Colour-phases in juvenile Honey-buzzard in Northumberland

Particular attention was placed in 2016 on determining the colour phases of the juvenile Honey-buzzard, as shown in Table 28:

Table 28. Incidence of colour phases in juvenile Honey-buzzard in 2016 bred in the Study Area in SW Northumberland and in migrants passing through northern England

Colour phases were noted in the field followed by checks made on photographs obtained. The colour was assessed on the coverts of the underwing and on the underbody; the head colour was also noted, particularly on paler individuals. The difference between dark- and brown-phase birds is subjective to some extent with the shade on a cline from almost black to a neutral brown; the distinction was often made when studying a pair of juveniles where one looked significantly darker than the other. In the study area in SW Northumberland, 82 juveniles were studied in post-fledging situations from 5/8-25/9, including one juvenile just outside the current study area; the colour phase for 15 of these birds (18%) could not be determined as the birds were too distant or the lighting was too poor such as the bird being in silhouette. Presumed migrants were also studied with 26 juveniles being assessed from 9/9-4/11; the colour phase could not be determined for 1 of these birds (4%). These presumed migrants included 22 in Northumberland over the whole quoted period and 4 in the Lake District from 9/10-12/10. These presumed migrants are considered to be almost entirely Scottish-bred, moving slowly through northern England. For the overall figures,108 juvenile were studied of which 16 could not be assessed (15%).

The results in the study area show that 66% were dark-phase, with 15% brown, 7% rufous/ruddy, 7% pale, 3% purple and 1% barred. There is therefore a wide range of plumage but 81% are of the dark/brown types, indicating that to a casual observer, juvenile Honey-buzzard appear sombre colour-wise. The smaller sample of migrants shows a broadly similar picture except for a significantly higher proportion in the rufous/ruddy category (28%). Adding the figures together to give an overall picture gives 64% dark and 11% brown, making 75% in the sombre class. Minor but still significant phases are 13% rufous/ruddy and 9% pale, with just 2% purple and 1% barred.

Worth quoting from Ferguson-Lees, James, and Christie, David A, Raptors of the World, Christopher Helm (2001):

p.341. Western Honey Buzzard is then monotypic but, in parallel with its eastern counterpart, polymorphic to extent of being arguably more diversified in colour and pattern (chiefly head, underbody and wing-linings) than almost any other wild bird species. Often lumped simply as 'typical', 'pale' and 'dark', about ten adult and seven morphs are distinguishable -- some common, some much scarcer -- with further intergrades; adults can be classed as 'white-bellied' (male only), 'densely barred', 'thinly barred', 'black-blotched', 'black-spotted', 'sparsely-spotted', and the more unicolorous 'rufous, 'olive-brown', 'dark brown' and 'melanistic'; counterparts of the last four are also found in juvenile plumages, along with 'buff', 'light' and 'whitish'.

This quote shows the variability and complexity of the juvenile plumage. My classification focuses on what can be seen at some distance in the field. Dick Forsman, in Flight Identification of Raptors of Europe, North Africa and the Middle East, Christopher Helm (2016), notes:

p.56-57. Although juveniles come in many different plumage-types, uniformly dark brown birds predominate and account for about 90% of all juveniles. The remaining birds are lighter below, from sandy buff to pure white, variably streaked or mottled, often with a pale or whitish head, with showy dark 'sunglasses' in many. The underwing coverts vary in accordance with the general plumage colour and pattern … The upperparts also vary according to the general colouration of the plumage …

On my classification the pale birds make up 9%, in line with the estimate by Forsman of 10%. The sombre birds make up 75%, less than the 90% estimated by Forsman, but some of my darker ruddy-plumaged birds and the purple-phase birds would I feel be considered as simply dark by Forsman so our estimates seem to be in reasonable agreement. Forsman does show a ruddy-tinged dark juvenile in picture 27 (p.61).

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