Bird watching (and gull watching in particular), is all about learning how to recognize and age a bird by looking for the relevant clues in the bird’s plumage. After having learned what a standard plumage looks like for a given species at a given age, any plumage which does not adhere to this standard can be easily recognized.
One non-standard feature though stands out more than the rest and will be picked up by any birder, regardless of its interest in plumage detail: leucism. In short, leucism is a condition in which the color of a feather is absent, thereby resulting in a white feather. Birds which are affected this way can show anything from just a few white feathers to being completely white.
Depending on how the bird is normally supposed to look, the result can be spectacular and some of the birds which I have enjoyed observing most are indeed leucistic.
Partly leucistic Black-headed Gull, in between two regular Black-headed Gulls in winter (basic) plumage. This individual lacks pigment on the head, the mantle, and in the scapulars. Full black pigmentation is present though in the primaries. White LSB, Amsterdam, The Netherlands, January 2011.
There is a lot of confusion about what leucism actually is and about the types of leucism that exist, and the general public (but also birders), often mistake leucism by albinism.
What is important to know is that plumage coloration in birds is mostly determined by the pigments melanines and carotenoids.
Two types of melanin are present in birds:
- Eumelanin. Responsible for black, gray, and dark-brown feathers.
- Phaeomelanin. Responsible for reddish-brown feathers (when present in a high concentration), or yellow-brown to white feathers (when present in low concentrations).
When combined, they can create colors such as grayish-brown.
Relevant to know also is that in skin and eyes, only eumalin is present (we’ll see the importance of this when discussing albinism.)
Carotenoids are taken in through feeding and are transformed into color pigments by enzymes. They range in color from pale yellow to scarlet red. Deposition of pigments occurs when the feather starts growing and any abnormal changes to this process are usually caused due to a change in food intake. (Black-headed Gulls can sometimes be seen with a pinkish-colored body; it is understood that this is caused by a change in diet.)
Black-headed Gull with distinctive pink underparts. Leiden, The Netherlands, January 2010.
Leucism means that the eumalanin and phaeomelanin pigments are partly or completely missing in the feathers. As a result, white or less-colored feathers appear at random parts in the plumage. This can range from just a few feathers to a completely white plumage.
First-calendar year leucistic Western Lesser Black-backed Gull, with white patches in the scapulars, tertials, wing coverts, secondaries, and primaries. Leiden, The Netherlands, August 2011.
Adult Western Lesser Black-backed Gull (Larus fuscus graellsii) with one greater primary covert in each wing without any pigment. Interestingly, this individual did not show any leucism the previous season. Texel, The Netherlands, June 2011.
Adult Western Lesser Black-backed Gull (Larus fuscus graellsii) with several greater primary coverts in each wing without any pigment. Texel, The Netherlands, June 2011.
Adult Black-headed Gull in full winter (basic) plumage, with white feathers in the lower scapulars. Leiden, The Netherlands, December 2010.
Adult Black-headed Gull in full winter (basic) plumage, of which some of the scapulars, tertial #3 (on both sides) and greater covert #2 (on the right-hand side) are white. This individual also shows a pink colouration on the breast and in the outer primary shafts. Leiden, The Netherlands, November 2010.
Partly leucistic Black-headed Gull. This individual lacks pigment on the head, the mantle, and in the scapulars. Full black pigmentation is present though in the primaries. White LSB, Amsterdam, The Netherlands, January 2011.
Partly leucistic Black-headed Gull, moulting to summer (alternate) plumage, showing a partially formed hood. White LSB, Amsterdam, The Netherlands, March 2011.
Partly leucistic Black-headed Gull. White LSB, Amsterdam, The Netherlands, November 2011. More images can be found here.
In albinism, the melanin pigment cannot be produced at all, resulting in a completely colorless bird. Remember the remark at the top of this post about skin and eyes only having eumalin? Because an albino bird cannot produce that pigment, its skin and eyes are also colorless. These therefore appear red or pinkish because of the blood that can be seen through the tissue. They are therefore easily distinguishable from leucistic birds, which will always have colored eyes.
Albinos also suffer from a reduced eyesight, making them less likely to survive. This also makes it less likely that birders will encounter such individuals in the field.
Another form of leucism is the ‘Brown’ abbaration, in which the eumalin pigment (responsible for black, gray, and dark-brown feathers) is reduced. Such feathers are very sensitive to sunlight though and can bleach quickly and strongly, turning these feathers white.
Leucistic European Herring Gull (Larus argentatus), brown aberration. This individual appeared almost completely white in the field, with just some brown patches on the outer primaries and primary coverts, and some dark scapulars. It was not until studying the photos that much more brown markings could be observed. The tail markings for instance is fully present. A very striking and beautiful bird to watch. More images can be seen here. Barneveld waste dump, The Netherlands, October 2011.
Source: “Not every white bird is an albino: sense and nonsense about colour aberrations in birds”
Author: Hein van Grouw
Publication: Dutch Birding, vol. 28, no. 2, 2006 (pages 79 – 89)
|vanGrouwHein2006 NotEveryWhiteBirdIsAnAlbinoSenseAndNonsenseAboutColourAberrationsInBirds pdf|
|Found at ebookbrowse.com|