Topic : Rare Birds
Article 25 08 March 2007
Avian Colour Oddities
Whilst on a recent visit to Madikwe Game Reserve in South Africa’s North West Province, we encountered an aberrant coloured bird that prompted me to delve further into plumage anomalies in birds. A photographed rare yellow morph of the Crimson-breasted Shrike Laniarius atrococcineus caused much debate as to what was the cause for its differing colour. The only other locally recorded example of another yellow morph bird is the Black-collared Barbet Lybius torquatus. When the plumage colouration deviates from the norm, perplexed birders are often left wondering if they are looking at a new possibly undescribed species!
Plumage anomalies can often be the result of disease, dietary deficiencies and often genetic mutation. This should not be confused with colour changes caused by diet such as the pinkish wash to Lesser Flamingo Phoenicopterus minor from the carotenoids extracted with some of the food as they filter feed.
Leucism is a reduction in intensity of pigments in the feathers, and is fairly common. There has been various letters in Africa Birds & Birding regarding leucistic birds over the years from Blue Crane Anthropoides paradiseus to Cape Sugarbird Promerops cafer and Natal Francolin Pternistes natalensis. (See Vol. 5(5):21, Vol. 10(1):14, Vol. 10(4):9). Whilst a large proportion of the bird’s feathers are white, the eyes remain dark and there is a vestige of some other feather coloration as well. In short, leucism is a genetic mutation in which the bird may have anywhere from one white feather to all white feathers, but coloured eyes or legs or beak. The dangers of leucism are numerous although it is often recorded that the bird appears to lead a ‘normal’ life, albeit in a different coloured dress to his mates.
The bird is obviously more conspicuous, thus a greater risk to predation. It is also generally accepted that leucistic birds have a shorter lifespan. The loss in pigments leads to a weakened feather structure over time with resultant increased wear. The retina of the eye is also often affected resulting in increased light sensitivity and even blindness from the sun. The success at mate attraction, based on visual feather cues could also be compromised, although leucistic birds often breed and rear young. Flock living species will often not accept a leucistic bird. Albinism on the other hand is the complete absence of pigmentation due to inability to produce melanin. All the feathers are snow white and the soft parts are pinkish. The eye colour is also pink as a result of blood vessels showing through in the absence of darker colours. Albinism is normally caused by a genetic mutation that can be inherited if both parents have the albino gene. These birds generally do not survive more than a few days and are rarely seen.
The other extreme to plumage colouration is excessive pigmentation, when the bird’s body synthesizes too much of a certain pigment in the skin and growing feathers. Melanism is due to an excess of the eumelanins resulting in an abnormally dark plumaged bird. In Southern Africa the Gabar Goshawk Melierax gabar is often seen in his all black ‘batman suit’. Melanism has also been recorded in Cattle Egret. (See Africa Birds & Birding Vol. 10(2): 16). Erythrism is an excess of red pigmentation. A good example of this for local birders is probably the rare rufous form of the female Common Cuckoo Cuculus canorus.
So by now you may be asking what caused the strange colour of the shrike. Colour morphs are a type of plumage abnormality in that the bird is born with a certain different feather colour to the norm for that species. A more technical term for this is Xanthochroism or Flavism. In xanthochroistic birds, either there is excessive yellow pigment in the feathers or yellow replaces another colour, typically red. This resulted in the yellow throat, chest and belly on the photographed Crimson-breasted Shrike above, with the normal red pigment failing to develop.
So for whatever the rhyme or ‘...ism’ aberrant coloured birds definitely attract our attention.