Adult blackbirds show a clear sexual dimorphism: The plumage of males is solid black, and the bill is conspicuously pale yellow to orange. In addition, males show a distinct ring around the eyes, the color of which resembles that of the beak, but can go somewhat brownish. This eye ring contrasts strongly with the dark brown iris. This eye ring is less distinct in the female, and the bill is also less conspicuous and light horn-colored instead of yellow. The plumage colouring of the female is much more variable and predominantly dark brown, sometimes going into grey or reddish brown. In both sexes the barrel and toes are dark brown. Compared to the smaller starling, which also has dark plumage and often stays on the ground, the blackbird has a much longer tail.
Plumage of adult blackbirds
The plumage of adult males is quite uniformly black. Especially on the underside, but also on the back and shoulder region, the feathers not infrequently show a grey to bronze-coloured terminal fringe, which is, however, quite inconspicuous and only makes a slightly scaly impression in favourable light conditions. The wing feathers can appear bleached out in spring.
The upper side of the females is dark olive-brown to olive-gray colored, the forehead often somewhat less dark. The coloration of the lighter underside is individually very different. The chin and throat are light dirty grey to reddish brown with dark stripes. The chest is brown-grey, yellow-brown to reddish-brown with more or less distinct speckles. The belly is brown, grey-brown or grey, sometimes with a distinctly scaled appearance due to a pale terminal fringe of feathers. The tail feathers are dark to blackish brown, hand and arm feathers dark brown with olive tinted outer plumes.
Young birds and moult
Fledged juveniles resemble females, but are more strongly spotted on the underside and can be recognized above all by the conspicuous light shaft lines on the back, shoulder feathers and wing coverts. The bill is brown. The feathering of wings and tail appears distinct from the rest of the plumage and is dark brown to brown-black in young males, somewhat lighter in females and tending to brown.
During the juvenile moult, which takes place between summer and autumn of the first calendar year, the birds change the small plumage and part of the wing feathers. Thereby, mainly the older young birds change into a so-called progress dress, the later hatched ones into an inhibition dress, in which they continue to resemble rather young birds. The one-year-old males in the inhibition dress are also called mallards. In all one-year-old males, i.e. also the males in progress dress, the wing feathers and the un-moused wing covers are brown in contrast to the brown-black or black colouring of the already moused feather parts. Also in the one-year-old females the already moulted and the unmoulted plumage parts contrast; the latter are clearly lighter.
As a rule, the regular annual moult, which is a full moult, begins two weeks after the young have finished rearing. The moult of a blackbird population extends over a period of five months, excluding extreme cases. In Europe, the moulting period is typically between June and the end of October, with almost all adult blackbirds moulting in August.
Deviating colorations
The drop pattern typical of thrushes is also detectable in blackbirds, but in males it is covered by intense melanin deposits in the feathers. Thus melanism can be considered normal for the species. What is not normal, on the other hand, is the reduction of pigment or the loss of pigment, which can occur in different forms and intensities: Pale colorations (chlorochroism, flavism) are caused by albinism. Completely albinotic white birds with red eyes are unlikely to survive in the wild because of their reduced vision. White birds with brown or black eyes are leucistic. Patches are due to attenuated leucism.
In some years, pied animals occur locally clustered. The resulting symmetrical or asymmetrical patterns are extremely variable. On the one hand, this white colouration may apparently be a hereditary or permanent disorder during the creation or development of the follicles. On the other hand, it has been shown experimentally that the white feathers in the plumage may depend on the composition of the diet, especially during the moult. Low-protein diets seem to favour albinism.
Aberrant colorations are observed today mainly in birds in the settlement area, but were known long before blackbirds were found near humans, for example Aristotle already described white blackbirds.
Body measurements and weight
With a body length of between 24 and 27 centimetres, blackbirds of the nominate form are only marginally smaller than the largest Central European thrush species, the mistle thrush. Males are slightly larger than females. The wing length of the male averages 133 mm and 128 mm for the female, roughly corresponding to a wingspan between 34 and 38.5 centimetres. The tail length is between 104 and 116 millimetres.
Weight fluctuations during the year are considerable in European blackbirds. In multi-year surveys conducted in the UK, weights ranged from 71 to 150 grams, with adult males averaging 102.8 grams and adult females slightly lighter at 100.3 grams. One-year-old birds averaged just under 3 grams lighter. Throughout the year, females are slightly heavier than males only during the laying season. Central European blackbirds have their greatest weight in January and their lowest in July or August, after the breeding season. The weight gain results from the build-up of fat reserves.
