackbirds migrate primarily during daytime. In general, males’ migration flocks arrive prior to females in the spring and after females in the fall.

Conservazione

It is a species of least-concern. Being one of the largest and most widely distributed birds in North America, little has been done to protect it from the effects of habitat destruction and urbanization. It can survive in a wide range of environments, many populations manage to overcome the loss of natural habitats. However, red-winged blackbirds thrive in wetland areas and with the destruction of natural wetlands their population is likely to shrink. The species is protected under the Migratory Bird Treaty Act 1918, a formal treaty between the United States and Canada that was later expanded to include Mexico. This law gives them legal protection in the United States, but they can be killed "when they are found preying or about to prey on ornamental trees or trees planted for shade, crops, livestock or wildlife."<p>A study in Illinois indicated that red-winged blackbird populations doubled between 1908 and 1958. It had traditionally reproduced in wetlands, with Ohio primarily inhabiting swamps associated with lakes and rivers. During the 20th century, however, it adapted to man-made habitat changes and now often nests in hayfields, along roads and ditches, and elsewhere in the highlands.<p>Despite its successful adaptation to changes in practices related to land use, populations of red-winged blackbird have reduced the width of its range during the second half of the twentieth century, and Changes in the abundance and adequacy of grasslands have been implicated in this. In Ohio, the red-winged blackbird was negatively affected between 1966 and 1996 by the decrease in hay production, the earlier harvest of hay and the increase in crops planted in furrows, a situation homologous to that suffered by other species of birds from less numerous grasslands. Although the negative effect of the increased efficiency and the diminishing diversity of modern agricultural practices on the populations of red-winged blackbird in this state may be perceived as a positive event by the producers of corn and sunflower, the agricultural practices that have precipitated the numerical decline of this species may have caused more severe repercussions for birds from less common grasslands, such as the upland sandpiper and the Grasshopper sparrow.<p>In the late 1970s, Ohio hosted the highest density of blackbirds during the breeding season among all US states and Canadian provinces, However, between 1966 and 1996, the reproductive populations in this state showed a marked decrease. Decreasing the area under hay other than alfalfa between those same years was likely to reduce the availability of quality nesting habitats. Likewise, corn even though it attracts insects that red-winged blackbirds consume and despite the fact that their grains represent in themselves an energy source and soybeans which do not constitute a food source for the species, which together made up 1966 to 1996, an average of 70% of the cultivated area in Ohio, do not provide adequate nesting habitat. In turn, the large annual fluctuations in the area cultivated with hay, both from one year to the next and within the same year of a certain reproductive season, Similarly, in Ontario between 1974 and 1995, the average size of harems in a given year was positively related to hay production from the previous year, although the percentage variation in the size of harems from one season to the next the following was not linearly related to the annual percentage change in hay production; Hay production declined during this period and the size of the harems decreased from approximately three females per male to 1.6 females.<p>In addition in Ontario, a negative relationship was found between the North Atlantic Oscillation Index (NAO) in the six months prior to a certain reproductive season and the size of the harems. If winter mortality contributes to the decrease in the size of the harems, the annual variations in their size should be related to the annual changes in the rates of return of marked territorial males. Variations in the size of harems and male return rates per year were indeed found to be positively correlated. Although not as pronounced, the relationship between male return rates and winter NAO values was similar to that between changes in harem size and NAO.<p>On the other hand, in southwestern Quebec, red-winged blackbird populations doubled between 1966 and 1981, apparently in response to the development of maize production. The increased availability of residual grains in spring and summer. the reproductive season will probably play a fundamental role in the growth of this bird's population. Likewise, in the mid-1990s, the reproductive population of North Dakota which at least between 1994 and 2002 was at the forefront among all the American states and Canadian provinces in terms of population density of this species began to increase rapidly.

Tendenza della popolazione

declining